Oracle Database Application Developer’s Guide Fundamentals Developer 10g Release 2

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Oracle® Database
Application Developer's Guide - Fundamentals
10g Release 2 (10.2)
B14251-01

November 2005

Oracle Database Application Developer’s Guide - Fundamentals, 10g Release 2 (10.2)
B14251-01
Copyright © 1996, 2005, Oracle. All rights reserved.
Primary Author:

Lance Ashdown

Contributing Authors:

D. Adams, M. Cowan, R. Moran, J. Melnick, E. Paapanen, J. Russell, R. Strohm

Contributors: D. Alpern, G. Arora, C. Barclay, D. Bronnikov, T. Chang, M. Davidson, G. Doherty, D. Elson,
A. Ganesh, M. Hartstein, J. Huang, N. Jain, R. Jenkins Jr., S. Kotsovolos, S. Kumar, C. Lei, D. Lorentz, V.
Moore, J. Muller, R. Murthy, R. Pang, B. Sinha, S. Vemuri, W. Wang, D. Wong, A. Yalamanchi, Q. Yu
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Contents
Preface ............................................................................................................................................................... xix
Audience.....................................................................................................................................................
Documentation Accessibility ...................................................................................................................
Related Documents ...................................................................................................................................
Conventions ...............................................................................................................................................

xix
xix
xx
xxi

What's New in Application Development? ................................................................................ xxiii
Oracle Database 10g Release 2 (10.2) New Features...........................................................................
Oracle Database 10g Release 1 (10.1) New Features...........................................................................

1

xxiii
xxiv

Orientation to Oracle Programmatic Environments
Overview of Oracle Application Development ................................................................................. 1-1
Overview of PL/SQL................................................................................................................................ 1-2
What Is PL/SQL? ............................................................................................................................... 1-2
Advantages of PL/SQL..................................................................................................................... 1-3
Integration with Oracle Database............................................................................................. 1-4
High Performance....................................................................................................................... 1-4
High Productivity ....................................................................................................................... 1-4
Scalability ..................................................................................................................................... 1-4
Manageability .............................................................................................................................. 1-5
Object-Oriented Programming Support .................................................................................. 1-5
Portability..................................................................................................................................... 1-5
Security ......................................................................................................................................... 1-5
Built-In Packages......................................................................................................................... 1-6
PL/SQL Web Development Tools................................................................................................... 1-6
Overview of Java Support Built Into the Database ........................................................................... 1-6
Overview of Oracle JVM ................................................................................................................... 1-7
Overview of Oracle Extensions to JDBC......................................................................................... 1-8
JDBC Thin Driver........................................................................................................................ 1-8
JDBC OCI Driver......................................................................................................................... 1-8
JDBC Server-Side Internal Driver............................................................................................. 1-9
Oracle Database Extensions to JDBC Standards .................................................................... 1-9
Sample JDBC 2.0 Program ......................................................................................................... 1-9
Sample Pre-2.0 JDBC Program............................................................................................... 1-10
JDBC in SQLJ Applications..................................................................................................... 1-10

iii

Overview of Oracle SQLJ ...............................................................................................................
Benefits of SQLJ........................................................................................................................
Comparing SQLJ with JDBC ..................................................................................................
SQLJ Stored Procedures in the Server...................................................................................
Overview of Oracle JPublisher......................................................................................................
Overview of Java Stored Procedures............................................................................................
Overview of Oracle Database Web Services ...............................................................................
Oracle Database as a Web Service Provider ........................................................................
Overview of Writing Procedures and Functions in Java...........................................................
Overview of Writing Database Triggers in Java .................................................................
Why Use Java for Stored Procedures and Triggers?...........................................................
Overview of Pro*C/C++ .......................................................................................................................
Implementing a Pro*C/C++ Application....................................................................................
Highlights of Pro*C/C++ Features...............................................................................................
Overview of Pro*COBOL ....................................................................................................................
How You Implement a Pro*COBOL Application ......................................................................
Highlights of Pro*COBOL Features .............................................................................................
Overview of OCI and OCCI................................................................................................................
Advantages of OCI..........................................................................................................................
Parts of the OCI ...............................................................................................................................
Procedural and Non-Procedural Elements .................................................................................
Building an OCI Application.........................................................................................................
Overview of Oracle Data Provider for .NET (ODP.NET) ..............................................................
Using ODP.NET in a Simple Application ...................................................................................
Overview of Oracle Objects for OLE (OO4O).................................................................................
OO4O Automation Server .............................................................................................................
OO4O Object Model .......................................................................................................................
OraSession.................................................................................................................................
OraServer ..................................................................................................................................
OraDatabase..............................................................................................................................
OraDynaset ...............................................................................................................................
OraField.....................................................................................................................................
OraMetaData and OraMDAttribute......................................................................................
OraParameters and OraParameter ........................................................................................
OraParamArray........................................................................................................................
OraSQLStmt..............................................................................................................................
OraAQ .......................................................................................................................................
OraAQMsg................................................................................................................................
OraAQAgent.............................................................................................................................
Support for Oracle LOB and Object Datatypes...........................................................................
OraBLOB and OraCLOB .........................................................................................................
OraBFILE...................................................................................................................................
Oracle Data Control ........................................................................................................................
Oracle Objects for OLE C++ Class Library..................................................................................
Additional Sources of Information ...............................................................................................
Choosing a Programming Environment ...........................................................................................
Choosing Whether to Use OCI or a Precompiler .......................................................................

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Using Built-In Packages and Libraries.........................................................................................
Java Compared to PL/SQL............................................................................................................
PL/SQL Is Optimized for Database Access .........................................................................
PL/SQL Is Integrated with the Database .............................................................................
Both Java and PL/SQL Have Object-Oriented Features ....................................................
Java Is Used for Open Distributed Applications.................................................................

Part I
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SQL for Application Developers

SQL Processing for Application Developers
Grouping Operations into Transactions .............................................................................................. 2-1
Deciding How to Group Operations in Transactions................................................................... 2-1
Improving Transaction Performance ............................................................................................. 2-2
Committing Transactions.................................................................................................................. 2-2
Managing Commit Redo Behavior........................................................................................... 2-3
Rolling Back Transactions ................................................................................................................ 2-4
Defining Transaction Savepoints .................................................................................................... 2-5
An Example of COMMIT, SAVEPOINT, and ROLLBACK ................................................. 2-5
Ensuring Repeatable Reads with Read-Only Transactions ............................................................ 2-5
Using Cursors within Applications ...................................................................................................... 2-6
Declaring and Opening Cursors ..................................................................................................... 2-6
Using a Cursor to Execute Statements Again ................................................................................ 2-7
Closing Cursors ................................................................................................................................. 2-7
Cancelling Cursors ............................................................................................................................ 2-8
Locking Data Explicitly .......................................................................................................................... 2-8
Choosing a Locking Strategy ........................................................................................................... 2-9
When to Lock with ROW SHARE and ROW EXCLUSIVE Mode ....................................... 2-9
When to Lock with SHARE Mode............................................................................................ 2-9
When to Lock with SHARE ROW EXCLUSIVE Mode....................................................... 2-10
When to Lock in EXCLUSIVE Mode..................................................................................... 2-11
Privileges Required ................................................................................................................. 2-11
Letting Oracle Database Control Table Locking......................................................................... 2-11
Explicitly Acquiring Row Locks .................................................................................................. 2-12
About User Locks .................................................................................................................................. 2-13
When to Use User Locks ................................................................................................................ 2-13
Example of a User Lock.................................................................................................................. 2-13
Viewing and Monitoring Locks .................................................................................................... 2-14
Using Serializable Transactions for Concurrency Control ........................................................... 2-14
How Serializable Transactions Interact ....................................................................................... 2-15
Setting the Isolation Level of a Transaction ................................................................................ 2-16
The INITRANS Parameter ..................................................................................................... 2-16
Referential Integrity and Serializable Transactions .................................................................. 2-17
Using SELECT FOR UPDATE................................................................................................ 2-18
READ COMMITTED and SERIALIZABLE Isolation ............................................................... 2-18
Transaction Set Consistency .................................................................................................. 2-18
Comparison of READ COMMITTED and SERIALIZABLE Transactions....................... 2-19

v

Choosing an Isolation Level for Transactions......................................................................
Application Tips for Transactions ................................................................................................
Autonomous Transactions ...................................................................................................................
Examples of Autonomous Transactions ......................................................................................
Entering a Buy Order ..............................................................................................................
Example: Making a Bank Withdrawal ..................................................................................
Defining Autonomous Transactions ............................................................................................
Restrictions on Autonomous Transactions..................................................................................
Resuming Execution After a Storage Error Condition...................................................................
What Operations Can Be Resumed After an Error Condition?................................................
Limitations on Resuming Operations After an Error Condition .............................................
Writing an Application to Handle Suspended Storage Allocation..........................................
Example of Resumable Storage Allocation .................................................................................

3

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Using SQL Datatypes in Application Development
Representing Data with SQL Datatypes: Overview.......................................................................... 3-1
Representing Character Data ................................................................................................................. 3-2
Representing Character Data: Overview ........................................................................................ 3-2
Specifying Column Lengths as Bytes or Characters ..................................................................... 3-3
Choosing Between the CHAR and VARCHAR2 Datatypes........................................................ 3-3
Using Character Literals in SQL Statements .................................................................................. 3-4
Quoting Character Literals ........................................................................................................ 3-5
Representing Numeric Data................................................................................................................... 3-5
What Are the Numeric Datatypes? ................................................................................................. 3-5
Using Floating-Point Number Formats .......................................................................................... 3-6
Using a Floating-Point Binary Format ..................................................................................... 3-7
Representing Special Values with Native Floating-Point Formats ..................................... 3-8
Using Comparison Operators for Native Floating-Point Datatypes .......................................... 3-9
Performing Arithmetic Operations with Native Floating-Point Datatypes ........................... 3-10
Using Conversion Functions with Native Floating-Point Datatypes ...................................... 3-10
Client Interfaces for Native Floating-Point Datatypes .............................................................. 3-11
OCI Native Floating-Point Datatypes SQLT_BFLOAT and SQLT_BDOUBLE .............. 3-11
Native Floating-Point Datatypes Supported in Oracle OBJECT Types ........................... 3-12
Pro*C/C++ Support for Native Floating-Point Datatypes ................................................ 3-12
Representing Datetime Data............................................................................................................... 3-12
Representing Datetime Data: Overview ...................................................................................... 3-12
Using the DATE Datatype ...................................................................................................... 3-12
Using the TIMESTAMP Datatype ......................................................................................... 3-12
Using the TIMESTAMP WITH TIME ZONE Datatype...................................................... 3-12
Using the TIMESTAMP WITH LOCAL TIME ZONE Datatype....................................... 3-13
Representing the Difference Between Datetime Values..................................................... 3-13
Manipulating the Date Format ..................................................................................................... 3-13
Changing the Default Date Format ....................................................................................... 3-13
Displaying the Current Date and Time ................................................................................ 3-14
Manipulating the Time Format .................................................................................................... 3-14
Performing Date Arithmetic .......................................................................................................... 3-14
Converting Between Datetime Types .......................................................................................... 3-15

vi

Importing and Exporting Datetime Types ..................................................................................
Representing Specialized Data...........................................................................................................
Representing Geographic Data .....................................................................................................
Representing Multimedia Data .....................................................................................................
Representing Large Amounts of Data..........................................................................................
Using RAW and LONG RAW Datatypes.............................................................................
Representing Searchable Text .......................................................................................................
Representing XML ..........................................................................................................................
Representing Dynamically Typed Data.......................................................................................
Representing Data with ANSI/ISO, DB2, and SQL/DS Datatypes .......................................
Representing Conditional Expressions as Data ..............................................................................
Identifying Rows by Address ............................................................................................................
Querying the ROWID Pseudocolumn .........................................................................................
Accessing the ROWID Datatype ...................................................................................................
Restricted ROWID ...................................................................................................................
Extended ROWID ....................................................................................................................
External Binary ROWID..........................................................................................................
Accessing the UROWID Datatype................................................................................................
How Oracle Database Converts Datatypes .....................................................................................
Datatype Conversion During Assignments ................................................................................
Datatype Conversion During Expression Evaluation ..............................................................

4

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Using Regular Expressions in Oracle Database
Using Regular Expressions with Oracle Database: Overview ........................................................ 4-1
What Are Regular Expressions?....................................................................................................... 4-1
How Are Oracle Database Regular Expressions Useful?............................................................. 4-2
Oracle Database Implementation of Regular Expressions........................................................... 4-2
Oracle Database Support for the POSIX Regular Expression Standard..................................... 4-3
Regular Expression Metacharacters in Oracle Database .................................................................. 4-4
POSIX Metacharacters in Oracle Database Regular Expressions................................................ 4-4
Regular Expression Operator Multilingual Enhancements ......................................................... 4-6
Perl-Influenced Extensions in Oracle Regular Expressions ......................................................... 4-7
Using Regular Expressions in SQL Statements: Scenarios .............................................................. 4-9
Using an Integrity Constraint to Enforce a Phone Number Format........................................... 4-9
Using Back References to Reposition Characters ....................................................................... 4-10

5

Using Indexes in Application Development
Guidelines for Application-Specific Indexes .....................................................................................
Create Indexes After Inserting Table Data ....................................................................................
Switch Your Temporary Tablespace to Avoid Space Problems Creating Indexes...................
Index the Correct Tables and Columns ..........................................................................................
Limit the Number of Indexes for Each Table .................................................................................
Choose the Order of Columns in Composite Indexes ..................................................................
Gather Statistics to Make Index Usage More Accurate ................................................................
Drop Indexes That Are No Longer Required ...............................................................................
Privileges Required to Create an Index .........................................................................................

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Creating Indexes: Basic Examples ........................................................................................................
When to Use Domain Indexes ...............................................................................................................
When to Use Function-Based Indexes ..................................................................................................
Advantages of Function-Based Indexes..........................................................................................
Examples of Function-Based Indexes..............................................................................................
Example: Function-Based Index for Case-Insensitive Searches ...........................................
Example: Precomputing Arithmetic Expressions with a Function-Based Index...............
Example: Function-Based Index for Language-Dependent Sorting....................................
Restrictions for Function-Based Indexes.........................................................................................

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Maintaining Data Integrity in Application Development
Overview of Integrity Constraints ........................................................................................................ 6-1
When to Enforce Business Rules with Integrity Constraints ...................................................... 6-1
Example of an Integrity Constraint for a Business Rule ....................................................... 6-2
When to Enforce Business Rules in Applications ......................................................................... 6-2
Creating Indexes for Use with Constraints .................................................................................... 6-2
When to Use NOT NULL Integrity Constraints ........................................................................... 6-2
When to Use Default Column Values ............................................................................................ 6-3
Setting Default Column Values ....................................................................................................... 6-4
Choosing a Table's Primary Key ..................................................................................................... 6-4
When to Use UNIQUE Key Integrity Constraints ........................................................................ 6-5
Constraints On Views: for Performance, Not Data Integrity....................................................... 6-5
Enforcing Referential Integrity with Constraints .............................................................................. 6-6
About Nulls and Foreign Keys......................................................................................................... 6-7
Defining Relationships Between Parent and Child Tables .......................................................... 6-8
Rules for Multiple FOREIGN KEY Constraints ............................................................................ 6-9
Deferring Constraint Checks ............................................................................................................ 6-9
Guidelines for Deferring Constraint Checks .......................................................................... 6-9
Managing Constraints That Have Associated Indexes .................................................................. 6-10
Minimizing Space and Time Overhead for Indexes Associated with Constraints................ 6-10
Guidelines for Indexing Foreign Keys.............................................................................................. 6-11
About Referential Integrity in a Distributed Database................................................................. 6-11
When to Use CHECK Integrity Constraints..................................................................................... 6-11
Restrictions on CHECK Constraints ............................................................................................ 6-12
Designing CHECK Constraints .................................................................................................... 6-12
Rules for Multiple CHECK Constraints ...................................................................................... 6-12
Choosing Between CHECK and NOT NULL Integrity Constraints ...................................... 6-13
Examples of Defining Integrity Constraints ................................................................................... 6-13
Example: Defining Integrity Constraints with the CREATE TABLE Command .................. 6-13
Example: Defining Constraints with the ALTER TABLE Command...................................... 6-14
Privileges Required to Create Constraints ................................................................................. 6-14
Naming Integrity Constraints ...................................................................................................... 6-14
Enabling and Disabling Integrity Constraints ............................................................................... 6-14
Why Disable Constraints? ............................................................................................................. 6-15
About Exceptions to Integrity Constraints.................................................................................. 6-15
Enabling Constraints ..................................................................................................................... 6-15
Creating Disabled Constraints ..................................................................................................... 6-16

viii

Enabling and Disabling Existing Integrity Constraints.............................................................
Enabling Existing Constraints ...............................................................................................
Disabling Existing Constraints ..............................................................................................
Tip: Using the Data Dictionary to Find Constraints ...........................................................
Guidelines for Enabling and Disabling Key Integrity Constraints..........................................
Fixing Constraint Exceptions .......................................................................................................
Altering Integrity Constraints ............................................................................................................
Renaming Integrity Constraints....................................................................................................
Dropping Integrity Constraints..........................................................................................................
Managing FOREIGN KEY Integrity Constraints ...........................................................................
Datatypes and Names for Foreign Key Columns.......................................................................
Limit on Columns in Composite Foreign Keys ..........................................................................
Foreign Key References Primary Key by Default.......................................................................
Privileges Required to Create FOREIGN KEY Integrity Constraints......................................
Choosing How Foreign Keys Enforce Referential Integrity ....................................................
Viewing Definitions of Integrity Constraints ................................................................................
Examples of Defining Integrity Constraints................................................................................

Part II
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PL/SQL for Application Developers

Coding PL/SQL Procedures and Packages
Overview of PL/SQL Program Units ................................................................................................... 7-1
Anonymous Blocks ........................................................................................................................... 7-2
Stored Program Units (Procedures, Functions, and Packages) .................................................. 7-3
Naming Procedures and Functions ......................................................................................... 7-4
Parameters for Procedures and Functions ............................................................................. 7-4
Creating Stored Procedures and Functions ........................................................................... 7-7
Altering Stored Procedures and Functions ............................................................................ 7-8
Dropping Procedures and Functions ...................................................................................... 7-8
External Procedures ................................................................................................................... 7-9
PL/SQL Packages ...................................................................................................................... 7-9
PL/SQL Object Size Limitation ............................................................................................ 7-11
Creating Packages ................................................................................................................... 7-11
Naming Packages and Package Objects .............................................................................. 7-12
Package Invalidations and Session State ............................................................................. 7-12
Packages Supplied With Oracle Database ........................................................................... 7-12
Overview of Bulk Binds .......................................................................................................... 7-12
When to Use Bulk Binds ......................................................................................................... 7-13
Triggers ..................................................................................................................................... 7-15
Compiling PL/SQL Procedures for Native Execution .................................................................... 7-15
Remote Dependencies ......................................................................................................................... 7-15
Timestamps ..................................................................................................................................... 7-16
Disadvantages of the Timestamp Model ............................................................................. 7-16
Signatures ........................................................................................................................................ 7-16
When Does a Signature Change?........................................................................................... 7-18
Examples of Changing Procedure Signatures .................................................................... 7-19

ix

Controlling Remote Dependencies ..............................................................................................
Dependency Resolution .........................................................................................................
Suggestions for Managing Dependencies ...........................................................................
Cursor Variables ...................................................................................................................................
Declaring and Opening Cursor Variables ..................................................................................
Examples of Cursor Variables ......................................................................................................
Fetching Data ...........................................................................................................................
Implementing Variant Records .............................................................................................
Handling PL/SQL Compile-Time Errors .........................................................................................
Handling Run-Time PL/SQL Errors ..................................................................................................
Declaring Exceptions and Exception Handling Routines ........................................................
Unhandled Exceptions ..................................................................................................................
Handling Errors in Distributed Queries .....................................................................................
Handling Errors in Remote Procedures ......................................................................................
Debugging Stored Procedures ............................................................................................................
Calling Stored Procedures ...................................................................................................................
A Procedure or Trigger Calling Another Procedure..................................................................
Interactively Calling Procedures From Oracle Database Tools ...............................................
Calling Procedures within 3GL Applications ............................................................................
Name Resolution When Calling Procedures ..............................................................................
Privileges Required to Execute a Procedure ..............................................................................
Specifying Values for Procedure Arguments .............................................................................
Calling Remote Procedures ................................................................................................................
Remote Procedure Calls and Parameter Values .........................................................................
Referencing Remote Objects ..........................................................................................................
Synonyms for Procedures and Packages ....................................................................................
Calling Stored Functions from SQL Expressions ...........................................................................
Using PL/SQL Functions ..............................................................................................................
Syntax for SQL Calling a PL/SQL Function ...............................................................................
Naming Conventions .....................................................................................................................
Name Precedence ....................................................................................................................
Arguments ...............................................................................................................................
Using Default Values ..............................................................................................................
Privileges ..................................................................................................................................
Requirements for Calling PL/SQL Functions from SQL Expressions ....................................
Controlling Side Effects .................................................................................................................
Restrictions................................................................................................................................
Declaring a Function ...............................................................................................................
Parallel Query and Parallel DML .........................................................................................
PRAGMA RESTRICT_REFERENCES – for Backward Compatibility .............................
Serially Reusable PL/SQL Packages ...........................................................................................
Package States ..........................................................................................................................
Why Serially Reusable Packages? .........................................................................................
Syntax of Serially Reusable Packages ...................................................................................
Semantics of Serially Reusable Packages..............................................................................
Examples of Serially Reusable Packages ..............................................................................
Returning Large Amounts of Data from a Function.......................................................................

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Coding Your Own Aggregate Functions ........................................................................................... 7-52

8

Coding Dynamic SQL
What Is Dynamic SQL? ........................................................................................................................... 8-1
Programming with Dynamic SQL ................................................................................................... 8-1
Why Use Dynamic SQL?......................................................................................................................... 8-2
Executing DDL and SCL Statements in PL/SQL .......................................................................... 8-3
Executing Dynamic Queries ............................................................................................................. 8-4
Referencing Database Objects that Do Not Exist at Compilation ............................................... 8-4
Optimizing Execution Dynamically ............................................................................................... 8-5
Executing Dynamic PL/SQL Blocks ............................................................................................... 8-5
Performing Dynamic Operations Using Invoker's Rights .......................................................... 8-6
Developing with Native Dynamic SQL: Scenario ............................................................................. 8-7
Sample DML Operation Using Native Dynamic SQL .................................................................. 8-7
Sample DDL Operation Using Native Dynamic SQL................................................................... 8-8
Sample Single-Row Query Using Native Dynamic SQL.............................................................. 8-8
Sample Multiple-Row Query with Native Dynamic SQL............................................................ 8-9
Choosing Between Native Dynamic SQL and the DBMS_SQL Package ..................................... 8-9
Advantages of Native Dynamic SQL ........................................................................................... 8-10
Native Dynamic SQL is Easy to Use ..................................................................................... 8-10
Native Dynamic SQL is Faster than DBMS_SQL ................................................................ 8-11
Native Dynamic SQL Supports User-Defined Types ......................................................... 8-12
Native Dynamic SQL Supports Fetching into Records ...................................................... 8-12
Advantages of the DBMS_SQL Package...................................................................................... 8-13
DBMS_SQL is Supported in Client-Side Programs ............................................................ 8-13
DBMS_SQL Supports Statements with Unknown Number of Inputs or Outputs ........ 8-13
DBMS_SQL Supports SQL Statements Larger than 32 KB ................................................ 8-13
DBMS_SQL Lets You Reuse SQL Statements ...................................................................... 8-13
Examples of DBMS_SQL Package Code and Native Dynamic SQL Code ............................. 8-14
Querying with Dynamic SQL: Example ............................................................................... 8-14
Performing DML with Dynamic SQL: Example ................................................................. 8-15
Performing DML with RETURNING Clause Using Dynamic SQL: Example ............... 8-16
Avoiding SQL Injection in PL/SQL ................................................................................................... 8-17
Overview of SQL Injection Techniques ....................................................................................... 8-17
Statement Modification ........................................................................................................... 8-18
Statement Injection .................................................................................................................. 8-19
Guarding Against SQL Injection................................................................................................... 8-21
Using Bind Variables to Guard Against SQL Injection ...................................................... 8-21
Using Validation Checks to Guard Against SQL Injection................................................ 8-22

9

Coding Triggers
Designing Triggers...................................................................................................................................
Creating Triggers .....................................................................................................................................
Types of Triggers ...............................................................................................................................
Overview of System Events.......................................................................................................
Getting the Attributes of System Events .................................................................................

9-1
9-2
9-3
9-3
9-3

xi

Naming Triggers ............................................................................................................................... 9-3
When Is the Trigger Fired? .............................................................................................................. 9-3
Do Import and SQL*Loader Fire Triggers?............................................................................. 9-4
How Column Lists Affect UPDATE Triggers ........................................................................ 9-4
Controlling When a Trigger Is Fired (BEFORE and AFTER Options) ...................................... 9-4
Ordering of Triggers ......................................................................................................................... 9-5
Modifying Complex Views (INSTEAD OF Triggers) ................................................................... 9-6
Views that Require INSTEAD OF Triggers............................................................................. 9-6
INSTEAD OF Trigger Example................................................................................................. 9-7
Object Views and INSTEAD OF Triggers .............................................................................. 9-8
Triggers on Nested Table View Columns ............................................................................... 9-8
Firing Triggers One or Many Times (FOR EACH ROW Option) .............................................. 9-9
Firing Triggers Based on Conditions (WHEN Clause) ............................................................. 9-10
Coding the Trigger Body ..................................................................................................................... 9-10
Accessing Column Values in Row Triggers ............................................................................... 9-12
Example: Modifying LOB Columns with a Trigger............................................................ 9-12
INSTEAD OF Triggers on Nested Table View Columns ................................................... 9-13
Avoiding Name Conflicts with Triggers (REFERENCING Option) ............................... 9-13
Detecting the DML Operation That Fired a Trigger ........................................................... 9-14
Error Conditions and Exceptions in the Trigger Body ...................................................... 9-14
Triggers on Object Tables............................................................................................................... 9-14
Triggers and Handling Remote Exceptions ............................................................................... 9-15
Restrictions on Creating Triggers ................................................................................................ 9-16
Who Is the Trigger User? ............................................................................................................... 9-19
Privileges Needed to Work with Triggers .................................................................................. 9-19
Compiling Triggers .............................................................................................................................. 9-20
Dependencies for Triggers ............................................................................................................ 9-20
Recompiling Triggers .................................................................................................................... 9-20
Modifying Triggers .............................................................................................................................. 9-21
Debugging Triggers ....................................................................................................................... 9-21
Enabling and Disabling Triggers ....................................................................................................... 9-21
Enabling Triggers ........................................................................................................................... 9-21
Disabling Triggers .......................................................................................................................... 9-21
Viewing Information About Triggers ............................................................................................... 9-22
Examples of Trigger Applications ..................................................................................................... 9-23
Responding to System Events through Triggers ............................................................................ 9-37
How Events Are Published Through Triggers ........................................................................... 9-37
Publication Context......................................................................................................................... 9-38
Error Handling ................................................................................................................................ 9-38
Execution Model.............................................................................................................................. 9-38
Event Attribute Functions.............................................................................................................. 9-38
List of Database Events .................................................................................................................. 9-41
System Events........................................................................................................................... 9-41
Client Events............................................................................................................................. 9-42

10

Developing Flashback Applications
Overview of Flashback Features ....................................................................................................... 10-1

xii

Application Development Features..............................................................................................
Database Administration Features ...............................................................................................
Database Administration Tasks Before Using Flashback Features .............................................
Using Flashback Query (SELECT ... AS OF) ...................................................................................
Examining Past Data: Example .....................................................................................................
Tips for Using Flashback Query ...................................................................................................
Using the DBMS_FLASHBACK Package.........................................................................................
Using ORA_ROWSCN.........................................................................................................................
Using Flashback Version Query ........................................................................................................
Using Flashback Transaction Query................................................................................................
Flashback Transaction Query and Flashback Version Query: Example ...............................
Flashback Tips .....................................................................................................................................
Flashback Tips – Performance.....................................................................................................
Flashback Tips – General .............................................................................................................

11

Developing Applications with the PL/SQL Web Toolkit
Developing PL/SQL Web Applications: Overview ........................................................................
Invoking a PL/SQL Web Application..........................................................................................
Implementing a PL/SQL Web Application ................................................................................
PL/SQL Web Toolkit...............................................................................................................
Using the mod_plsql Gateway ...........................................................................................................
Generating HTML Output with PL/SQL..........................................................................................
Passing Parameters to a PL/SQL Web Application .........................................................................
Passing List and Dropdown List Parameters from an HTML Form .......................................
Passing Radio Button and Checkbox Parameters from an HTML Form ................................
Passing Entry Field Parameters from an HTML Form ..............................................................
Passing Hidden Parameters from an HTML Form ....................................................................
Uploading a File from an HTML Form........................................................................................
Submitting a Completed HTML Form.........................................................................................
Handling Missing Input from an HTML Form ..........................................................................
Maintaining State Information Between Web Pages .................................................................
Performing Network Operations within PL/SQL Stored Procedures.......................................
Sending E-Mail from PL/SQL.....................................................................................................
Getting a Host Name or Address from PL/SQL......................................................................
Working with TCP/IP Connections from PL/SQL .................................................................
Retrieving the Contents of an HTTP URL from PL/SQL .......................................................
Working with Tables, Image Maps, Cookies, and CGI Variables from PL/SQL ...............

12

10-2
10-2
10-3
10-4
10-5
10-5
10-6
10-7
10-8
10-10
10-10
10-12
10-12
10-13

11-1
11-1
11-2
11-2
11-3
11-4
11-5
11-5
11-6
11-6
11-8
11-8
11-8
11-9
11-9
11-10
11-10
11-10
11-11
11-11
11-13

Developing PL/SQL Server Pages
PL/SQL Server Pages: Overview ........................................................................................................
What Are PL/SQL Server Pages and Why Use Them?.............................................................
Prerequisites for Developing and Deploying PL/SQL Server Pages......................................
PSP and the HTP Package..............................................................................................................
PSP and Other Scripting Solutions ...............................................................................................
Writing a PL/SQL Server Page ............................................................................................................
Specifying Basic Server Page Characteristics ..............................................................................

12-1
12-1
12-2
12-3
12-3
12-4
12-5

xiii

Specifying the Scripting Language........................................................................................
Returning Data to the Client ..................................................................................................
Handling Script Errors ............................................................................................................
Accepting User Input......................................................................................................................
Naming the PL/SQL Stored Procedure.......................................................................................
Including the Contents of Other Files ..........................................................................................
Declaring Global Variables in a PSP Script .................................................................................
Specifying Executable Statements in a PSP Script......................................................................
Substituting an Expression Result in a PSP Script ...................................................................
Quoting and Escaping Strings in a PSP Script ..........................................................................
Including Comments in a PSP Script .........................................................................................
Loading a PL/SQL Server Page into the Database ........................................................................
Querying PSP Source Code .........................................................................................................
Executing a PL/SQL Server Page Through a URL.........................................................................
Examples of PL/SQL Server Pages ...................................................................................................
Setup for PL/SQL Server Pages Examples................................................................................
Printing the Sample Table with a Loop .....................................................................................
Allowing a User Selection............................................................................................................
Using an HTML Form to Call a PL/SQL Server Page.............................................................
Including JavaScript in a PSP File .......................................................................................
Debugging PL/SQL Server Page Problems ....................................................................................
Putting PL/SQL Server Pages into Production ..............................................................................

13

Developing Applications with Database Change Notification
What Is Database Change Notification? ...........................................................................................
Using Database Change Notification in the Middle Tier .............................................................
Registering Queries for Database Change Notification ................................................................
Privileges ..........................................................................................................................................
What Is a Database Change Registration?...................................................................................
Supported Query Types .................................................................................................................
Registration Properties ...................................................................................................................
Drop Table........................................................................................................................................
Interfaces for Database Change Registration ..............................................................................
Creating a PL/SQL Stored Procedure as the Change Notification Recipient........................
Registering Queries for Change Notification Through PL/SQL .............................................
Creating a CHNF$_REG_INFO Object .................................................................................
Creating a Registration with DBMS_CHANGE_NOTIFICATION................................
Adding Objects to an Existing Registration .......................................................................
Querying Change Notification Registrations................................................................................
Interpreting a Database Change Notification................................................................................
Interpreting a CHNF$_DESC Object..........................................................................................
Interpreting a CHNF$_TDESC Object ................................................................................
Interpreting a CHNF$_RDESC Object ................................................................................
Configuring Database Change Notification: Scenario ................................................................
Creating a PL/SQL Callback Procedure....................................................................................
Registering the Query...................................................................................................................
Best Practices ........................................................................................................................................

xiv

12-5
12-5
12-7
12-7
12-8
12-8
12-9
12-9
12-10
12-11
12-11
12-12
12-13
12-14
12-15
12-15
12-16
12-17
12-18
12-19
12-20
12-21

13-1
13-2
13-5
13-5
13-5
13-6
13-6
13-7
13-8
13-8
13-9
13-9
13-11
13-12
13-12
13-13
13-13
13-13
13-14
13-14
13-15
13-16
13-17

Troubleshooting................................................................................................................................... 13-18

Part III
14

Advanced Topics for Application Developers

Calling External Procedures
Overview of Multi-Language Programs ...........................................................................................
What Is an External Procedure? ..........................................................................................................
Overview of The Call Specification for External Procedures.......................................................
Loading External Procedures ..............................................................................................................
Loading Java Class Methods .........................................................................................................
Loading External C Procedures ....................................................................................................
Publishing External Procedures .........................................................................................................
The AS LANGUAGE Clause for Java Class Methods ...............................................................
The AS LANGUAGE Clause for External C Procedures ..........................................................
LIBRARY ..................................................................................................................................
NAME .....................................................................................................................................
LANGUAGE ..........................................................................................................................
CALLING STANDARD .......................................................................................................
WITH CONTEXT ..................................................................................................................
PARAMETERS ......................................................................................................................
AGENT IN ..............................................................................................................................
Publishing Java Class Methods........................................................................................................
Publishing External C Procedures ...................................................................................................
Locations of Call Specifications .......................................................................................................
Passing Parameters to External C Procedures with Call Specifications ...................................
Specifying Datatypes ....................................................................................................................
External Datatype Mappings.......................................................................................................
BY VALUE/REFERENCE for IN and IN OUT Parameter Modes .......................................
The PARAMETERS Clause..........................................................................................................
Overriding Default Datatype Mapping .....................................................................................
Specifying Properties ....................................................................................................................
INDICATOR ...........................................................................................................................
LENGTH and MAXLEN.......................................................................................................
CHARSETID and CHARSETFORM....................................................................................
Repositioning Parameters.....................................................................................................
Using SELF..............................................................................................................................
Passing Parameters by Reference ........................................................................................
WITH CONTEXT ...................................................................................................................
Inter-Language Parameter Mode Mappings......................................................................
Executing External Procedures with the CALL Statement ..........................................................
Preconditions for External Procedures ......................................................................................
Privileges of External Procedures........................................................................................
Managing Permissions ..........................................................................................................
Creating Synonyms for External Procedures.....................................................................
CALL Statement Syntax ...............................................................................................................
Calling Java Class Methods .........................................................................................................

14-1
14-2
14-3
14-3
14-3
14-4
14-8
14-9
14-9
14-9
14-10
14-10
14-10
14-10
14-10
14-10
14-10
14-11
14-11
14-14
14-15
14-16
14-18
14-18
14-19
14-19
14-20
14-21
14-21
14-22
14-22
14-24
14-25
14-25
14-25
14-26
14-26
14-27
14-27
14-27
14-28

xv

How the Database Server Calls External C Procedures ..........................................................
Handling Errors and Exceptions in Multi-Language Programs.................................................
Using Service Procedures with External C Procedures................................................................
Doing Callbacks with External C Procedures................................................................................
Object Support for OCI Callbacks...............................................................................................
Restrictions on Callbacks .............................................................................................................
Debugging External Procedures .................................................................................................
Using Package DEBUG_EXTPROC ....................................................................................
Demo Program ..............................................................................................................................
Guidelines for External C Procedures........................................................................................
Restrictions on External C Procedures.......................................................................................

15

Developing Applications with Oracle XA
X/Open Distributed Transaction Processing (DTP)........................................................................
DTP Terminology............................................................................................................................
Required Public Information .........................................................................................................
Oracle XA Library Interface Subroutines.........................................................................................
XA Library Subroutines .................................................................................................................
Extensions to the XA Interface ......................................................................................................
Developing and Installing XA Applications ...................................................................................
Responsibilities of the DBA or System Administrator ..............................................................
Responsibilities of the Application Developer ...........................................................................
Defining the xa_open() String .......................................................................................................
Syntax of the xa_open() String ...............................................................................................
Required Fields for the xa_open() String..............................................................................
Optional Fields for the xa_open() String ..............................................................................
Interfacing XA with Precompilers and OCI ..............................................................................
Using Precompilers with the Oracle XA Library ..............................................................
Using OCI with the Oracle XA Library ..............................................................................
Managing Transaction Control with XA ...................................................................................
Examples of Precompiler Applications ..............................................................................
Migrating Precompiler or OCI Applications to TPM Applications.......................................
Managing XA Library Thread Safety .........................................................................................
Specifying Threading in the Open String...........................................................................
Restrictions on Threading in XA .........................................................................................
Troubleshooting XA Applications ...................................................................................................
Accessing XA Trace Files .............................................................................................................
The xa_open() String DbgFl..................................................................................................
Trace File Locations ...............................................................................................................
Managing In-Doubt or Pending Transactions ..........................................................................
Using SYS Account Tables to Monitor XA Transactions.........................................................
XA Issues and Restrictions ................................................................................................................
Using Database Links in XA Applications ................................................................................
Managing Transaction Branches in XA Applications..............................................................
Using XA with Oracle Real Application Clusters ....................................................................
Managing Transaction Branches on Oracle Real Application Clusters (RAC).............
Managing Instance Recovery in Real Application Clusters ............................................

xvi

14-28
14-29
14-29
14-35
14-37
14-37
14-38
14-38
14-39
14-39
14-40

15-1
15-2
15-4
15-4
15-5
15-5
15-6
15-6
15-7
15-7
15-7
15-8
15-8
15-10
15-10
15-11
15-12
15-13
15-14
15-15
15-15
15-15
15-15
15-16
15-16
15-17
15-17
15-17
15-18
15-18
15-19
15-19
15-19
15-20

Global Uniqueness of XIDs in Real Application Clusters................................................
SQL-Based XA Restrictions..........................................................................................................
Rollbacks and Commits ........................................................................................................
DDL Statements .....................................................................................................................
Session State............................................................................................................................
EXEC SQL ...............................................................................................................................
Miscellaneous Restrictions...........................................................................................................

16

15-21
15-21
15-21
15-22
15-22
15-22
15-22

Developing Applications on the Publish-Subscribe Model
Introduction to Publish-Subscribe ....................................................................................................
Publish-Subscribe Architecture .........................................................................................................
Publish-Subscribe Concepts ...............................................................................................................
Examples of a Publish-Subscribe Mechanism ................................................................................

16-1
16-2
16-3
16-5

Index

xvii

xviii

Preface
The Oracle Database Application Developer's Guide - Fundamentals describes basic
application development features of Oracle Database 10g. Information in this guide
applies to features that work the same on all supported platforms, and does not
include system-specific information.

Audience
Oracle Database Application Developer's Guide - Fundamentals is intended for
programmers developing new applications or converting existing applications to run
in the Oracle Database environment. This book will also be valuable to systems
analysts, project managers, and others interested in the development of database
applications.
To use this document, you need a working knowledge of application programming,
and that you are acquainted with using the Structured Query Language (SQL) to
access information in relational database systems. Some sections of this guide assume
a familiar with object-oriented programming.

Documentation Accessibility
Our goal is to make Oracle products, services, and supporting documentation
accessible, with good usability, to the disabled community. To that end, our
documentation includes features that make information available to users of assistive
technology. This documentation is available in HTML format, and contains markup to
facilitate access by the disabled community. Accessibility standards will continue to
evolve over time, and Oracle is actively engaged with other market-leading
technology vendors to address technical obstacles so that our documentation can be
accessible to all of our customers. For more information, visit the Oracle Accessibility
Program Web site at
http://www.oracle.com/accessibility/
Accessibility of Code Examples in Documentation
Screen readers may not always correctly read the code examples in this document. The
conventions for writing code require that closing braces should appear on an
otherwise empty line; however, some screen readers may not always read a line of text
that consists solely of a bracket or brace.

xix

Accessibility of Links to External Web Sites in Documentation
This documentation may contain links to Web sites of other companies or
organizations that Oracle does not own or control. Oracle neither evaluates nor makes
any representations regarding the accessibility of these Web sites.
TTY Access to Oracle Support Services
Oracle provides dedicated Text Telephone (TTY) access to Oracle Support Services
within the United States of America 24 hours a day, seven days a week. For TTY
support, call 800.446.2398.

Related Documents
For more information, see these Oracle resources:
■

■

■

■

■

Oracle Database PL/SQL User's Guide and Reference to learn PL/SQL and to get a
complete description of the PL/SQL high-level programming language, which is
Oracle's procedural extension to SQL.
Oracle Call Interface Programmer's Guide and Oracle C++ Call Interface Programmer's
Guide to learn about the Oracle Call Interface (OCI). You can use the OCI to build
third-generation language (3GL) applications that access the Oracle Database.
Oracle Database Security Guide to learn about security features of the database that
application developers and database administrators need to be aware of.
The Oracle documentation for the Pro* series of precompilers, which allow you to
embed SQL and PL/SQL in your programs. If you write 3GL application
programs in C, C++, COBOL, or FORTRAN that incorporate embedded SQL, then
refer to the corresponding precompiler manual. For example, if you program in C
or C++, then refer to the Pro*C/C++ Programmer's Guide.
Oracle JDeveloper 10g is an Integrated Development Environment (IDE) for
building applications and Web services using the latest industry standards for
Java, XML, and SQL. You can access the JDeveloper documentation at the
following product page:
http://www.oracle.com/technology/products/jdev

■

■
■

Oracle Database SQL Reference and Oracle Database Administrator's Guide for SQL
information.
Oracle Database Concepts for basic Oracle Database concepts
Oracle XML Developer's Kit Programmer's Guide and Oracle XML DB Developer's
Guide for developing applications that manipulate XML data.

Many of the examples in this book use the sample schemas, which are installed by
default when you select the Basic Installation option with an Oracle Database
installation. Refer to Oracle Database Sample Schemas for information on how these
schemas were created and how you can use them yourself.
Printed documentation is available for sale in the Oracle Store at
http://oraclestore.oracle.com/

To download free release notes, installation documentation, white papers, or other
collateral, please visit the Oracle Technology Network (OTN). You must register online
before using OTN; registration is free and can be done at
http://www.oracle.com/technology/membership/

xx

If you already have a username and password for OTN, then you can go directly to the
documentation section of the OTN Web site at
http://www.oracle.com/technology/documentation/

For additional information, see:
■
■

■
■

■

■

Oracle JDeveloper 10g: Empowering J2EE Development by Harshad Oak. Apress, 2004.
Oracle JDeveloper 10g Handbook by Avrom Faderman, Peter Koletzke, and Paul
Dorsey. Oracle Press, 2004.
Oracle PL/SQL Tips and Techniques by Joseph C. Trezzo. Oracle Press, 1999.
Oracle PL/SQL Programming by Steven Feuerstein. 3rd Edition. O'Reilly &
Associates, 2002.
Oracle PL/SQL Developer's Workbook by Steven Feuerstein. O'Reilly & Associates,
2000.
Oracle PL/SQL Best Practices by Steven Feuerstein. O'Reilly & Associates, 2001.

Conventions
The following text conventions are used in this document:
Convention

Meaning

boldface

Boldface type indicates graphical user interface elements associated
with an action, or terms defined in text or the glossary.

italic

Italic type indicates book titles, emphasis, or placeholder variables for
which you supply particular values.

monospace

Monospace type indicates commands within a paragraph, URLs, code
in examples, text that appears on the screen, or text that you enter.

xxi

xxii

What's New in Application Development?
This section describes new features of the Oracle Database 10g Release 2 (10.2) and
provides pointers to additional information. New features information from previous
releases is also retained to help those users migrating to the current release.
The following sections describe the new features in Oracle Oracle Database 10g:
■

Oracle Database 10g Release 2 (10.2) New Features

■

Oracle Database 10g Release 1 (10.1) New Features

Oracle Database 10g Release 2 (10.2) New Features
This section discusses new features introduced in Oracle Database 10g Release 1 (10.1).
■

Regular expression enhancements
This release adds SQL support for common Perl-based extensions that are not
included but do not conflict with the POSIX standard.
See Also: Chapter 4, "Using Regular Expressions in Oracle
Database" for more information

■

Unicode datatype literal enhancement
You can avoid data loss if the database character set cannot represent all characters
in the client character set.
See Also:

■

"Using Character Literals in SQL Statements" on page 3-4

Database Change Notification
Database Change Notification enables client applications to receive notifications
when the result set of a registered query changes. For example, if the client
registers a query of the hr.employees table, and if a user adds an employee,
then the application can receive a database change notification when a new row is
added to the table. A new query of hr.employees returns the changed result set.
Database Change Notification supports both mid-tier caches and server-side
stored procedures. For example, a stored procedure that performs DML on
registered tables can automatically send change notifications to a mid-tier
application, which can keep cached data up to date.
See Also: Chapter 13, "Developing Applications with Database
Change Notification"

xxiii

■

Asynchronous Commit
Oracle Database enables you to change the handling of commit redo depending on
the needs of your application. You can change the default COMMIT options so that
the application does not need to wait for Oracle Database to write data to the
online redo logs.
See Also:

■

"Managing Commit Redo Behavior" on page 2-3

Automatic Undo Retention Enhancement
This feature provides maximum retention for the fixed-size undo tablespace,
thereby improving flashback capability as well as other statistic collection queries
in which the query length is unknown.
See Also: "Database Administration Tasks Before Using Flashback
Features" on page 10-3

■

Failover Improvements for Real Application Clusters (RAC) Distributed
Transactions
Distributed Transactions in a RAC environment detect failures and start the
failover and failback processes automatically.
See Also:

"Using XA with Oracle Real Application Clusters" on

page 15-19
■

XML DB Web Services
Enables direct access to the Oracle database through a native Web Service.
Developers can write and deploy web services that can query the database with
SQL or XQuery or execute stored procedures. You can access web services through
the Oracle XML DB listener.
See Also:

"Overview of Oracle Database Web Services" on page 1-14

Oracle Database 10g Release 1 (10.1) New Features
This section discusses new features introduced in Oracle Database 10g Release 1 (10.1).
■

Regular Expression Support
A set of SQL functions introduced in this release let you perform queries and
manipulate string data by means of regular expressions. Refer to Chapter 4, "Using
Regular Expressions in Oracle Database" for more information.

■

Oracle Expression Filter
Oracle Expression Filter lets you store conditional expressions in a column that
you can use in the WHERE clause of a database query. Refer to "Representing
Conditional Expressions as Data" on page 3-22 for more information.
See Also:

■

Oracle Database SQL Reference

Native floating-point datatypes
Column datatypes BINARY_FLOAT and BINARY_DOUBLE are introduced in this
release. These datatypes provide an alternative to using the Oracle NUMBER
datatype, with the following benefits:

xxiv

■

More efficient use of storage resources

■

Faster arithmetic operations

■

Support for numerical algorithms specified in the IEEE 754 Standard

Support for native floating-point datatypes in bind and fetch operations is
provided for the following client interfaces:
■

SQL

■

PL/SQL

■

OCI

■

OCCI

■

Pro*C/C++

■

JDBC
See Also:

■

"Representing Numeric Data" on page 3-5

Terabyte-Size Large Object (LOB) support
This release provides support for terabyte-size LOB values (from 8 to 128
terabytes) in the following programmatic environments:
■

Java (JDBC)

■

OCI

■

PL/SQL (package DBMS_LOB)

You can store and manipulate LOB (BLOB, CLOB, and NCLOB) datatypes larger
than 4GB.
See Also:

■

For details on terabyte-size LOB support:

■

Oracle Database Application Developer's Guide - Large Objects

■

Oracle Call Interface Programmer's Guide

Flashback
This release has new and enhanced flashback features. You can now do the
following:
■
■

■

Query the transaction history of a row.
Obtain the SQL undo syntax for a row and perform row-level flashback
operations.
Perform remote queries of past data.
See Also:

■

Chapter 10, "Developing Flashback Applications"

Oracle Data Provider for .NET
Oracle Data Provider for .NET (ODP.NET) is a new programmatic environment
that implements a data provider for Oracle Database. It uses APIs native to Oracle
Database to offer fast and reliable access from any .NET application to database
features and data. ODP.NET also uses and inherits classes and interfaces available
in the Microsoft .NET Framework Class Library.
See Also:

Oracle Data Provider for .NET Developer's Guide
xxv

xxvi

1
Orientation to Oracle Programmatic
Environments
This chapter contains these topics:
■

Overview of Oracle Application Development

■

Overview of PL/SQL

■

Overview of Java Support Built Into the Database

■

Overview of Pro*C/C++

■

Overview of Pro*COBOL

■

Overview of OCI and OCCI

■

Overview of Oracle Data Provider for .NET (ODP.NET)

■

Overview of Oracle Objects for OLE (OO4O)

■

Choosing a Programming Environment

Overview of Oracle Application Development
As an application developer, you have many choices when writing a program to
interact with an Oracle database.

Client/Server Model
In a traditional client/server program, the code of your application runs on a machine
other than the database server. Database calls are transmitted from this client machine
to the database server. Data is transmitted from the client to the server for insert and
update operations and returned from the server to the client for query operations. The
data is processed on the client machine. Client/server programs are typically written
by using precompilers, whereas SQL statements are embedded within the code of
another language such as C, C++, or COBOL.

Server-Side Coding
You can develop application logic that resides entirely inside the database by using
triggers that are executed automatically when changes occur in the database or stored
procedures that are called explicitly. Off-loading the work from your application lets
you reuse code that performs verification and cleanup and control database operations
from a variety of clients. For example, by making stored procedures callable through a
Web server, you can construct a Web-based user interface that performs the same
functions as a client/server application.

Orientation to Oracle Programmatic Environments

1-1

Overview of PL/SQL

Two-Tier Versus Three-Tier Models
Client/server computing is often referred to as a two-tier model: your application
communicates directly with the database server. In the three-tier model, a separate
application server processes the requests. The application server might be a basic Web
server, or might perform advanced functions like caching and load-balancing.
Increasing the processing power of this middle tier lets you lessen the resources
needed by client systems, resulting in a thin client configuration in which the client
machine might need only a Web browser or other means of sending requests over the
TCP/IP or HTTP protocols.

User Interface
The interface that your application displays to end users depends on the technology
behind the application as well as the needs of the users themselves. Experienced users
can enter SQL commands that are passed on to the database. Novice users can be
shown a graphical user interface that uses the graphics libraries of the client system
(such as Windows or X-Windows). Any of these traditional user interfaces can also be
provided in a Web browser by means of HTML and Java.

Stateful Versus Stateless User Interfaces
In traditional client/server applications, the application can keep a record of user
actions and use this information over the course of one or more sessions. For example,
past choices can be presented in a menu so that they do not have to be entered again.
When the application is able to save information in this way, the application is
considered stateful.
Web or thin-client applications that are stateless are easier to develop. Stateless
applications gather all the required information, process it using the database, and
then start over with the next user. This is a popular way to process single-screen
requests such as customer registration.
There are many ways to add stateful behavior to Web applications that are stateless by
default. For example, an entry form on one Web page can pass information to
subsequent Web pages, allowing you to construct a wizard-like interface that
remembers the user's choices through several different steps. Cookies can be used to
store small items of information on the client machine, and retrieve them when the
user returns to a Web site. Servlets can be used to keep a database session open and
store variables between requests from the same client.

Overview of PL/SQL
This section contains the following topics:
■

What Is PL/SQL?

■

Advantages of PL/SQL

■

PL/SQL Web Development Tools

What Is PL/SQL?
PL/SQL is Oracle's procedural extension to SQL, the standard database access
language. It is an advanced 4GL (fourth-generation programming language), which
means that it is an application-specific language. PL/SQL and SQL have built-in
treatment of the relational database domain.
In PL/SQL, you can manipulate data with SQL statements and control program flow
with procedural constructs such as loops. You can also do the following:
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Overview of PL/SQL

■

Declare constants and variables

■

Define procedures and functions

■

Use collections and object types

■

Trap runtime errors

Applications written in any of the Oracle programmatic interfaces can call PL/SQL
stored procedures and send blocks of PL/SQL code to Oracle Database for execution.
3GL applications can access PL/SQL scalar and composite datatypes through host
variables and implicit datatype conversion. A 3GL language is easier than assembler
language for a human to understand and includes features such as named variables.
Unlike 4GL, it is not specific to an application domain.
Example 1–1 provides an example of a simple PL/SQL procedure. The procedure
debit_account withdraws money from a bank account. It accepts an account
number and an amount of money as parameters. It uses the account number to
retrieve the account balance from the database, then computes the new balance. If this
new balance is less than zero, then the procedure jumps to an error routine; otherwise,
it updates the bank account.
Example 1–1 Simple PL/SQL Example
PROCEDURE debit_account (p_acct_id INTEGER, p_debit_amount REAL)
IS
v_old_balance REAL;
v_new_balance REAL;
e_overdrawn
EXCEPTION;
BEGIN
SELECT bal
INTO v_old_balance
FROM accts
WHERE acct_no = p_acct_id;
v_new_balance := v_old_balance - p_debit_amount;
IF v_new_balance < 0 THEN
RAISE e_overdrawn;
ELSE
UPDATE accts SET bal = v_new_balance
WHERE acct_no = p_acct_id;
END IF;
COMMIT;
EXCEPTION
WHEN e_overdrawn THEN
-- handle the error
END debit_account;

See Also:
■

Oracle Database PL/SQL User's Guide and Reference

■

Oracle Database SQL Reference

Advantages of PL/SQL
PL/SQL is a portable, high-performance transaction processing language with the
following advantages:
■

Integration with Oracle Database

■

High Performance

■

High Productivity
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Overview of PL/SQL

■

Scalability

■

Manageability

■

Object-Oriented Programming Support

■

Portability

■

Security

■

Built-In Packages

Integration with Oracle Database
PL/SQL enables you use all of the Oracle Database SQL data manipulation, cursor
control, and transaction control statements. PL/SQL also supports the SQL functions,
operators, and pseudocolumns. So, you can manipulate data in Oracle Database
flexibly and safely.
PL/SQL supports all SQL datatypes. Combined with the direct access that SQL
provides, these shared datatypes integrate PL/SQL with the Oracle Database data
dictionary.
PL/SQL supports Dynamic SQL, which is a programming technique that enables you
to build and process SQL statements "on the fly" at run time. It gives PL/SQL
flexibility comparable to scripting languages such as Perl, Korn shell, and Tcl.
The %TYPE and %ROWTYPE attributes enable your code to adapt as table definitions
change. For example, the %TYPE attribute declares a variable based on the type of a
database column. If the column datatype changes, then the variable uses the correct
type at runtime. This provides data independence and reduces maintenance costs.

High Performance
If your application is database intensive, then you can use PL/SQL blocks to group
SQL statements before sending them to Oracle Database for execution. This coding
strategy can drastically reduce the communication overhead between your application
and Oracle Database.
PL/SQL stored procedures are compiled once and stored in executable form, so
procedure calls are quick and efficient. A single call can start a compute-intensive
stored procedure, reducing network traffic and improving round-trip response times.
Executable code is automatically cached and shared among users, lowering memory
requirements and invocation overhead.

High Productivity
PL/SQL adds procedural capabilities such as Oracle Forms and Oracle Reports. For
example, you can use an entire PL/SQL block in an Oracle Forms trigger instead of
multiple trigger steps, macros, or user exits.
PL/SQL is the same in all environments. As soon as you master PL/SQL with one
Oracle tool, you can transfer your knowledge to others, and so multiply the
productivity gains. For example, scripts written with one tool can be used by other
tools.

Scalability
PL/SQL stored procedures increase scalability by centralizing application processing
on the server. Automatic dependency tracking helps you develop scalable
applications.

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Overview of PL/SQL

The shared memory facilities of the shared server enable Oracle Database to support
many thousands of concurrent users on a single node. For more scalability, you can
use the Oracle Connection Manager to multiplex network connections.

Manageability
After being validated, you can use a PL/SQL stored procedure in any number of
applications. If its definition changes, then only the procedure is affected, not the
applications that call it. This simplifies maintenance and enhancement. Also,
maintaining a procedure on the Oracle Database is easier than maintaining copies on
various client machines.

Object-Oriented Programming Support
You can use PL/SQL object types and collections for object-oriented programming.
Object Types An object type is a user-defined composite datatype that encapsulates a
data structure along with the functions and procedures needed to manipulate the data.
The variables that form the data structure are called attributes. The functions and
procedures that characterize the behavior of the object type are called methods, which
you can implement in PL/SQL.
Object types are an ideal object-oriented modeling tool, which you can use to reduce
the cost and time required to build complex applications. Besides allowing you to
create software components that are modular, maintainable, and reusable, object types
allow different teams of programmers to develop software components concurrently.
Collections A collection is an ordered group of elements, all of the same type (for
example, the grades for a class of students). Each element has a unique subscript that
determines its position in the collection. PL/SQL offers two kinds of collections:
nested tables and varrays (variable-size arrays).
Collections work like the set, queue, stack, and hash table data structures found in
most third-generation programming languages. Collections can store instances of an
object type and can also be attributes of an object type. Collections can be passed as
parameters. So, you can use them to move columns of data into and out of database
tables or between client-side applications and stored subprograms. You can define
collection types in a PL/SQL package, then use the same types across many
applications.

Portability
Applications written in PL/SQL can run on any operating system and hardware
platform on which Oracle Database runs. You can write portable program libraries and
reuse them in different environments.

Security
PL/SQL stored procedures enable you to divide application logic between the client
and the server, which prevents client applications from manipulating sensitive Oracle
Database data. Database triggers written in PL/SQL can prevent applications from
making specified updates and can audit user queries.
You can restrict access to Oracle Database data by allowing users to manipulate it only
through stored procedures that have a restricted set of privileges. For example, you
can grant users access to a procedure that updates a table but not grant them access to
the table itself.

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Overview of Java Support Built Into the Database

See Also: Oracle Database Security Guide for details on database
security features

Built-In Packages
A package is an encapsulated collection of related program objects stored together in
the database. Program objects are procedures, functions, variables, constants, cursors,
and exceptions.
The following packages are especially useful in application development for Oracle
Database:
■

DBMS_PIPE is used to communicate between sessions.

■

DBMS_ALERT is used to broadcast alerts to users.

■

DBMS_LOCK and DBMS_TRANSACTION are used for lock and transaction
management.

■

DBMS_AQ is used for Advanced Queuing.

■

DBMS_LOB is used to manipulate large objects.

■

DBMS_ROWID is used for employing ROWID values.

■

UTL_RAW is the RAW facility.

■

UTL_REF is for work with REF values.

The following packages are useful for server manageability:
■
■

■

DBMS_SESSION is for session management by DBAs.
DBMS_SPACE and DBMS_SHARED_POOL provide space information and reserve
shared pool resources.
DBMS_JOB is used to schedule jobs in the server.

PL/SQL Web Development Tools
Oracle Database provides built-in tools and technologies that enable you to deploy
PL/SQL applications over the Web. Thus, PL/SQL serves as an alternative to Web
application frameworks such as CGI.
The PL/SQL Web Toolkit is a set of PL/SQL packages that you can use to develop
stored procedures that can be invoked by a Web client. The PL/SQL Gateway enables
an HTTP client to invoke a PL/SQL stored procedure through mod_plsql, which is a
plug-in to Oracle HTTP Server. This module performs the following actions:
1.

Translates a URL passed by a browser client

2.

Calls an Oracle Database stored procedure with the parameters in the URL

3.

Returns output (typically HTML) to the client
See Also: Chapter 11, "Developing Applications with the PL/SQL
Web Toolkit" to learn how to use PL/SQL in Web development

Overview of Java Support Built Into the Database
This section provides an overview of built-in database features that support Java
applications. The database includes the core JDK libraries such as java.lang,
java.io, and so on. The database supports client-side Java standards such as JDBC

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and SQLJ, and provides server-side JDBC and SQLJ drivers that allow data-intensive
Java code to run within the database.
This section contains the following topics:
■

Overview of Oracle JVM

■

Overview of Oracle Extensions to JDBC

■

Overview of Oracle SQLJ

■

Overview of Oracle JPublisher

■

Overview of Java Stored Procedures

■

Overview of Oracle Database Web Services

■

Overview of Writing Procedures and Functions in Java
See Also:
■

Oracle Database Concepts for background information about Java
and how the database supports it

■

Oracle Database Java Developer's Guide

■

Oracle Database JDBC Developer's Guide and Reference

■

Oracle Database JPublisher User's Guide

Overview of Oracle JVM
Oracle JVM, the Java Virtual Machine provided with the Oracle Database, is compliant
with the J2SE version 1.4.x specification and supports the database session
architecture.
Any database session can activate a dedicated JVM. All sessions share the same JVM
code and statics; however, private states for any given session are held, and
subsequently garbage collected, in an individual session space.
This design provides the following benefits:
■

Java applications have the same session isolation and data integrity as SQL
operations.

■

There is no need to run Java in a separate process for data integrity.

■

Oracle JVM is a robust JVM with a small memory footprint.

■

The JVM has the same linear Symmetric Multiprocessing (SMP) scalability as the
database and can support thousands of concurrent Java sessions.

Oracle JVM works consistently with every platform supported by Oracle Database.
Java applications that you develop with Oracle JVM can easily be ported to any
supported platform.
Oracle JVM includes a deployment-time native compiler that enables Java code to be
compiled once, stored in executable form, shared among users, and invoked more
quickly and efficiently.
Security features of the database are also available with Oracle JVM. Java classes must
be loaded in a database schema (by using Oracle JDeveloper, a third-party IDE,
SQL*Plus, or the loadjava utility) before they can be invoked. Java class invocation is
secured and controlled through database authentication and authorization, Java 2
security, and invoker's or definer's rights.

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Overview of Java Support Built Into the Database

Overview of Oracle Extensions to JDBC
JDBC (Java Database Connectivity) is an API (Applications Programming Interface)
that allows Java to send SQL statements to an object-relational database such as Oracle
Database.
The JDBC standard defines four types of JDBC drivers:
■
■

■
■

Type 1. A JDBC-ODBC bridge. Software must be installed on client systems.
Type 2. Native methods (calls C or C++) and Java methods. Software must be
installed on the client.
Type 3. Pure Java. The client uses sockets to call middleware on the server.
Type 4. The most pure Java solution. Talks directly to the database by using Java
sockets.

JDBC is based on the X/Open SQL Call Level Interface, and complies with the SQL92
Entry Level standard.
You can use JDBC to do dynamic SQL. In dynamic SQL, the embedded SQL statement
to be executed is not known before the application is run and requires input to build
the statement.
The drivers that are implemented by Oracle have extensions to the capabilities in the
JDBC standard that was defined by Sun Microsystems. Oracle's implementations of
JDBC drivers are described in the following sections. Oracle Database support of and
extensions to various levels of the JDBC standard are described in "Oracle Database
Extensions to JDBC Standards" on page 1-9.

JDBC Thin Driver
The JDBC thin driver is a Type 4 (100% pure Java) driver that uses Java sockets to
connect directly to a database server. It has its own implementation of a Two-Task
Common (TTC), a lightweight implementation of TCP/IP from Oracle Net. It is
written entirely in Java and is therefore platform-independent.
The thin driver does not require Oracle software on the client side. It does need a
TCP/IP listener on the server side. Use this driver in Java applets that are downloaded
into a Web browser or in applications for which you do not want to install Oracle
client software. The thin driver is self-contained, but it opens a Java socket, and thus
can only run in a browser that supports sockets.

JDBC OCI Driver
The OCI driver is a Type 2 JDBC driver. It makes calls to the OCI (Oracle Call
Interface) written in C to interact with Oracle Database, thus using native and Java
methods.
The OCI driver allows access to more features than the thin driver, such as Transparent
Application Fail-Over, advanced security, and advanced LOB manipulation.
The OCI driver provides the highest compatibility between different Oracle Database
versions. It also supports all installed Oracle Net adapters, including IPC, named
pipes, TCP/IP, and IPX/SPX.
Because it uses native methods (a combination of Java and C) the OCI driver is
platform-specific. It requires a client installation of version Oracle8i or later including
Oracle Net, OCI libraries, CORE libraries, and all other dependent files. The OCI
driver usually executes faster than the thin driver.

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The OCI driver is not appropriate for Java applets, because it uses a C library that is
platform-specific and cannot be downloaded into a Web browser. It is usable in J2EE
components running in middle-tier application servers, such as Oracle Application
Server. Oracle Application Server provides middleware services and tools that support
access between applications and browsers.

JDBC Server-Side Internal Driver
The JDBC server-side internal driver is a Type 2 driver that runs inside the database
server, reducing the number of round-trips needed to access large amounts of data.
The driver, the Java server VM, the database, the Java native compiler which speeds
execution by as much as 10 times, and the SQL engine all run within the same address
space.
This driver provides server-side support for any Java program used in the database:
SQLJ stored procedures, functions, triggers, and Java stored procedures. You can also
call PL/SQL stored procedures, functions, and triggers.
The server driver fully supports the same features and extensions as the client-side
drivers.

Oracle Database Extensions to JDBC Standards
Oracle Database includes the following extensions to the JDBC 1.22 standard:
■

Support for Oracle datatypes

■

Performance enhancement by row prefetching

■

Performance enhancement by execution batching

■

Specification of query column types to save round-trips

■

Control of DatabaseMetaData calls

Oracle Database supports all APIs from the JDBC 2.0 standard, including the core
APIs, optional packages, and numerous extensions. Some of the highlights include
datasources, JTA, and distributed transactions.
Oracle Database supports the following features from the JDBC 3.0 standard:
■

Support for JDK 1.4.

■

Toggling between local and global transactions.

■

Transaction savepoints.

■

Reuse of prepared statements by connection pools.

Sample JDBC 2.0 Program
The following example shows the recommended technique for looking up a data
source using JNDI in JDBC 2.0:
// import the JDBC packages
import java.sql.*;
import javax.sql.*;
import oracle.jdbc.pool.*;
...
InitialContext ictx = new InitialContext();
DataSource ds = (DataSource)ictx.lookup("jdbc/OracleDS");
Connection conn = ds.getConnection();
Statement stmt = conn.createStatement();
ResultSet rs = stmt.executeQuery("SELECT last_name FROM employees");

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Overview of Java Support Built Into the Database

while ( rs.next() ) {
out.println( rs.getString("ename") + "
");
}
conn.close();

Sample Pre-2.0 JDBC Program
The following source code registers an Oracle JDBC thin driver, connects to the
database, creates a Statement object, executes a query, and processes the result set.
The SELECT statement retrieves and lists the contents of the last_name column of
the hr.employees table.
import
import
import
import

java.sql.*
java.math.*
java.io.*
java.awt.*

class JdbcTest {
public static void main (String args []) throws SQLException {
// Load Oracle driver
DriverManager.registerDriver (new oracle.jdbc.OracleDriver());
// Connect to the local database
Connection conn =
DriverManager.getConnection ("jdbc:oracle:thin:@myhost:1521:orcl",
"hr", "hr");
// Query the employee names
Statement stmt = conn.createStatement ();
ResultSet rset = stmt.executeQuery ("SELECT last_name FROM employees");
// Print the name out
while (rset.next ())
System.out.println (rset.getString (1));
// Close the result set, statement, and the connection
rset.close();
stmt.close();
conn.close();
}
}

One Oracle Database extension to the JDBC drivers is a form of the
getConnection() method that uses a Properties object. The Properties object
lets you specify user, password, and database information as well as row prefetching
and execution batching.
To use the OCI driver in this code, replace the Connection statement with the
following, where MyHostString is an entry in the tnsnames.ora file:
Connection conn = DriverManager.getConnection ("jdbc:oracle:oci8:@MyHostString",
"hr", "hr");

If you are creating an applet, then the getConnection() and registerDriver()
strings will be different.

JDBC in SQLJ Applications
JDBC code and SQLJ code (see "Overview of Oracle SQLJ" on page 1-11) interoperate,
allowing dynamic SQL statements in JDBC to be used with both static and dynamic
SQL statements in SQLJ. A SQLJ iterator class corresponds to the JDBC result set.

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See Also: Oracle Database JDBC Developer's Guide and Reference for
more information on JDBC

Overview of Oracle SQLJ
SQLJ is an ANSI SQL-1999 standard for embedding SQL statements in Java source
code. SQLJ provides a simpler alternative to JDBC for both client-side and server-side
SQL data access from Java.
A SQLJ source file contains Java source with embedded SQL statements. Oracle SQLJ
supports dynamic as well as static SQL. Support for dynamic SQL is an Oracle
extension to the SQLJ standard.
The term "SQLJ," when used in this manual, refers to the
Oracle SQLJ implementation, including Oracle SQLJ extensions.

Note:

Oracle Database provides a translator and a run time driver to support SQLJ. The SQLJ
translator is 100% pure Java and is portable to any JVM that is compliant with JDK
version 1.1 or higher.
The Oracle SQLJ translator performs the following tasks:
■

■
■

Translates SQLJ source to Java code with calls to the SQLJ run time driver. The
SQLJ translator converts the source code to pure Java source code and can check
the syntax and semantics of static SQL statements against a database schema and
verify the type compatibility of host variables with SQL types.
Compiles the generated Java code with the Java compiler.
(Optional) Creates profiles for the target database. SQLJ generates "profile" files
with customization specific to Oracle Database.

Oracle Database supports SQLJ stored procedures, functions, and triggers which
execute in the Oracle JVM. SQLJ is integrated with JDeveloper. Source-level debugging
support for SQLJ is available in JDeveloper.
The following is an example of a simple SQLJ executable statement, which returns one
value because employee_id is unique in the employee table:
String name;
#sql { SELECT first_name INTO :name FROM employees WHERE employee_id=112 };
System.out.println("Name is " + name + ", employee number = " + employee_id);

Each host variable (or qualified name or complex Java host expression) included in a
SQL expression is preceded by a colon (:). Other SQLJ statements declare Java types.
For example, you can declare an iterator (a construct related to a database cursor) for
queries that retrieve many values, as follows:
#sql iterator EmpIter (String EmpNam, int EmpNumb);

See Also:
■
■

For more examples and details on Oracle SQLJ syntax:

Oracle Database JPublisher User's Guide
Sample SQLJ code available on the Oracle Technology Network
Web site: http://www.oracle.com/technology/

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Overview of Java Support Built Into the Database

Benefits of SQLJ
Oracle SQLJ extensions to Java allow rapid development and easy maintenance of
applications that perform database operations through embedded SQL.
In particular, Oracle SQLJ does the following:
■

■

■

■

■

■
■

■

Provides a concise, legible mechanism for database access from static SQL. Most
SQL in applications is static. SQLJ provides more concise and less error-prone
static SQL constructs than JDBC does.
Provides an SQL Checker module for verification of syntax and semantics at
translate time.
Provides flexible deployment configurations, which makes it possible to
implement SQLJ on the client, server, or middle tier.
Supports a software standard. SQLJ is an effort of a group of vendors and will be
supported by all of them. Applications can access multiple database vendors.
Provides source code portability. Executables can be used with all of the vendor
DBMSs if the code does not rely on any vendor-specific features.
Enforces a uniform programming style for the clients and the servers.
Integrates the SQLJ translator with Oracle JDeveloper, a graphical IDE that
provides SQLJ translation, Java compilation, profile customizing, and debugging
at the source code level, all in one step.
Includes Oracle type extensions. Datatypes supported include: LOB datatypes,
ROWID, REF CURSOR, VARRAY, nested table, user-defined object types, RAW, and
NUMBER.

Comparing SQLJ with JDBC
JDBC provides a complete dynamic SQL interface from Java to databases. It gives
developers full control over database operations. SQLJ simplifies Java database
programming to improve development productivity.
JDBC provides fine-grained control of the execution of dynamic SQL from Java,
whereas SQLJ provides a higher-level binding to SQL operations in a specific database
schema. Following are some differences between JDBC and SQLJ:
■
■

■

■

■

SQLJ source code is more concise than equivalent JDBC source code.
SQLJ uses database connections to type-check static SQL code. JDBC, being a
completely dynamic API, does not.
SQLJ provides strong typing of query outputs and return parameters and allows
type-checking on calls. JDBC passes values to and from SQL without compile-time
type checking.
SQLJ programs allow direct embedding of Java bind expressions within SQL
statements. JDBC requires a separate get or set statement for each bind variable
and specifies the binding by position number.
SQLJ provides simplified rules for calling SQL stored procedures and functions.
For example, the following JDBC excerpt requires a generic call to a stored
procedure or function, in this case fun, to have the following syntax. (This
example shows SQL92 and Oracle JDBC syntaxes. Both are allowed.)
prepStmt.prepareCall("{call fun(?,?)}");
//stored procedure SQL92
prepStmt.prepareCall("{? = call fun(?,?)}");
//stored function SQL92
prepStmt.prepareCall("begin fun(:1,:2);end;"); //stored procedure Oracle
prepStmt.prepareCall("begin :1 := fun(:2,:3);end;");//stored func Oracle

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Following is the SQLJ equivalent:
#sql {call fun(param_list) }; //Stored procedure
// Declare x
...
#sql x = {VALUES(fun(param_list)) }; // Stored function
// where VALUES is the SQL construct

The following benefits are common to SQLJ and JDBC:
■
■

■

SQLJ source files can contain JDBC calls. SQLJ and JDBC are interoperable.
Oracle JPublisher generates custom Java classes to be used in your SQLJ or JDBC
application for mappings to Oracle object types and collections.
Java and PL/SQL stored procedures can be used interchangeably.

SQLJ Stored Procedures in the Server
SQLJ applications can be stored and executed in the server by using the following
techniques:
■

■

Translate, compile, and customize the SQLJ source code on a client and load the
generated classes and resources into the server with the loadjava utility. The
classes are typically stored in a Java archive (.jar) file.
Load the SQLJ source code into the server, also using loadjava, where it is
translated and compiled by the server's embedded translator.
See Also: Oracle Database JPublisher User's Guide for more
information on using stored procedures with Oracle SQLJ

Overview of Oracle JPublisher
Oracle JPublisher is a code generator that automates the process of creating
database-centric Java classes by hand. Oracle JPublisher is a client-side utility and is
built into the database system. You can run Oracle JPublisher from the command line
or directly from the Oracle JDeveloper IDE.
Oracle JPublisher inspects PL/SQL packages and database object types such as SQL
object types, VARRAY types, and nested table types, and then generates a Java class
that is a wrapper around the PL/SQL package with corresponding fields and
methods.
The generated Java class can be incorporated and used by Java clients or J2EE
components to exchange and transfer object type instances to and from the database
transparently.
See Also:

Oracle Database JPublisher User's Guide

Overview of Java Stored Procedures
Java stored procedures enable you to implement programs that run in the database
server and which are independent of programs that run in the middle tier. Structuring
applications in this way reduces complexity and increases reuse, security,
performance, and scalability.
For example, you can create a Java stored procedure that performs operations that
require data persistence and a separate program to perform presentation or business
logic operations.

Orientation to Oracle Programmatic Environments 1-13

Overview of Java Support Built Into the Database

Java stored procedures interface with SQL by using a similar execution model as
PL/SQL.
See Also:

Oracle Database Java Developer's Guide

Overview of Oracle Database Web Services
Web services represent a distributed computing paradigm for Java application
development that is an alternative to earlier Java protocols such as JDBC. It allows
application-to-application interaction by means of the XML and Web protocols. For
example, an electronics parts vendor can provide a Web-based programmatic interface
to its suppliers for inventory management. The vendor can call a Web service as part
of a program and automatically order new stock based on the data returned.
The key technologies used in Web services are:
■

■

■

Web Services Description Language (WSDL), which is a standard format for
creating an XML document. WSDL describes what a web service can do, where it
resides, and how to invoke it. Specifically, it describes the operations and
parameters, including parameter types, provided by a Web service. In addition, a
WSDL document describes the location, the transport protocol, and the invocation
style for the Web service.
Simple Object Access Protocol (SOAP) messaging, which is an XML-based
message protocol used by Web services. SOAP does not prescribe a specific
transport mechanism such as HTTP, FTP, SMTP, or JMS; however, most Web
services accept messages that use HTTP or HTTPS.
Universal Description, Discovery, and Integration (UDDI) business registry, which
is a directory that lists Web services on the internet. The UDDI registry is often
compared to a telephone directory, listing unique identifiers (white pages),
business categories (yellow pages), and instructions for binding to a service
protocol (green pages).

Web services can use a variety of techniques and protocols. For example:
■
■

■

Dispatching can occur in a synchronous (typical) or asynchronous manner.
You can perform invocation in an RPC-style operation in which arguments are
sent and a response returned, or in a message style such as a one-way SOAP
document exchange.
You can use different encoding rules: literal or encoded.

You can invoke a Web service statically, in which case you may know everything about
it beforehand, or dynamically, in which case you can discover its operations and
transport endpoints on the fly.

Oracle Database as a Web Service Provider
Oracle Database can function as either a Web service provider or as a Web service
consumer. When used as a provider, the database enables sharing and disconnected
access to stored procedures, data, metadata, and other database resources such as the
queuing and messaging systems.
As a Web service provider, Oracle Database provides a disconnected and
heterogeneous environment that:
■

■

Exposes stored procedures independently of the language in which the procedures
are written
Exposes SQL Queries and XQuery

1-14 Oracle Database Application Developer’s Guide - Fundamentals

Overview of Pro*C/C++

Overview of Writing Procedures and Functions in Java
Functions and procedures are named blocks that encapsulate a sequence of statements.
They are like building blocks that you can use to construct modular, maintainable
applications. You write these named blocks and then define them by using the
loadjava command or SQL CREATE FUNCTION, CREATE PROCEDURE, or CREATE
PACKAGE statements. These Java methods can accept arguments and are callable from:
■

SQL CALL statements

■

Embedded SQL CALL statements

■

PL/SQL blocks, subprograms, and packages

■

DML statements (INSERT, UPDATE, DELETE, and SELECT)

■

Oracle development tools such as OCI, Pro*C/C++, and Oracle Developer

■

■

Oracle Java interfaces such as JDBC, SQLJ statements, CORBA, and Enterprise
Java Beans
Method calls from object types

Overview of Writing Database Triggers in Java
A database trigger is a stored procedure that Oracle Database invokes ("fires")
automatically when certain events occur, for example, when a DML operation
modifies a certain table. Triggers enforce business rules, prevent incorrect values from
being stored, and reduce the need to perform checking and cleanup operations in each
application.

Why Use Java for Stored Procedures and Triggers?
■

■

■
■

■
■

■

Stored procedures and triggers are compiled once, are easy to use and maintain,
and require less memory and computing overhead.
Network bottlenecks are avoided, and response time is improved. Distributed
applications are easier to build and use.
Computation-bound procedures run faster in the server.
Data access can be controlled by letting users have only stored procedures and
triggers that execute with their definer's privileges instead of invoker's rights.
PL/SQL and Java stored procedures can call each other.
Java in the server follows the Java language specification and can use the SQLJ
standard, so that databases other than Oracle Database are also supported.
Stored procedures and triggers can be reused in different applications as well as
different geographic sites.

Overview of Pro*C/C++
The Pro*C/C++ precompiler is a software tool that allows the programmer to embed
SQL statements in a C or C++ source file. Pro*C/C++ reads the source file as input and
outputs a C or C++ source file that replaces the embedded SQL statements with Oracle
runtime library calls and is then compiled by the C or C++ compiler.
When there are errors found during the precompilation or the subsequent compilation,
modify your precompiler input file and re-run the two steps.

Orientation to Oracle Programmatic Environments 1-15

Overview of Pro*C/C++

Implementing a Pro*C/C++ Application
The following is a simple code fragment from a C source file that queries the table
employees in the schema hr:
...
#define UNAME_LEN
10
...
int
emp_number;
/* Define a host structure for the output values of a SELECT statement. */
/* No declare section needed if precompiler option MODE=ORACLE
*/
struct {
VARCHAR last_name[UNAME_LEN];
float
salary;
float
commission_pct;
} emprec;
/* Define an indicator structure to correspond to the host output structure. */
struct {
short emp_name_ind;
short sal_ind;
short comm_ind;
} emprec_ind;
...
/* Select columns last_name, salary, and commission_pct given the user's input
/* for employee_id. */
EXEC SQL SELECT last_name, salary, commission_pct
INTO :emprec INDICATOR :emprec_ind
FROM employees
WHERE employee_id = :emp_number;
...

The embedded SELECT statement is only slightly different from an interactive
(SQL*Plus) SELECT statement. Every embedded SQL statement begins with EXEC
SQL. The colon (:), precedes every host (C) variable. The returned values of data and
indicators (set when the data value is NULL or character columns have been truncated)
can be stored in structs (such as in the preceding code fragment), in arrays, or in arrays
of structs. Multiple result set values are handled very simply in a manner that
resembles the case shown, where there is only one result, because of the unique
employee number. You use the actual names of columns and tables in embedded SQL.
Use the default precompiler option values, or you can enter values which give you
control over the use of resources, how errors are reported, the formatting of output,
and how cursors (which correspond to a particular connection or SQL statement) are
managed. Cursors are used when there are multiple result set values.
Enter the options either in a configuration file, on the command line, or in-line inside
your source code with a special statement that begins with EXEC ORACLE. If there are
no errors found, you can then compile, link, and execute the output source file, like
any other C program that you write.
Use the precompiler to create server database access from clients that can be on many
different platforms. Pro*C/C++ allows you the freedom to design your own user
interfaces and to add database access to existing applications.
Before writing your embedded SQL statements, you may want to test interactive
versions of the SQL in SQL*Plus. You then make only minor changes to start testing
your embedded SQL application.

1-16 Oracle Database Application Developer’s Guide - Fundamentals

Overview of Pro*C/C++

Highlights of Pro*C/C++ Features
The following is a short subset of the capabilities of Pro*C/C++. For complete details,
refer to the Pro*C/C++ Precompiler Programmer's Guide.
■
■

■

■

■

■

■

■

■
■

■

■

■

■
■

■
■

■

You can write your application in either C or C++.
You can write multithreaded programs if your platform supports a threads
package. Concurrent connections are supported in either single-threaded or
multithreaded applications.
You can improve performance by embedding PL/SQL blocks. These blocks can
call functions or procedures in Java or PL/SQL that are written by you or
provided in Oracle Database packages.
Using precompiler options, you can check the syntax and semantics of your SQL
or PL/SQL statements during precompilation, as well as at runtime.
You can call stored PL/SQL and Java subprograms. Modules written in COBOL or
in C can be called from Pro*C/C++. External C procedures in shared libraries are
callable by your program.
You can conditionally precompile sections of your code so that they can execute in
different environments.
You can use arrays, or structures, or arrays of structures as host and indicator
variables in your code to improve performance.
You can deal with errors and warnings so that data integrity is guaranteed. As a
programmer, you control how errors are handled.
Your program can convert between internal datatypes and C language datatypes.
The Oracle Call Interface (OCI) and Oracle C++ Call Interface (OCCI), lower-level
C and C++ interfaces, are available for use in your precompiler source.
Pro*C/C++ supports dynamic SQL, a technique that allows users to input variable
values and statement syntax.
Pro*C/C++ can use special SQL statements to manipulate tables containing
user-defined object types. An Object Type Translator (OTT) will map the object
types and named collection types in your database to structures and headers that
you will then include in your source.
Two kinds of collection types, nested tables and VARRAY, are supported with a set
of SQL statements that allow a high degree of control over data.
Large Objects are accessed by another set of SQL statements.
A new ANSI SQL standard for dynamic SQL is supported for new applications, so
that you can execute SQL statements with a varying number of host variables. An
older technique for dynamic SQL is still usable by pre-existing applications.
Globalization support lets you use multibyte characters and UCS2 Unicode data.
Using scrollable cursors, you can move backward and forward through a result
set. For example, you can fetch the last row of the result set, or jump forward or
backward to an absolute or relative position within the result set.
A connection pool is a group of physical connections to a database that can be
shared by several named connections. Enabling the connection pool option
can
help to optimize the performance of Pro*C/C++ application. The connection pool
option is not enabled by default.

Orientation to Oracle Programmatic Environments 1-17

Overview of Pro*COBOL

Overview of Pro*COBOL
The Pro*COBOL precompiler is a software tool that allows the programmer to embed
SQL statements in a COBOL source code file. Pro*COBOL reads the source file as
input and outputs a COBOL source file that replaces the embedded SQL statements
with Oracle Database runtime library calls, and is then compiled by the COBOL
compiler.
When there are errors found during the precompilation or the subsequent compilation,
modify your precompiler input file and re-run the two steps.

How You Implement a Pro*COBOL Application
Here is a simple code fragment from a source file that queries the table employees in
the schema hr:
...
WORKING-STORAGE SECTION.
*
* DEFINE HOST INPUT AND OUTPUT HOST AND INDICATOR VARIABLES.
* NO DECLARE SECTION NEEDED IF MODE=ORACLE.
*
01 EMP-REC-VARS.
05 EMP-NAME
PIC X(10) VARYING.
05 EMP-NUMBER PIC S9(4) COMP VALUE ZERO.
05 SALARY
PIC S9(5)V99 COMP-3 VALUE ZERO.
05 COMMISSION PIC S9(5)V99 COMP-3 VALUE ZERO.
05 COMM-IND
PIC S9(4) COMP VALUE ZERO.
...
PROCEDURE DIVISION.
...
EXEC SQL
SELECT last_name, salary, commission_pct
INTO :EMP-NAME, :SALARY, :COMMISSION:COMM-IND
FROM employees
WHERE employee_id = :EMP-NUMBER
END-EXEC.
...

The embedded SELECT statement is only slightly different from an interactive
(SQL*Plus) SELECT statement. Every embedded SQL statement begins with EXEC
SQL. The colon (:) precedes every host (COBOL) variable. The SQL statement is
terminated by END-EXEC. The returned values of data and indicators (set when the
data value is NULL or character columns have been truncated) can be stored in group
items (such as in the preceding code fragment), in tables, or in tables of group items.
Multiple result set values are handled very simply in a manner that resembles the case
shown, where there is only one result, given the unique employee number. You use the
actual names of columns and tables in embedded SQL.
Use the default precompiler option values, or enter values that give you control over
the use of resources, how errors are reported, the formatting of output, and how
cursors are managed (cursors correspond to a particular connection or SQL statement).
Enter the options in a configuration file, on the command line, or in-line inside your
source code with a special statement that begins with EXEC ORACLE. If there are no
errors found, you can then compile, link, and execute the output source file, like any
other COBOL program that you write.

1-18 Oracle Database Application Developer’s Guide - Fundamentals

Overview of OCI and OCCI

Use the precompiler to create server database access from clients that can be on many
different platforms. Pro*COBOL allows you the freedom to design your own user
interfaces and to add database access to existing COBOL applications.
The embedded SQL statements available conform to an ANSI standard, so that you
can access data from many databases in a program, including remote servers
networked through Oracle Net.
Before writing your embedded SQL statements, you may want to test interactive
versions of the SQL in SQL*Plus. You then make only minor changes to start testing
your embedded SQL application.

Highlights of Pro*COBOL Features
The following is a short subset of the capabilities of Pro*COBOL.
■

■

■

■

■

■

■

You can call stored PL/SQL or Java subprograms. You can improve performance
by embedding PL/SQL blocks. These blocks can call PL/SQL functions or
procedures written by you or provided in Oracle Database packages.
Precompiler options allow you to define how cursors, errors, syntax-checking, file
formats, and so on, are handled.
Using precompiler options, you can check the syntax and semantics of your SQL
or PL/SQL statements during precompilation, as well as at runtime.
You can conditionally precompile sections of your code so that they can execute in
different environments.
Use tables, or group items, or tables of group items as host and indicator variables
in your code to improve performance.
You can program how errors and warnings are handled, so that data integrity is
guaranteed.
Pro*COBOL supports dynamic SQL, a technique that allows users to input
variable values and statement syntax.
See Also:

Pro*COBOL Programmer's Guide for complete details

Overview of OCI and OCCI
The Oracle Call Interface (OCI) and Oracle C++ Call Interface (OCCI) are application
programming interfaces (APIs) that allow you to create applications that use native
procedures or function calls of a third-generation language to access Oracle Database
and control all phases of SQL statement execution. These APIs provide:
■

■

Improved performance and scalability through the efficient use of system memory
and network connectivity
Consistent interfaces for dynamic session and transaction management in a
two-tier client/server or multitier environment

■

N-tiered authentication

■

Comprehensive support for application development using Oracle objects

■

Access to external databases

■

Ability to develop applications that service an increasing number of users and
requests without additional hardware investments

Orientation to Oracle Programmatic Environments 1-19

Overview of OCI and OCCI

OCI lets you manipulate data and schemas in a database using a host programming
language, such as C. OCCI is an object-oriented interface suitable for use with C++.
These APIs provide a library of standard database access and retrieval functions in the
form of a dynamic runtime library (OCILIB) that can be linked in an application at
runtime. This eliminates the need to embed SQL or PL/SQL within 3GL programs.
See Also:

For more information about OCI and OCCI calls:

■

Oracle Call Interface Programmer's Guide

■

Oracle C++ Call Interface Programmer's Guide

■

Oracle Streams Advanced Queuing User's Guide and Reference

■

Oracle Database Globalization Support Guide

■

Oracle Database Data Cartridge Developer's Guide

Advantages of OCI
OCI provides significant advantages over other methods of accessing Oracle Database:
■

More fine-grained control over all aspects of the application design.

■

High degree of control over program execution.

■

■
■

Use of familiar 3GL programming techniques and application development tools
such as browsers and debuggers.
Support of dynamic SQL, method 4.
Availability on the broadest range of platforms of all the Oracle programmatic
interfaces.

■

Dynamic bind and define using callbacks.

■

Describe functionality to expose layers of server metadata.

■

Asynchronous event notification for registered client applications.

■

Enhanced array data manipulation language (DML) capability for array INSERTs,
UPDATEs, and DELETEs.

■

Ability to associate a commit request with an execute to reduce round-trips.

■

Optimization for queries using transparent prefetch buffers to reduce round-trips.

■

■

Thread safety, so you do not have to implement mutual exclusion (mutex) locks on
OCI handles.
The server connection in nonblocking mode means that control returns to the OCI
code when a call is still executing or could not complete.

Parts of the OCI
The OCI encompasses four main sets of functionality:
■

■

■

■

OCI relational functions, for managing database access and processing SQL
statements
OCI navigational functions, for manipulating objects retrieved from an Oracle
Database
OCI datatype mapping and manipulation functions, for manipulating data attributes
of Oracle types
OCI external procedure functions, for writing C callbacks from PL/SQL

1-20 Oracle Database Application Developer’s Guide - Fundamentals

Overview of OCI and OCCI

Procedural and Non-Procedural Elements
The Oracle Call Interface (OCI) enables you to develop applications that combine the
non-procedural data access power of SQL with the procedural capabilities of most
programming languages, including C and C++. Note the following characteristics of
procedural and non-procedural languages:
■

■

In a non-procedural language program, the set of data to be operated on is
specified, but what operations will be performed and how the operations are to be
carried out is not specified. The non-procedural nature of SQL makes it an easy
language to learn and to use to perform database transactions. It is also the
standard language used to access and manipulate data in modern relational and
object-relational database systems.
In a procedural language program, the execution of most statements depends on
previous or subsequent statements and on control structures, such as loops or
conditional branches, which are not available in SQL. The procedural nature of
these languages makes them more complex than SQL, but it also makes them very
flexible and powerful.

The combination of both non-procedural and procedural language elements in an OCI
program provides easy access to Oracle Database in a structured programming
environment.
The OCI supports all SQL data definition, data manipulation, query, and transaction
control facilities that are available through Oracle Database. For example, an OCI
program can run a query against Oracle Database. The queries can require the
program to supply data to the database using input (bind) variables, as follows:
SELECT name FROM employees WHERE empno = :empnumber

In the preceding SQL statement,:empnumber is a placeholder for a value that will be
supplied by the application.
You can alternatively use PL/SQL, Oracle's procedural extension to SQL. The
applications you develop can be more powerful and flexible than applications written
in SQL alone. The OCI also provides facilities for accessing and manipulating objects
in Oracle Database.

Building an OCI Application
As Figure 1–1 shows, you compile and link an OCI program in the same way that you
compile and link a non-database application. There is no need for a separate
preprocessing or precompilation step.

Orientation to Oracle Programmatic Environments 1-21

Overview of Oracle Data Provider for .NET (ODP.NET)

Figure 1–1 The OCI Development Process

Source Files

Host Language Compiler

Object Files

OCI Library

Host Linker

Application

Object
Server

To properly link your OCI programs, it may be necessary on
some platforms to include other libraries, in addition to the OCI
library. Check your Oracle platform-specific documentation for
further information about extra libraries that may be required.

Note:

Overview of Oracle Data Provider for .NET (ODP.NET)
Oracle Data Provider for .NET (ODP.NET) is an implementation of a data provider for
Oracle Database.
ODP.NET uses APIs native to Oracle Database to offer fast and reliable access from
any .NET application to database features and data. It also uses and inherits classes
and interfaces available in the Microsoft .NET Framework Class Library.
For programmers using Oracle Provider for OLE DB, ADO (ActiveX Data Objects)
provides an automation layer that exposes an easy programming model. ADO.NET
provides a similar programming model, but without the automation layer, for better
performance. More importantly, the ADO.NET model allows native providers such as
ODP.NET to expose specific features and datatypes specific to Oracle Database.
See Also:

Oracle Data Provider for .NET Developer's Guide

Using ODP.NET in a Simple Application
The following is a simple C# application that connects to Oracle Database and displays
its version number before disconnecting.
using System;
using Oracle.DataAccess.Client;
class Example
{
OracleConnection con;
void Connect()
1-22 Oracle Database Application Developer’s Guide - Fundamentals

Overview of Oracle Objects for OLE (OO4O)

{
con = new OracleConnection();
con.ConnectionString = "User Id=hr;Password=hr;Data Source=oracle";
con.Open();
Console.WriteLine("Connected to Oracle" + con.ServerVersion);
}
void Close()
{
con.Close();
con.Dispose();
}
static void Main()
{
Example example = new Example();
example.Connect();
example.Close();
}
}

Note: Additional samples are provided in directory ORACLE_
BASE\ORACLE_HOME\ODP.NET\Samples.

Overview of Oracle Objects for OLE (OO4O)
Oracle Objects for OLE (OO4O) is a product designed to allow easy access to data
stored in Oracle Database with any programming or scripting language that supports
the Microsoft COM Automation and ActiveX technology. This includes Visual Basic,
Visual C++, Visual Basic For Applications (VBA), IIS Active Server Pages (VBScript
and JavaScript), and others.
See the OO4O online help for detailed information about using OO4O.
Oracle Objects for OLE consists of the following software layers:
■

OO4O "In-Process" Automation Server

■

Oracle Data Control

■

Oracle Objects for OLE C++ Class Library

Figure 1–2, "Software Layers" illustrates the OO4O software components.

Orientation to Oracle Programmatic Environments 1-23

Overview of Oracle Objects for OLE (OO4O)

Figure 1–2 Software Layers
Data Aware
ActiveX
Controls

C++ Class
Libraries

Oracle Data
Control

Automation
Controllers
(VB, Excel, ASP)

COM/DCOM
OO4O
In-Process
Automation
Server
Oracle Client
Libraries
(OCI, CORE,
NLS)

Oracle
Database

OO4O Automation Server
The OO4O Automation Server is a set of COM Automation objects for connecting to
Oracle Database, executing SQL statements and PL/SQL blocks, and accessing the
results.
Unlike other COM-based database connectivity APIs, such as Microsoft ADO, the
OO4O Automation Server has been developed and evolved specifically for use with
Oracle Database.
It provides an optimized API for accessing features that are unique to Oracle Database
and are otherwise cumbersome or inefficient to use from ODBC or OLE
database-specific components.
OO4O provides key features for accessing Oracle Database efficiently and easily in
environments ranging from the typical two-tier client/server applications, such as
those developed in Visual Basic or Excel, to application servers deployed in
multitiered application server environments such as Web server applications in
Microsoft Internet Information Server (IIS) or Microsoft Transaction Server (MTS).
Features include:
■

■

■

■

Support for execution of PL/SQL and Java stored procedures, and PL/SQL
anonymous blocks. This includes support for Oracle datatypes used as parameters
to stored procedures, including PL/SQL cursors. Refer to "Support for Oracle LOB
and Object Datatypes" on page 1-28.
Support for scrollable and updatable cursors for easy and efficient access to result
sets of queries.
Thread-safe objects and Connection Pool Management Facility for developing
efficient Web server applications.
Full support for Oracle object-relational and LOB datatypes.

1-24 Oracle Database Application Developer’s Guide - Fundamentals

Overview of Oracle Objects for OLE (OO4O)

■

Full support for Advanced Queuing.

■

Support for array inserts and updates.

■

Support for Microsoft Transaction Server (MTS).

OO4O Object Model
The Oracle Objects for OLE object model is illustrated in Figure 1–3, "Objects and Their
Relations".
Figure 1–3 Objects and Their Relations
OraSession
OraDatabase
OraServer
OraDynaset

OraField

OraMetaData

OraMDAttribute

OraParameter
OraParameters
OraParamArray

OraSQLStmt

OraAQ

OraAQMsg

OraSession
An OraSession object manages collections of OraDatabase, OraConnection, and
OraDynaset objects used within an application.
Typically, a single OraSession object is created for each application, but you can create
named OraSession objects for shared use within and between applications.
The OraSession object is the top-most object for an application. It is the only object
created by the CreateObject VB/VBA API and not by an Oracle Objects for OLE
method. The following code fragment shows how to create an OraSession object:
Dim OraSession as Object
Set OraSession = CreateObject("OracleInProcServer.XOraSession")

OraServer
OraServer represents a physical network connection to Oracle Database.
The OraServer interface is introduced to expose the connection-multiplexing feature
provided in the Oracle Call Interface. After an OraServer object is created, multiple
user sessions (OraDatabase) can be attached to it by invoking the OpenDatabase
method. This feature is particularly useful for application components, such as
Internet Information Server (IIS), that use Oracle Objects for OLE in n-tier distributed
environments.

Orientation to Oracle Programmatic Environments 1-25

Overview of Oracle Objects for OLE (OO4O)

The use of connection multiplexing when accessing Oracle Database with a large
number of user sessions active can help reduce server processing and resource
requirements while improving server scalability.
OraServer is used to share a single connection across multiple OraDatabase objects
(multiplexing), whereas each OraDatabase obtained from an OraSession has its own
physical connection.

OraDatabase
An OraDatabase interface adds additional methods for controlling transactions and
creating interfaces representing of Oracle object types. Attributes of schema objects can
be retrieved using the Describe method of the OraDatabase interface.
In releases prior to Oracle8i, an OraDatabase object is created by invoking the
OpenDatabase method of an OraSession interface. The Oracle Net alias, user
name, and password are passed as arguments to this method. In Oracle8i and later,
invocation of this method results in implicit creation of an OraServer object.
An OraDatabase object can also be created using the OpenDatabase method of the
OraServer interface.
Transaction control methods are available at the OraDatabase (user session) level.
Transactions may be started as Read-Write (default), Serializable, or
Read-only. Transaction control methods include:
■

BeginTrans

■

CommitTrans

■

RollbackTrans

For example:
UserSession.BeginTrans(OO4O_TXN_READ_WRITE)
UserSession.ExecuteSQL("delete emp where empno = 1234")
UserSession.CommitTrans

OraDynaset
An OraDynaset object permits browsing and updating of data created from a SQL
SELECT statement.
The OraDynaset object can be thought of as a cursor, although in actuality several
real cursors may be used to implement the semantics of OraDynaset. An
OraDynaset object automatically maintains a local cache of data fetched from the
server and transparently implements scrollable cursors within the browse data. Large
queries may require significant local disk space; application developers are
encouraged to refine queries to limit disk usage.

OraField
An OraField object represents a single column or data item within a row of a
dynaset.
If the current row is being updated, then the OraField object represents the currently
updated value, although the value may not yet have been committed to the database.
Assignment to the Value property of a field is permitted only if a record is being
edited (using Edit) or a new record is being added (using AddNew). Other attempts to
assign data to a field's Value property results in an error.

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Overview of Oracle Objects for OLE (OO4O)

OraMetaData and OraMDAttribute
An OraMetaData object is a collection of OraMDAttribute objects that represent the
description information about a particular schema object in the database.
The OraMetaData object can be visualized as a table with three columns:
■

Metadata Attribute Name

■

Metadata Attribute Value

■

Flag specifying whether the Value is another OraMetaData object

The OraMDAttribute objects contained in the OraMetaData object can be accessed
by subscripting using ordinal integers or by using the name of the property.
Referencing a subscript that is not in the collection results in the return of a NULL
OraMDAttribute object.

OraParameters and OraParameter
An OraParameter object represents a bind variable in a SQL statement or PL/SQL
block.
OraParameter objects are created, accessed, and removed indirectly through the
OraParameters collection of an OraDatabase object. Each parameter has an
identifying name and an associated value. You can automatically bind a parameter to
SQL and PL/SQL statements of other objects (as noted in the object descriptions), by
using the parameter name as a placeholder in the SQL or PL/SQL statement. Such use
of parameters can simplify dynamic queries and increase program performance.

OraParamArray
An OraParamArray object represents an array-type bind variable in a SQL statement
or PL/SQL block, as opposed to a scalar-type bind variable represented by the
OraParameter object.
OraParamArray objects are created, accessed, and removed indirectly through the
OraParameters collection of an OraDatabase object. Each OraParamArray object
has an identifying name and an associated value.

OraSQLStmt
An OraSQLStmt object represents a single SQL statement. Use the CreateSQL
method to create an OraSQLStmt object from an OraDatabase object.
During create and refresh, OraSQLStmt objects automatically bind all relevant,
enabled input parameters to the specified SQL statement, using the parameter names
as placeholders in the SQL statement. This can improve the performance of SQL
statement execution without re-parsing the SQL statement.
The OraSQLStmt object can be used later to execute the same query using a different
value for the :SALARY placeholder. This is done as follows (updateStmt is the
OraSQLStmt object here):
OraDatabase.Parameters("SALARY").value = 200000
updateStmt.Parameters("ENAME").value = "KING"
updateStmt.Refresh

OraAQ
An OraAQ object is instantiated by invoking the CreateAQ method of the
OraDatabase interface. It represents a queue that is present in the database.

Orientation to Oracle Programmatic Environments 1-27

Overview of Oracle Objects for OLE (OO4O)

Oracle Objects for OLE provides interfaces for accessing Oracle Advanced Queuing
(AQ) feature. It makes AQ accessible from popular COM-based development
environments such as Visual Basic. For a detailed description of Oracle Advanced
Queuing, refer to Oracle Streams Advanced Queuing User's Guide and Reference.

OraAQMsg
The OraAQMsg object encapsulates the message to be enqueued or dequeued. The
message can be of any user-defined or raw type.
For a detailed description of Oracle Advanced Queuing, refer to Oracle Streams
Advanced Queuing User's Guide and Reference.

OraAQAgent
The OraAQAgent object represents a message recipient and is only valid for queues
that allow multiple consumers. It is a child of OraAQMsg.
An OraAQAgent object can be instantiated by invoking the AQAgent method. For
example:
Set agent = qMsg.AQAgent(name)

An OraAQAgent object can also be instantiated by invoking the AddRecipient
method. For example:
Set agent = qMsg.AddRecipient(name, address, protocol).

Support for Oracle LOB and Object Datatypes
Oracle Objects for OLE provides full support for accessing and manipulating instances
of object datatypes and LOBs in Oracle Database. Figure 1–4, "Supported Oracle
Datatypes" illustrates the datatypes supported by OO4O.
Instances of these types can be fetched from the database or passed as input or output
variables to SQL statements and PL/SQL blocks, including stored procedures and
functions. All instances are mapped to COM Automation Interfaces that provide
methods for dynamic attribute access and manipulation.
Figure 1–4 Supported Oracle Datatypes
OraObject

OraAttribute

OraRef

OraAttribute

OraCollection

Element Values

OraField

OraParameter

OraParamArray
OraBLOB

OraCLOB

OraBFILE

Value of all other scalar types

1-28 Oracle Database Application Developer’s Guide - Fundamentals

Overview of Oracle Objects for OLE (OO4O)

OraBLOB and OraCLOB
The OraBlob and OraClob interfaces in Oracle Objects for OLE provide methods for
performing operations on large database objects of datatype BLOB, CLOB, and NCLOB.
BLOB, CLOB, and NCLOB datatypes are also referred to here as LOB datatypes.
LOB data is accessed using Read and the CopyToFile methods.
LOB data is modified using Write, Append, Erase, Trim, Copy, CopyFromFile,
and CopyFromBFile methods. Before modifying the content of a LOB column in a
row, a row lock must be obtained. If the LOB column is a field of an OraDynaset,
object, then the lock is obtained by invoking the Edit method.

OraBFILE
The OraBFile interface in Oracle Objects for OLE provides methods for performing
operations on large database objects of datatype BFILE.
BFILE objects are large binary data objects stored in operating system files outside of
the database tablespaces.

Oracle Data Control
Oracle Data Control (ODC) is an ActiveX Control that is designed to simplify the
exchange of data between Oracle Database and visual controls such edit, text, list, and
grid controls in Visual Basic and other development tools that support custom
controls.
ODC acts as an agent to handle the flow of information from Oracle Database and a
visual data-aware control, such as a grid control, that is bound to it. The data control
manages various user interface (UI) tasks such as displaying and editing data. It also
executes and manages the results of database queries.
Oracle Data Control is compatible with the Microsoft data control included with
Visual Basic. If you are familiar with the Visual Basic data control, learning to use
Oracle Data Control is quick and easy. Communication between data-aware controls
and a Data Control is governed by a protocol that has been specified by Microsoft.

Oracle Objects for OLE C++ Class Library
Oracle Objects for OLE C++ Class Library is a collection of C++ classes that provide
programmatic access to the Oracle Object Server. Although the class library is
implemented using OLE Automation, neither the OLE development kit nor any OLE
development knowledge is necessary to use it. This library helps C++ developers
avoid the chore of writing COM client code for accessing the OO4O interfaces.

Additional Sources of Information
For detailed information about Oracle Objects for OLE refer to the online help
provided with the OO4O product:
■

Oracle Objects for OLE Help

■

Oracle Objects for OLE C++ Class Library Help

To view examples of how to use Oracle Objects for OLE, refer to the samples located in
the ORACLE_HOME\OO4O directory of the Oracle Database installation. Additional
OO4O examples can be found in the following Oracle publications:
■

Oracle Database Application Developer's Guide - Large Objects

Orientation to Oracle Programmatic Environments 1-29

Choosing a Programming Environment

■
■

Oracle Streams Advanced Queuing User's Guide and Reference
Oracle Database PL/SQL Packages and Types Reference

Choosing a Programming Environment
To choose a programming environment for a new development project:
■
■

■

■

Review the preceding overviews and the manuals for each environment.
Read the platform-specific manual that explains which compilers are approved for
use with your platforms.
If a particular language does not provide a feature you need, remember that
PL/SQL and Java stored procedures can both be called from code written in any of
the languages in this chapter. Stored procedures include triggers and object type
methods.
External procedures written in C can be called from OCI, Java, PL/SQL or SQL.
The external procedure itself can call back into the database using either SQL, OCI,
or Pro*C (but not C++).

The following examples illustrate easy choices:
■

■

Pro*COBOL does not support object types or collection types, while Pro*C/C++
does.
SQLJ does not support dynamic SQL the way that JDBC does.

Choosing Whether to Use OCI or a Precompiler
Precompiler applications typically contain less code than equivalent OCI applications,
which can help productivity.
Some situations require detailed control of the database and are suited for OCI
applications (either pure OCI or a precompiler application with embedded OCI calls):
■
■

■
■

■

■

■

OCI provides more detailed control over multiplexing and migrating sessions.
OCI provides dynamic bind and define using callbacks that can be used for any
arbitrary structure, including lists.
OCI has many calls to handle metadata.
OCI allows asynchronous event notifications to be received by a client application.
It provides a means for clients to generate notifications for propagation to other
clients.
OCI allows DML statements to use arrays to complete as many iterations as
possible before returning any error messages.
OCI calls for special purposes include Advanced Queuing, globalization support,
Data Cartridges, and support of the date and time datatypes.
OCI calls can be embedded in a Pro*C/C++ application.

Using Built-In Packages and Libraries
Both Java and PL/SQL have built-in packages and libraries.
PL/SQL and Java interoperate in the server. You can execute a PL/SQL package from
Java or wrap a PL/SQL class with a Java wrapper so that it can be called from

1-30 Oracle Database Application Developer’s Guide - Fundamentals

Choosing a Programming Environment

distributed CORBA and EJB clients. The following table shows PL/SQL packages and
their Java equivalents:
Table 1–1

PL/SQL and Java Equivalent Software

PL/SQL Package

Java Equivalent

DBMS_ALERT

Call package with SQLJ or JDBC.

DBMS_DDL

JDBC has this functionality.

DBMS_JOB

Schedule a job that has a Java Stored procedure.

DBMS_LOCK

Call with SQLJ or JDBC.

DBMS_MAIL

Use JavaMail.

DBMS_OUTPUT

Use subclass
oracle.aurora.rdbms.OracleDBMSOutputStream or Java
stored procedure DBMS_JAVA.SET_STREAMS.

DBMS_PIPE

Call with SQLJ or JDBC.

DBMS_SESSION

Use JDBC to execute an ALTER SESSION statement.

DBMS_SNAPSHOT

Call with SQLJ or JDBC.

DBMS_SQL

Use JDBC.

DBMS_TRANSACTION

Use JDBC to execute an ALTER SESSION statement.

DBMS_UTILITY

Call with SQLJ or JDBC.

UTL_FILE

Grant the JAVAUSERPRIV privilege and then use Java I/O entry
points.

Java Compared to PL/SQL
Both Java and PL/SQL can be used to build applications in the database. Here are
some guidelines for their use:

PL/SQL Is Optimized for Database Access
PL/SQL uses the same datatypes as SQL. SQL datatypes are thus easier to use and
SQL operations are faster than with Java, especially when a large amount of data is
involved, when mostly database access is done, or when bulk operations are used.

PL/SQL Is Integrated with the Database
PL/SQL is an extension to SQL offering data encapsulation, information hiding,
overloading, and exception-handling.
Some advanced PL/SQL capabilities are not available for Java in Oracle9i. Examples
are autonomous transactions and the dblink facility for remote databases. Code
development is usually faster in PL/SQL than in Java.

Both Java and PL/SQL Have Object-Oriented Features
Java has inheritance, polymorphism, and component models for developing
distributed systems. PL/SQL has inheritance and type evolution, the ability to change
methods and attributes of a type while preserving subtypes and table data that use the
type.

Orientation to Oracle Programmatic Environments 1-31

Choosing a Programming Environment

Java Is Used for Open Distributed Applications
Java has a richer type system than PL/SQL and is an object-oriented language. Java
can use CORBA (which can have many different computer languages in its clients) and
EJB. PL/SQL packages can be called from CORBA or EJB clients.
You can run XML tools, the Internet File System, or JavaMail from Java.
Many Java-based development tools are available throughout the industry.

1-32 Oracle Database Application Developer’s Guide - Fundamentals

Part I
SQL for Application Developers
This part contains the following chapters:
■

Chapter 2, "SQL Processing for Application Developers"

■

Chapter 3, "Using SQL Datatypes in Application Development"

■

Chapter 4, "Using Regular Expressions in Oracle Database"

■

Chapter 5, "Using Indexes in Application Development"

■

Chapter 6, "Maintaining Data Integrity in Application Development"

2
SQL Processing for Application Developers
This chapter describes how Oracle Database processes SQL statements. Before reading
this chapter you should read the section "SQL Processing" in Oracle Database Concepts.
Topics include the following:
■

Grouping Operations into Transactions

■

Ensuring Repeatable Reads with Read-Only Transactions

■

Using Cursors within Applications

■

Locking Data Explicitly

■

About User Locks

■

Using Serializable Transactions for Concurrency Control

■

Autonomous Transactions

■

Resuming Execution After a Storage Error Condition

Grouping Operations into Transactions
This section contains the following topics:
■

Deciding How to Group Operations in Transactions

■

Improving Transaction Performance

■

Committing Transactions

■

Rolling Back Transactions

■

Defining Transaction Savepoints

Deciding How to Group Operations in Transactions
In general, only application designers who use the programming interfaces to Oracle
Database are concerned with which types of actions should be grouped together as
one transaction. You should use the following principles when deciding how to group
transactions:
■

■

■

Transactions must be defined properly so that work is accomplished in logical
units and data is kept consistent.
Data in all referenced tables should be in a consistent state before the transaction
begins and after it ends.
Transactions should consist of only the SQL statements or PL/SQL blocks that
comprise one consistent change to the data.
SQL Processing for Application Developers

2-1

Grouping Operations into Transactions

For example, suppose that you write a Web application that enables users to transfer
funds between accounts. The transaction should include the debit to one account,
which is executed by one SQL statement, and the credit to another account, which is
executed by a second SQL statement. Both statements should fail or succeed together
as a unit of work; the credit should not be committed without the debit. Other
non-related actions, such as a new deposit to one account, should not be included in
the same transaction.

Improving Transaction Performance
As an application developer, you should consider whether you can improve
performance. Consider the following performance enhancements when designing and
writing your application:
■

■

■

■

■

■
■

■

Use the SET TRANSACTION command with the USE ROLLBACK SEGMENT clause to
explicitly assign a transaction to a rollback segment. This technique can eliminate
the need to allocate additional extents dynamically, which can reduce system
performance. Note that this clause is relevant and valid only if you use rollback
segments for undo. If you use automatic undo management, then Oracle Database
ignores this clause.
Establish standards for writing SQL statements so that you can take advantage of
shared SQL areas. Oracle Database recognizes identical SQL statements and allows
them to share memory areas. This reduces memory usage on the database server
and increases system throughput.
Use the ANALYZE command to collect statistics that can be used by Oracle
Database to implement a cost-based approach to SQL statement optimization. You
can supply additional "hints" to the optimizer as needed.
Call the DBMS_APPLICATION_INFO.SET_ACTION procedure before beginning a
transaction to register and name a transaction for later use when measuring
performance across an application. You should specify which type of activity a
transaction performs so that the system tuners can later see which transactions are
taking up the most system resources.
Increase user productivity and query efficiency by including user-written PL/SQL
functions in SQL expressions as described in "Calling Stored Functions from SQL
Expressions" on page 7-36.
Create explicit cursors when writing a PL/SQL application.
Reduce frequency of parsing and improve performance in precompiler programs
by increasing the number of cursors with MAX_OPEN_CURSORS.
Use the SET TRANSACTION command with the ISOLATION LEVEL set to
SERIALIZABLE to get ANSI/ISO serializable transactions.
See Also:
■

"How Serializable Transactions Interact" on page 2-15

■

"Using Cursors within Applications" on page 2-6

■

Oracle Database Concepts for more information about transaction
tuning features

Committing Transactions
To commit a transaction, use the COMMIT statement. The following two statements are
equivalent and commit the current transaction:

2-2 Oracle Database Application Developer’s Guide - Fundamentals

Grouping Operations into Transactions

COMMIT WORK;
COMMIT;

The COMMIT statements lets you include the COMMENT parameter along with a
comment that provides information about the transaction being committed. This
option is useful for including information about the origin of the transaction when you
commit distributed transactions:
COMMIT COMMENT 'Dallas/Accts_pay/Trans_type 10B';

Managing Commit Redo Behavior
When a transaction updates the database, it generates a redo entry corresponding to
this update. Oracle Database buffers this redo in memory until the completion of the
transaction. When the transaction commits, the log writer (LGWR) process writes redo
for the commit, along with the accumulated redo of all changes in the transaction, to
disk. By default Oracle Database writes the redo to disk before the call returns to the
client. This behavior introduces a latency in the commit because the application must
wait for the redo to be persisted on disk.
Suppose that you are writing an application that requires very high transaction
throughput. If you are willing to trade commit durability for lower commit latency,
then you can change the default COMMIT options so that the application does not need
to wait for Oracle Database to write data to the online redo logs.
Oracle Database enables you to change the handling of commit redo depending on the
needs of your application. You can change the commit behavior in the following
locations:
■

COMMIT_WRITE initialization parameter at the system or session level

■

COMMIT statement

The options in the COMMIT statement override the current settings in the initialization
parameter. Table 2–1 describes redo persistence options that you can set in either
location.
Table 2–1

Initialization Parameter and COMMIT Options for Managing Commit Redo

Option

Specifies that . . .

WAIT

The commit does not return as successful until the redo corresponding to the
commit is persisted in the online redo logs (default).

NOWAIT

The commit should return to the application without waiting for the redo to
be written to the online redo logs.

IMMEDIATE

The log writer process should write the redo for the commit immediately
(default). In other words, this option forces a disk I/O.

BATCH

Oracle Database should buffer the redo. The log writer process is permitted to
write the redo to disk in its own time.

The following example shows how to set the commit behavior to BATCH and NOWAIT
in the initialization parameter file:
COMMIT_WRITE = BATCH, NOWAIT

You can change the commit behavior at the system level by executing ALTER SYSTEM
as in the following example:
ALTER SYSTEM SET COMMIT_WRITE = BATCH, NOWAIT

SQL Processing for Application Developers

2-3

Grouping Operations into Transactions

After the initialization parameter is set, a COMMIT statement with no options conforms
to the options specified in the parameter. Alternatively, you can override the current
initialization parameter setting by specifying options directly on the COMMIT
statement as in the following example:
COMMIT WRITE BATCH NOWAIT

In either case, your application specifies that log writer does not have to write the redo
for the commit immediately to the online redo logs and should not wait for
confirmation that the redo has been written to disk.
You cannot change the default IMMEDIATE and WAIT behavior
for distributed transactions.

Note:

If your application uses OCI, then you can modify redo behavior by setting the
following flags in the OCITransCommit() function within your application:
■

OCI_TRANS_WRITEBATCH

■

OCI_TRANS_WRITENOWAIT

■

OCI_TRANS_WRITEIMMED

■

OCI_TRANS_WRITEWAIT

Note that the specification of the NOWAIT and BATCH options allows a small window
of vulnerability in which Oracle Database can roll back a transaction that your
application view as committed. Your application must be able to tolerate the following
scenarios:
■

■

The database host crashes, which causes the database to lose redo that was
buffered but not yet written to the online redo logs.
A file I/O problem prevents log writer from writing buffered redo to disk. If the
redo logs are not multiplexed, then the commit is lost.
See Also:
■

■

Oracle Database SQL Reference for information on the COMMIT
statement
Oracle Call Interface Programmer's Guide for information about the
OCITransCommit() function

Rolling Back Transactions
To roll back an entire transaction, or to roll back part of a transaction to a savepoint,
use the ROLLBACK statement. For example, either of the following statements rolls
back the entire current transaction:
ROLLBACK WORK;
ROLLBACK;

The WORK option of the ROLLBACK command has no function.
To roll back to a savepoint defined in the current transaction, use the TO option of the
ROLLBACK command. For example, either of the following statements rolls back the
current transaction to the savepoint named POINT1:
SAVEPOINT Point1;
...

2-4 Oracle Database Application Developer’s Guide - Fundamentals

Ensuring Repeatable Reads with Read-Only Transactions

ROLLBACK TO SAVEPOINT Point1;
ROLLBACK TO Point1;

Defining Transaction Savepoints
To define a savepoint in a transaction, use the SAVEPOINT command. The following
statement creates the savepoint named ADD_EMP1 in the current transaction:
SAVEPOINT Add_emp1;

If you create a second savepoint with the same identifier as an earlier savepoint, the
earlier savepoint is erased. After creating a savepoint, you can roll back to the
savepoint.
There is no limit on the number of active savepoints for each session. An active
savepoint is one that has been specified since the last commit or rollback.

An Example of COMMIT, SAVEPOINT, and ROLLBACK
Table 2–4 shows a series of SQL statements that illustrates the use of COMMIT,
SAVEPOINT, and ROLLBACK statements within a transaction.
Table 2–2

Use of COMMIT, SAVEPOINT, and ROLLBACK

SQL Statement

Results

SAVEPOINT a;

First savepoint of this transaction

DELETE...;

First DML statement of this transaction

SAVEPOINT b;

Second savepoint of this transaction

INSERT INTO...; Second DML statement of this transaction
SAVEPOINT c;

Third savepoint of this transaction

UPDATE...;

Third DML statement of this transaction.

ROLLBACK TO c;

UPDATE statement is rolled back, savepoint C remains defined

ROLLBACK TO b;

INSERT statement is rolled back, savepoint C is lost, savepoint B remains
defined

ROLLBACK TO c;

ORA-01086 error; savepoint C no longer defined

INSERT INTO...; New DML statement in this transaction
COMMIT;

Commits all actions performed by the first DML statement (the DELETE
statement) and the last DML statement (the second INSERT statement)
All other statements (the second and the third statements) of the
transaction were rolled back before the COMMIT. The savepoint A is no
longer active.

Ensuring Repeatable Reads with Read-Only Transactions
By default, the consistency model for Oracle Database guarantees statement-level read
consistency, but does not guarantee transaction-level read consistency (repeatable
reads). If you want transaction-level read consistency, and if your transaction does not
require updates, then you can specify a read-only transaction. After indicating that your
transaction is read-only, you can execute as many queries as you like against any
database table, knowing that the results of each query in the read-only transaction are
consistent with respect to a single point in time.
A read-only transaction does not acquire any additional data locks to provide
transaction-level read consistency. The multi-version consistency model used for
SQL Processing for Application Developers

2-5

Using Cursors within Applications

statement-level read consistency is used to provide transaction-level read consistency;
all queries return information with respect to the system change number (SCN)
determined when the read-only transaction begins. Because no data locks are acquired,
other transactions can query and update data being queried concurrently by a
read-only transaction.
Long-running queries sometimes fail because undo information required for consistent
read operations is no longer available. This happens when committed undo blocks are
overwritten by active transactions. Automatic undo management provides a way to
explicitly control when undo space can be reused; that is, how long undo information
is retained. Your database administrator can specify a retention period by using the
parameter UNDO_RETENTION.
See Also: Oracle Database Administrator's Guide for information on
long-running queries and resumable space allocation

For example, if UNDO_RETENTION is set to 30 minutes, then all committed undo
information in the system is retained for at least 30 minutes. This ensures that all
queries running for 30 minutes or less, under usual circumstances, do not encounter
the OER error, "snapshot too old."
A read-only transaction is started with a SET TRANSACTION statement that includes
the READ ONLY option. For example:
SET TRANSACTION READ ONLY;

The SET TRANSACTION statement must be the first statement of a new transaction; if
any DML statements (including queries) or other non-DDL statements (such as SET
ROLE) precede a SET TRANSACTION READ ONLY statement, an error is returned. Once
a SET TRANSACTION READ ONLY statement successfully executes, only SELECT
(without a FOR UPDATE clause), COMMIT, ROLLBACK, or non-DML statements (such
as SET ROLE, ALTER SYSTEM, LOCK TABLE) are allowed in the transaction.
Otherwise, an error is returned. A COMMIT, ROLLBACK, or DDL statement terminates
the read-only transaction; a DDL statement causes an implicit commit of the read-only
transaction and commits in its own transaction.

Using Cursors within Applications
PL/SQL implicitly declares a cursor for all SQL data manipulation statements,
including queries that return only one row. For queries that return more than one row,
you can explicitly declare a cursor to process the rows individually.
A cursor is a handle to a specific private SQL area. In other words, a cursor can be
thought of as a name for a specific private SQL area. A PL/SQL cursor variable
enables the retrieval of multiple rows from a stored procedure. Cursor variables allow
you to pass cursors as parameters in your 3GL application. Cursor variables are
described in Oracle Database PL/SQL User's Guide and Reference.
Although most Oracle Database users rely on the automatic cursor handling of the
database utilities, the programmatic interfaces offer application designers more control
over cursors. In application development, a cursor is a named resource available to a
program, which can be specifically used for parsing SQL statements embedded within
the application.

Declaring and Opening Cursors
There is no absolute limit to the total number of cursors one session can have open at
one time, subject to two constraints:
2-6 Oracle Database Application Developer’s Guide - Fundamentals

Using Cursors within Applications

■

■

Each cursor requires virtual memory, so a session's total number of cursors is
limited by the memory available to that process.
A systemwide limit of cursors for each session is set by the initialization parameter
named OPEN_CURSORS found in the parameter file (such as INIT.ORA).
See Also:

Oracle Database Reference for descriptions of parameters

Explicitly creating cursors for precompiler programs can offer some advantages in
tuning those applications. For example, increasing the number of cursors can often
reduce the frequency of parsing and improve performance. If you know how many
cursors may be required at a given time, then you can make sure you can open that
many simultaneously.

Using a Cursor to Execute Statements Again
After each stage of execution, the cursor retains enough information about the SQL
statement to reexecute the statement without starting over, as long as no other SQL
statement has been associated with that cursor. The statement can be reexecuted
without including the parse stage.
By opening several cursors, the parsed representation of several SQL statements can
be saved. Repeated execution of the same SQL statements can thus begin at the
describe, define, bind, or execute step, saving the repeated cost of opening cursors and
parsing.
To understand the performance characteristics of a cursor, a DBA can retrieve the text
of the query represented by the cursor using the V$SQL catalog view. Because the
results of EXPLAIN PLAN on the original query might differ from the way the query is
actually processed, a DBA can get more precise information by examining the
V$SQL_PLAN, V$SQL_PLAN_STATISTICS, and V$SQL_PLAN_STATISTICS_ALL
catalog views.:
■

■

■

The V$SQL_PLAN view contains the execution plan information for each child
cursor loaded in the library cache.
The V$SQL_PLAN_STATISTICS view provides execution statistics at the row
source level for each child cursor.
The V$SQL_PLAN_STATISTICS_ALL view contains memory usage statistics for
row sources that use SQL memory (sort or hash-join). This view concatenates
information in V$SQL_PLAN with execution statistics from
V$SQL_PLAN_STATISTICS and V$SQL_WORKAREA.
See Also:

Oracle Database Reference for details on these views

Closing Cursors
Closing a cursor means that the information currently in the associated private area is
lost and its memory is deallocated. Once a cursor is opened, it is not closed until one of
the following events occurs:
■
■

The user program terminates its connection to the server.
If the user program is an OCI program or precompiler application, then it
explicitly closes any open cursor during the execution of that program. (However,
when this program terminates, any cursors remaining open are implicitly closed.)

SQL Processing for Application Developers

2-7

Locking Data Explicitly

Cancelling Cursors
Cancelling a cursor frees resources from the current fetch.The information currently in
the associated private area is lost but the cursor remains open, parsed, and associated
with its bind variables.
Note:

You cannot cancel cursors using Pro*C/C++ or PL/SQL.

See Also: Oracle Call Interface Programmer's Guide for more
information about cancelling cursors

Locking Data Explicitly
Oracle Database always performs necessary locking to ensure data concurrency,
integrity, and statement-level read consistency. You can override these default locking
mechanisms. For example, you might want to override the default locking of Oracle
Database if:
■

■

You want transaction-level read consistency or "repeatable reads"—where
transactions query a consistent set of data for the duration of the transaction,
knowing that the data has not been changed by any other transactions. This level
of consistency can be achieved by using explicit locking, read-only transactions,
serializable transactions, or overriding default locking for the system.
A transaction requires exclusive access to a resource. To proceed with its
statements, the transaction with exclusive access to a resource does not have to
wait for other transactions to complete.

The automatic locking mechanisms can be overridden at the transaction level.
Transactions including the following SQL commands override Oracle Database's
default locking:
■

LOCK TABLE

■

SELECT, including the FOR UPDATE clause

■

SET TRANSACTION with the READ ONLY or ISOLATION LEVEL SERIALIZABLE
options

Locks acquired by these statements are released after the transaction is committed or
rolled back.
The following sections describe each option available for overriding the default
locking of Oracle Database. The initialization parameter DML_LOCKS determines the
maximum number of DML locks allowed.
See Also:

Oracle Database Reference for a discussion of parameters

Although the default value is usually enough, you might need to increase it if you use
additional manual locks.
Caution: If you override the default locking of Oracle Database at
any level, be sure that the overriding locking procedures operate
correctly: Ensure that data integrity is guaranteed, data concurrency is
acceptable, and deadlocks are not possible or are appropriately
handled.

2-8 Oracle Database Application Developer’s Guide - Fundamentals

Locking Data Explicitly

Choosing a Locking Strategy
A transaction explicitly acquires the specified table locks when a LOCK TABLE
statement is executed. A LOCK TABLE statement manually overrides default locking.
When a LOCK TABLE statement is issued on a view, the underlying base tables are
locked. The following statement acquires exclusive table locks for the EMP_TAB and
DEPT_TAB tables on behalf of the containing transaction:
LOCK TABLE Emp_tab, Dept_tab
IN EXCLUSIVE MODE NOWAIT;

You can specify several tables or views to lock in the same mode; however, only a
single lock mode can be specified for each LOCK TABLE statement.
When a table is locked, all rows of the table are locked. No
other user can modify the table.

Note:

You can also indicate if you do or do not want to wait to acquire the lock. If you
specify the NOWAIT option, then you only acquire the table lock if it is immediately
available. Otherwise an error is returned to notify that the lock is not available at this
time. In this case, you can attempt to lock the resource at a later time. If NOWAIT is
omitted, then the transaction does not proceed until the requested table lock is
acquired. If the wait for a table lock is excessive, then you might want to cancel the
lock operation and retry at a later time; you can code this logic into your applications.

When to Lock with ROW SHARE and ROW EXCLUSIVE Mode
LOCK TABLE Emp_tab IN ROW SHARE MODE;
LOCK TABLE Emp_tab IN ROW EXCLUSIVE MODE;

ROW SHARE and ROW EXCLUSIVE table locks offer the highest degree of concurrency.
You might use these locks if:
■

■

Your transaction needs to prevent another transaction from acquiring an
intervening share, share row, or exclusive table lock for a table before the table can
be updated in your transaction. If another transaction acquires an intervening
share, share row, or exclusive table lock, no other transactions can update the table
until the locking transaction commits or rolls back.
Your transaction needs to prevent a table from being altered or dropped before the
table can be modified later in your transaction.

When to Lock with SHARE Mode
LOCK TABLE Emp_tab IN SHARE MODE;

SHARE table locks are rather restrictive data locks. You might use these locks if:
■

■

■

Your transaction only queries the table, and requires a consistent set of the table
data for the duration of the transaction.
You can hold up other transactions that try to update the locked table, until all
transactions that hold SHARE locks on the table either commit or roll back.
Other transactions may acquire concurrent SHARE table locks on the same table,
also allowing them the option of transaction-level read consistency.

SQL Processing for Application Developers

2-9

Locking Data Explicitly

Caution: Your transaction may or may not update the table later in
the same transaction. However, if multiple transactions concurrently
hold share table locks for the same table, no transaction can update
the table (even if row locks are held as the result of a SELECT... FOR
UPDATE statement). Therefore, if concurrent share table locks on the
same table are common, updates cannot proceed and deadlocks are
common. In this case, use share row exclusive or exclusive table locks
instead.

For example, assume that two tables, EMP_TAB and BUDGET_TAB, require a consistent
set of data in a third table, DEPT_TAB. For a given department number, you want to
update the information in both of these tables, and ensure that no new members are
added to the department between these two transactions.
Although this scenario is quite rare, it can be accommodated by locking the DEPT_TAB
table in SHARE MODE, as shown in the following example. Because the DEPT_TAB table
is rarely updated, locking it probably does not cause many other transactions to wait
long.
You may need to set up data structures similar to the following
for certain examples to work:

Note:

CREATE TABLE dept_tab(
deptno NUMBER(2) NOT NULL,
dname VARCHAR2(14),
loc VARCHAR2(13));
CREATE TABLE emp_tab (
empno NUMBER(4) NOT NULL,
ename VARCHAR2(10),
job VARCHAR2(9),
mgr NUMBER(4),
hiredate DATE,
sal NUMBER(7,2),
comm NUMBER(7,2),
deptno NUMBER(2));
CREATE TABLE Budget_tab (
totsal NUMBER(7,2),
deptno NUMBER(2) NOT NULL);

LOCK TABLE Dept_tab IN SHARE MODE;
UPDATE Emp_tab
SET sal = sal * 1.1
WHERE deptno IN
(SELECT deptno FROM Dept_tab WHERE loc = 'DALLAS');
UPDATE Budget_tab
SET Totsal = Totsal * 1.1
WHERE Deptno IN
(SELECT Deptno FROM Dept_tab WHERE Loc = 'DALLAS');
COMMIT; /* This releases the lock */

When to Lock with SHARE ROW EXCLUSIVE Mode
LOCK TABLE Emp_tab IN SHARE ROW EXCLUSIVE MODE;

2-10 Oracle Database Application Developer’s Guide - Fundamentals

Locking Data Explicitly

You might use a SHARE ROW EXCLUSIVE table lock if:
■

■

■

Your transaction requires both transaction-level read consistency for the specified
table and the ability to update the locked table.
You do not care if other transactions acquire explicit row locks (using SELECT...
FOR UPDATE), which might make UPDATE and INSERT statements in the locking
transaction wait and might cause deadlocks.
You only want a single transaction to have this behavior.

When to Lock in EXCLUSIVE Mode
LOCK TABLE Emp_tab IN EXCLUSIVE MODE;

You might use an EXCLUSIVE table if:
■

■

■

Your transaction requires immediate update access to the locked table. When your
transaction holds an exclusive table lock, other transactions cannot lock specific
rows in the locked table.
Your transaction also ensures transaction-level read consistency for the locked
table until the transaction is committed or rolled back.
You are not concerned about low levels of data concurrency, making transactions
that request exclusive table locks wait in line to update the table sequentially.

Privileges Required
You can automatically acquire any type of table lock on tables in your schema. To
acquire a table lock on a table in another schema, you must have the LOCK ANY TABLE
system privilege or any object privilege (for example, SELECT or UPDATE) for the
table.

Letting Oracle Database Control Table Locking
Letting Oracle Database control table locking means your application needs less
programming logic, but also has less control, than if you manage the table locks
yourself.
Issuing the command SET TRANSACTION ISOLATION LEVEL SERIALIZABLE or
ALTER SESSION ISOLATION LEVEL SERIALIZABLE preserves ANSI serializability
without changing the underlying locking protocol. This technique allows concurrent
access to the table while providing ANSI serializability. Getting table locks greatly
reduces concurrency.
See Also:
■

■

Oracle Database SQL Reference for information on the SET
TRANSACTION statement
Oracle Database SQL Reference for information on the ALTER
SESSION statements

The settings for these parameters should be changed only when an instance is shut
down. If multiple instances are accessing a single database, then all instances should
use the same setting for these parameters.

SQL Processing for Application Developers 2-11

Locking Data Explicitly

Explicitly Acquiring Row Locks
You can override default locking with a SELECT statement that includes the FOR
UPDATE clause. This statement acquires exclusive row locks for selected rows (as an
UPDATE statement does), in anticipation of updating the selected rows in a subsequent
statement.
You can use a SELECT... FOR UPDATE statement to lock a row without actually
changing it. For example, several triggers in Chapter 9, "Coding Triggers", show how
to implement referential integrity. In the EMP_DEPT_CHECK trigger (see "Foreign Key
Trigger for Child Table"), the row that contains the referenced parent key value is
locked to guarantee that it remains for the duration of the transaction; if the parent key
is updated or deleted, referential integrity would be violated.
SELECT... FOR UPDATE statements are often used by interactive programs that allow a
user to modify fields of one or more specific rows (which might take some time); row
locks are acquired so that only a single interactive program user is updating the rows
at any given time.
If a SELECT... FOR UPDATE statement is used when defining a cursor, the rows in the
return set are locked when the cursor is opened (before the first fetch) rather than
being locked as they are fetched from the cursor. Locks are only released when the
transaction that opened the cursor is committed or rolled back, not when the cursor is
closed.
Each row in the return set of a SELECT... FOR UPDATE statement is locked
individually; the SELECT... FOR UPDATE statement waits until the other transaction
releases the conflicting row lock. If a SELECT... FOR UPDATE statement locks many
rows in a table, and if the table experiences a lot of update activity, it might be faster to
acquire an EXCLUSIVE table lock instead.
The return set for a SELECT... FOR UPDATE may change
while the query is running; for example, if columns selected by the
query are updated or rows are deleted after the query started. When
this happens, SELECT... FOR UPDATE acquires locks on the rows
that did not change, gets a new read-consistent snapshot of the table
using these locks, and then restarts the query to acquire the remaining
locks.

Note:

This can cause a deadlock between sessions querying the table
concurrently with DML operations when rows are locked in a
non-sequential order. To prevent such deadlocks, design your
application so that any concurrent DML on the table does not affect
the return set of the query. If this is not feasible, you may want to
serialize queries in your application.
When acquiring row locks with SELECT... FOR UPDATE, you can specify the NOWAIT
option to indicate that you are not willing to wait to acquire the lock. If you cannot
acquire then lock immediately, an error is returned to signal that the lock is not
possible at this time. You can try to lock the row again later.
By default, the transaction waits until the requested row lock is acquired. If the wait
for a row lock is too long, you can code logic into your application to cancel the lock
operation and try again later.

2-12 Oracle Database Application Developer’s Guide - Fundamentals

About User Locks

About User Locks
You can use Oracle Lock Management services for your applications by making calls
to the DBMS_LOCK package. It is possible to request a lock of a specific mode, give it a
unique name recognizable in another procedure in the same or another instance,
change the lock mode, and release it. Because a reserved user lock is the same as an
Oracle Database lock, it has all the features of a database lock, such as deadlock
detection. Be certain that any user locks used in distributed transactions are released
upon COMMIT, or an undetected deadlock can occur.
See Also: Oracle Database PL/SQL Packages and Types Reference for
detailed information on the DBMS_LOCK package

When to Use User Locks
User locks can help to:
■

Provide exclusive access to a device, such as a terminal

■

Provide application-level enforcement of read locks

■

Detect when a lock is released and cleanup after the application

■

Synchronize applications and enforce sequential processing

Example of a User Lock
The following Pro*COBOL precompiler example shows how locks can be used to
ensure that there are no conflicts when multiple people need to access a single device.
*****************************************************************
* Print Check
*
* Any cashier may issue a refund to a customer returning goods. *
* Refunds under $50 are given in cash, more than $50 by check. *
* This code prints the check. The one printer is opened by all *
* the cashiers to avoid the overhead of opening and closing it *
* for every check. This means that lines of output from multiple*
* cashiers could become interleaved if we don't ensure exclusive*
* access to the printer. The DBMS_LOCK package is used to
*
* ensure exclusive access.
*
*****************************************************************
CHECK-PRINT
*
Get the lock "handle" for the printer lock.
MOVE "CHECKPRINT" TO LOCKNAME-ARR.
MOVE 10 TO LOCKNAME-LEN.
EXEC SQL EXECUTE
BEGIN DBMS_LOCK.ALLOCATE_UNIQUE ( :LOCKNAME, :LOCKHANDLE );
END; END-EXEC.
*
Lock the printer in exclusive mode (default mode).
EXEC SQL EXECUTE
BEGIN DBMS_LOCK.REQUEST ( :LOCKHANDLE );
END; END-EXEC.
*
We now have exclusive use of the printer, print the check.
...
*
Unlock the printer so other people can use it
EXEC SQL EXECUTE
BEGIN DBMS_LOCK.RELEASE ( :LOCKHANDLE );
END; END-EXEC.

SQL Processing for Application Developers 2-13

Using Serializable Transactions for Concurrency Control

Viewing and Monitoring Locks
Table 2–5 describes Oracle Database facilities to display locking information for
ongoing transactions within an instance.
Table 2–3

Ways to Display Locking Information

Tool

Description

Oracle Enterprise
Manager 10g Database
Control

From the Additional Monitoring Links section of the Database
Performance page, click Database Locks to display user blocks,
blocking locks, or the complete list of all database locks. Refer to
Oracle Database 2 Day DBA for more information.

UTLLOCKT.SQL

The UTLLOCKT.SQL script displays a simple character lock wait-for
graph in tree structured fashion. Using any ad hoc SQL tool (such as
SQL*Plus) to execute the script, it prints the sessions in the system
that are waiting for locks and the corresponding blocking locks. The
location of this script file is operating system dependent. (You must
have run the CATBLOCK.SQL script before using UTLLOCKT.SQL.)

Using Serializable Transactions for Concurrency Control
By default, Oracle Database permits concurrently executing transactions to modify,
add, or delete rows in the same table, and in the same data block. Changes made by
one transaction are not seen by another concurrent transaction until the transaction
that made the changes commits.
If a transaction A attempts to update or delete a row that has been locked by another
transaction B (by way of a DML or SELECT... FOR UPDATE statement), then A's DML
command blocks until B commits or rolls back. Once B commits, transaction A can see
changes that B has made to the database.
For most applications, this concurrency model is the appropriate one, because it
provides higher concurrency and thus better performance. But some rare cases require
transactions to be serializable. Serializable transactions must execute in such a way
that they appear to be executing one at a time (serially), rather than concurrently.
Concurrent transactions executing in serialized mode can only make database changes
that they could have made if the transactions ran one after the other.
Figure 2–1 shows a serializable transaction (B) interacting with another transaction
(A).
The ANSI/ISO SQL standard SQL92 defines three possible kinds of transaction
interaction, and four levels of isolation that provide increasing protection against these
interactions. These interactions and isolation levels are summarized in Table 2–4.
Table 2–4

Summary of ANSI Isolation Levels

Isolation Level

Dirty Read1 Non-Repeatable Read2 Phantom Read3

READ UNCOMMITTED Possible

Possible

Possible

READ COMMITTED

Not possible Possible

Possible

REPEATABLE READ

Not possible Not possible

Possible

SERIALIZABLE

Not possible Not possible

Not possible

1
2
3

A transaction can read uncommitted data changed by another transaction.
A transaction rereads data committed by another transaction and sees the new data.
A transaction can execute a query again, and discover new rows inserted by another
committed transaction.

2-14 Oracle Database Application Developer’s Guide - Fundamentals

Using Serializable Transactions for Concurrency Control

The behavior of Oracle Database with respect to these isolation levels is summarized
in Table 2–5.
Table 2–5

ANSI Isolation Levels and Oracle Database

Isolation Level

Description

READ UNCOMMITTED Oracle Database never permits "dirty reads." Although some other
database products use this undesirable technique to improve
thoughput, it is not required for high throughput with Oracle Database.
READ COMMITTED

Oracle Database meets the READ COMMITTED isolation standard. This is
the default mode for all Oracle Database applications. Because an
Oracle Database query only sees data that was committed at the
beginning of the query (the snapshot time), Oracle Database actually
offers more consistency than is required by the ANSI/ISO SQL92
standards for READ COMMITTED isolation.

REPEATABLE READ

Oracle Database does not normally support this isolation level, except
as provided by SERIALIZABLE.

SERIALIZABLE

Oracle Database does not normally support this isolation level, except
as provided by SERIALIZABLE.

How Serializable Transactions Interact
Figure 2–1 on page 2-16 shows how a serializable transaction (Transaction B) interacts
with another transaction (A, which can be either SERIALIZABLE or READ
COMMITTED).
When a serializable transaction fails with an ORA-08177 error ("cannot serialize
access"), the application can take any of several actions:
■
■

■

Commit the work executed to that point
Execute additional, different, statements, perhaps after rolling back to a prior
savepoint in the transaction
Roll back the entire transaction and try it again

Oracle Database stores control information in each data block to manage access by
concurrent transactions. To use the SERIALIZABLE isolation level, you must use the
INITRANS clause of the CREATE TABLE or ALTER TABLE command to set aside
storage for this control information. To use serializable mode, INITRANS must be set
to at least 3.

SQL Processing for Application Developers 2-15

Using Serializable Transactions for Concurrency Control

Figure 2–1 Time Line for Two Transactions

TRANSACTION A
(arbitrary)
begin work
update row 2
in block 1

insert row 4

Issue update "too recent"
for B to see

SET TRANSACTION
ISOLATION LEVEL
SERIALIZABLE
read row 1 in block 1

Change other row in
same block, see own
changes

update row 1 in block 1
read updated row 1 in
block 1

Create possible
"phantom" row

Uncommitted changes
invisible

commit

TRANSACTION B
(serializable)

read old row 2 in block 1
search for row 4
(notfound)

Make changes visible
to transactions that
begin later

Make changes
after A commits

update row 3 in block 1

B can see its own changes
but not the committed
changes of transaction A.

re-read updated row 1
in block 1
search for row 4 (not found)
read old row 2 in block 1

Failure on attempt to update
row updated and committed
since transaction B began

update row 2 in block 1
FAILS; rollback and retry

TIME

Setting the Isolation Level of a Transaction
You can change the isolation level of a transaction using the ISOLATION LEVEL clause
of the SET TRANSACTION command, which must be the first command issued in a
transaction.
Use the ALTER SESSION command to set the transaction isolation level on a
session-wide basis.
Oracle Database Reference for the complete syntax of the
SET TRANSACTION and ALTER SESSION commands

See Also:

The INITRANS Parameter
Oracle Database stores control information in each data block to manage access by
concurrent transactions. Therefore, if you set the transaction isolation level to

2-16 Oracle Database Application Developer’s Guide - Fundamentals

Using Serializable Transactions for Concurrency Control

SERIALIZABLE, then you must use the ALTER TABLE command to set INITRANS to
at least 3. This parameter causes Oracle Database to allocate sufficient storage in each
block to record the history of recent transactions that accessed the block. Higher values
should be used for tables that will undergo many transactions updating the same
blocks.

Referential Integrity and Serializable Transactions
Because Oracle Database does not use read locks, even in SERIALIZABLE
transactions, data read by one transaction can be overwritten by another. Transactions
that perform database consistency checks at the application level should not assume
that the data they read will not change during the execution of the transaction (even
though such changes are not visible to the transaction). Database inconsistencies can
result unless such application-level consistency checks are coded carefully, even when
using SERIALIZABLE transactions. Note, however, that the examples shown in this
section are applicable for both READ COMMITTED and SERIALIZABLE transactions.
Figure 2–2 on page 2-17 shows two different transactions that perform
application-level checks to maintain the referential integrity parent/child relationship
between two tables. One transaction checks that a row with a specific primary key
value exists in the parent table before inserting corresponding child rows. The other
transaction checks to see that no corresponding detail rows exist before deleting a
parent row. In this case, both transactions assume (but do not ensure) that data they
read will not change before the transaction completes.
Figure 2–2 Referential Integrity Check
B's query does
not prevent this
insert

TRANSACTION A

TRANSACTION B

read parent (it exists)

read child rows (not found)

insert child row(s)

delete parent

commit work

commit work

A's query does
not prevent this
delete

The read issued by transaction A does not prevent transaction B from deleting the
parent row, and transaction B's query for child rows does not prevent transaction A
from inserting child rows. This scenario leaves a child row in the database with no
corresponding parent row. This result occurs even if both A and B are SERIALIZABLE
transactions, because neither transaction prevents the other from making changes in
the data it reads to check consistency.

SQL Processing for Application Developers 2-17

Using Serializable Transactions for Concurrency Control

As this example shows, sometimes you must take steps to ensure that the data read by
one transaction is not concurrently written by another. This requires a greater degree
of transaction isolation than defined by SQL92 SERIALIZABLE mode.

Using SELECT FOR UPDATE
Fortunately, it is straightforward in Oracle Database to prevent the anomaly described:
■

■

Transaction A can use SELECT FOR UPDATE to query and lock the parent row and
thereby prevent transaction B from deleting the row.
Transaction B can prevent Transaction A from gaining access to the parent row by
reversing the order of its processing steps. Transaction B first deletes the parent
row, and then rolls back if its subsequent query detects the presence of
corresponding rows in the child table.

Referential integrity can also be enforced in Oracle Database using database triggers,
instead of a separate query as in Transaction A. For example, an INSERT into the child
table can fire a BEFORE INSERT row-level trigger to check for the corresponding
parent row. The trigger queries the parent table using SELECT FOR UPDATE, ensuring
that parent row (if it exists) remains in the database for the duration of the transaction
inserting the child row. If the corresponding parent row does not exist, the trigger
rejects the insert of the child row.
SQL statements issued by a database trigger execute in the context of the SQL
statement that caused the trigger to fire. All SQL statements executed within a trigger
see the database in the same state as the triggering statement. Thus, in a READ
COMMITTED transaction, the SQL statements in a trigger see the database as of the
beginning of the triggering statement execution, and in a transaction executing in
SERIALIZABLE mode, the SQL statements see the database as of the beginning of the
transaction. In either case, the use of SELECT FOR UPDATE by the trigger correctly
enforces referential integrity.

READ COMMITTED and SERIALIZABLE Isolation
Oracle Database gives you a choice of two transaction isolation levels with different
characteristics. Both the READ COMMITTED and SERIALIZABLE isolation levels
provide a high degree of consistency and concurrency. Both levels reduce contention,
and are designed for deploying real-world applications. The rest of this section
compares the two isolation modes and provides information helpful in choosing
between them.

Transaction Set Consistency
A useful way to describe the READ COMMITTED and SERIALIZABLE isolation levels in
Oracle Database is to consider:
■

A collection of database tables (or any set of data)

■

A sequence of reads of rows in those tables

■

The set of transactions committed at any moment

An operation (a query or a transaction) is transaction set consistent if its read
operations all return data written by the same set of committed transactions. When an
operation is not transaction set consistent, some reads reflect the changes of one set of
transactions, and other reads reflect changes made by other transactions. Such an
operation sees the database in a state that reflects no single set of committed
transactions.

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Using Serializable Transactions for Concurrency Control

Oracle Database transactions executing in READ COMMITTED mode are transaction-set
consistent on an individual-statement basis, because all rows read by a query must be
committed before the query begins.
Oracle Database transactions executing in SERIALIZABLE mode are transaction set
consistent on an individual-transaction basis, because all statements in a
SERIALIZABLE transaction execute on an image of the database as of the beginning of
the transaction.
In other database systems, a single query run in READ COMMITTED mode provides
results that are not transaction set consistent. The query is not transaction set
consistent, because it may see only a subset of the changes made by another
transaction. For example, a join of a master table with a detail table could see a master
record inserted by another transaction, but not the corresponding details inserted by
that transaction, or vice versa. The READ COMMITTED mode avoids this problem, and
so provides a greater degree of consistency than read-locking systems.
In read-locking systems, at the cost of preventing concurrent updates, SQL92
REPEATABLE READ isolation provides transaction set consistency at the statement
level, but not at the transaction level. The absence of phantom protection means two
queries issued by the same transaction can see data committed by different sets of
other transactions. Only the throughput-limiting and deadlock-susceptible
SERIALIZABLE mode in these systems provides transaction set consistency at the
transaction level.

Comparison of READ COMMITTED and SERIALIZABLE Transactions
Table 2–6 summarizes key similarities and differences between READ COMMITTED and
SERIALIZABLE transactions.
Table 2–6

Read Committed Versus Serializable Transaction

Operation

Read Committed Serializable

Dirty write

Not Possible

Not Possible

Dirty read

Not Possible

Not Possible

Non-repeatable read

Possible

Not Possible

Phantoms

Possible

Not Possible

Compliant with ANSI/ISO SQL 92

Yes

Yes

Read snapshot time

Statement

Transaction

Transaction set consistency

Statement level

Transaction level

Row-level locking

Yes

Yes

Readers block writers

No

No

Writers block readers

No

No

Different-row writers block writers

No

No

Same-row writers block writers

Yes

Yes

Waits for blocking transaction

Yes

Yes

Subject to "can't serialize access" error

No

Yes

Error after blocking transaction aborts

No

No

Error after blocking transaction commits No

Yes

SQL Processing for Application Developers 2-19

Autonomous Transactions

Choosing an Isolation Level for Transactions
Choose an isolation level that is appropriate to the specific application and workload.
You might choose different isolation levels for different transactions. The choice
depends on performance and consistency needs, and consideration of application
coding requirements.
For environments with many concurrent users rapidly submitting transactions, you
must assess transaction performance against the expected transaction arrival rate and
response time demands, and choose an isolation level that provides the required
degree of consistency while performing well. Frequently, for high performance
environments, you must trade-off between consistency and concurrency (transaction
throughput).
Both Oracle Database isolation modes provide high levels of consistency and
concurrency (and performance) through the combination of row-level locking and
Oracle Database's multi-version concurrency control system. Because readers and
writers do not block one another in Oracle Database, while queries still see consistent
data, both READ COMMITTED and SERIALIZABLE isolation provide a high level of
concurrency for high performance, without the need for reading uncommitted ("dirty")
data.
READ COMMITTED isolation can provide considerably more concurrency with a
somewhat increased risk of inconsistent results (due to phantoms and non-repeatable
reads) for some transactions. The SERIALIZABLE isolation level provides somewhat
more consistency by protecting against phantoms and non-repeatable reads, and may
be important where a read/write transaction executes a query more than once.
However, SERIALIZABLE mode requires applications to check for the "can't serialize
access" error, and can significantly reduce throughput in an environment with many
concurrent transactions accessing the same data for update. Application logic that
checks database consistency must take into account the fact that reads do not block
writes in either mode.

Application Tips for Transactions
When a transaction runs in serializable mode, any attempt to change data that was
changed by another transaction since the beginning of the serializable transaction
causes an error:
ORA-08177: Can't serialize access for this transaction.

When you get this error, roll back the current transaction and execute it again. The
transaction gets a new transaction snapshot, and the operation is likely to succeed.
To minimize the performance overhead of rolling back transactions and executing
them again, try to put DML statements that might conflict with other concurrent
transactions near the beginning of your transaction.

Autonomous Transactions
This section gives a brief overview of autonomous transactions and what you can do
with them.
See Also: Oracle Database PL/SQL User's Guide and Reference and
Chapter 9, "Coding Triggers" for detailed information on autonomous
transactions

2-20 Oracle Database Application Developer’s Guide - Fundamentals

Autonomous Transactions

At times, you may want to commit or roll back some changes to a table independently
of a primary transaction's final outcome. For example, in a stock purchase transaction,
you may want to commit a customer's information regardless of whether the overall
stock purchase actually goes through. Or, while running that same transaction, you
may want to log error messages to a debug table even if the overall transaction rolls
back. Autonomous transactions allow you to do such tasks.
An autonomous transaction (AT) is an independent transaction started by another
transaction, the main transaction (MT). It lets you suspend the main transaction, do
SQL operations, commit or roll back those operations, then resume the main
transaction.
An autonomous transaction executes within an autonomous scope. An autonomous
scope is a routine you mark with the pragma (compiler directive)
AUTONOMOUS_TRANSACTION. The pragma instructs the PL/SQL compiler to mark a
routine as autonomous (independent). In this context, the term routine includes:
■

Top-level (not nested) anonymous PL/SQL blocks

■

Local, standalone, and packaged functions and procedures

■

Methods of a SQL object type

■

PL/SQL triggers

Figure 2–3 shows how control flows from the main routine (MT) to an autonomous
routine (AT) and back again. As you can see, the autonomous routine can commit
more than one transaction (AT1 and AT2) before control returns to the main routine.
Figure 2–3 Transaction Control Flow

Main Routine

Autonomous Routine

PROCEDURE proc1 IS
emp_id NUMBER;
BEGIN
emp_id := 7788;
INSERT ...
SELECT ...
proc2;
DELETE ...
COMMIT;
END;

PROCEDURE proc2 IS
PRAGMA AUTON...
dept_id NUMBER;
BEGIN
dept_id := 20;
UPDATE ...
INSERT ...
UPDATE ...
COMMIT;
INSERT ...
INSERT ...
COMMIT;
END;

MT begins

MT ends

MT suspends
AT1 begins

AT1 ends
AT2 begins
AT2 ends
MT resumes

When you enter the executable section of an autonomous routine,
the main routine suspends. When you exit the routine, the main
routine resumes.
COMMIT and ROLLBACK end the active autonomous transaction but do not exit the
autonomous routine. As Figure 2–3 shows, when one transaction ends, the next SQL
statement begins another transaction.
A few more characteristics of autonomous transactions:
■

The changes autonomous transactions effect do not depend on the state or the
eventual disposition of the main transaction. For example:
–

An autonomous transaction does not see any changes made by the main
transaction.

SQL Processing for Application Developers 2-21

Autonomous Transactions

–
■

■

When an autonomous transaction commits or rolls back, it does not affect the
outcome of the main transaction.

The changes an autonomous transaction effects are visible to other transactions as
soon as that autonomous transaction commits. This means that users can access
the updated information without having to wait for the main transaction to
commit.
Autonomous transactions can start other autonomous transactions.

Figure 2–4 illustrates some of the possible sequences autonomous transactions can
follow.
Figure 2–4 Possible Sequences of Autonomous Transactions

A main transaction scope
(MT Scope) begins the main
transaction, MTx. MTx
invokes the first autonomous
transaction scope (AT
Scope1). MTx suspends. AT
Scope 1 begins the
transaction Tx1.1.
At Scope 1 commits or rolls
back Tx1.1, than ends. MTx
resumes.

MT Scope

AT Scope 1

AT Scope 2

AT Scope 3

AT Scope 4

MTx
Tx1.1
MTx

MTx invokes AT Scope 2. MT
suspends, passing control to
AT Scope 2 which, initially, is
performing queries.
AT Scope 2 then begins
Tx2.1 by, say, doing an
update. AT Scope 2 commits
or rolls back Tx2.1.

Tx2.1

Later, AT Scope 2 begins a
second transaction, Tx2.2,
then commits or rolls it back.
AT Scope 2 performs a few
queries, then ends, passing
control back to MTx.

Tx2.2

MTx

MTx invokes AT Scope 3.
MTx suspends, AT Scope 3
begins.

Tx3.1

AT Scope 3 begins Tx3.1
which, in turn, invokes AT
Scope 4. Tx3.1 suspends, AT
Scope 4 begins.

Tx4.1

AT Scope 4 begins Tx4.1,
commits or rolls it back, then
ends. AT Scope 3 resumes.
AT Scope 3 commits or rolls
back Tx3.1, then ends. MTx
resumes.
Finally, MT Scope commits or
rolls back MTx, then ends.

Tx3.1
MTx

Examples of Autonomous Transactions
The two examples in this section illustrate some of the ways you can use autonomous
transactions.

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Autonomous Transactions

As these examples illustrate, there are four possible outcomes that can occur when you
use autonomous and main transactions. Table 2–7 presents these possible outcomes.
As you can see, there is no dependency between the outcome of an autonomous
transaction and that of a main transaction.
Table 2–7

Possible Transaction Outcomes

Autonomous Transaction Main Transaction
Commits

Commits

Commits

Rolls back

Rolls back

Commits

Rolls back

Rolls back

Entering a Buy Order
In this example, illustrated by Figure 2–5, a customer enters a buy order. That
customer's information (such as name, address, phone) is committed to a customer
information table—even though the sale does not go through.
Figure 2–5 Example: A Buy Order
MT Scope begins the main
transaction, MTx inserts the
buy order into a table.

MT Scope

MTx invokes the autonomous
transaction scope (AT
Scope). When AT Scope
begins, MT Scope suspends.

AT Scope

ATx

ATx, updates the audit table
with customer information.
MTx seeks to validate the
order, finds that the selected
item is unavailable, and
therefore rolls back the main
transaction.

MTx

Example: Making a Bank Withdrawal
In this example, a customer tries to make a withdrawal from a bank account. In the
process, a main transaction calls one of two autonomous transaction scopes (AT Scope
1, and AT Scope 2).
The following diagrams illustrate three possible scenarios for this transaction.
■

■

■

Scenario 1: There are sufficient funds to cover the withdrawal and therefore the
bank releases the funds
Scenario 2: There are insufficient funds to cover the withdrawal, but the customer
has overdraft protection. The bank therefore releases the funds.
Scenario 3: There are insufficient funds to cover the withdrawal, the customer does
not have overdraft protection, and the bank therefore withholds the requested
funds.

Scenario 1
There are sufficient funds to cover the withdrawal and therefore the bank releases the
funds. This is illustrated by Figure 2–6.
SQL Processing for Application Developers 2-23

Autonomous Transactions

Figure 2–6 Example: Bank Withdrawal—Sufficient Funds
MTx generates a
transaction ID.

MT Scope

AT Scope 1

AT Scope 2

MTx
Tx1.1 inserts the transaction
ID into the audit table and
commits.

MTx validates the balance on
the account.

Tx1.1

MTx

Tx2.1, updates the audit table
using the transaction ID
generated above, then
commits.

MTx releases the funds. MT
Scope ends.

Tx2.1

MTx

Scenario 2
There are insufficient funds to cover the withdrawal, but the customer has overdraft
protection. The bank therefore releases the funds. This is illustrated by Figure 2–7.

2-24 Oracle Database Application Developer’s Guide - Fundamentals

Autonomous Transactions

Figure 2–7 Example: Bank Withdrawal—Insufficient Funds WITH Overdraft Protection

MT Scope

AT Scope 1

AT Scope 2

MTx
Tx1.1

MTx discovers that there are
insufficient funds to cover the
withdrawal. It finds that the
customer has overdraft
protection and sets a flag to
the appropriate value.

MTx

Tx2.1, updates the
audit table.

MTx, releases the funds. MT
Scope ends.

Tx2.1

MTx

Scenario 3
There are insufficient funds to cover the withdrawal, the customer does not have
overdraft protection, and the bank therefore withholds the requested funds. This is
illustrated by Figure 2–8.

SQL Processing for Application Developers 2-25

Autonomous Transactions

Figure 2–8 Example: Bank Withdrawal—Insufficient Funds WITHOUT Overdraft
Protection

MT Scope

AT Scope 1

AT Scope 2

MTx
Tx1.1

MTx discovers that there are
insufficient funds to cover the
withdrawal. It finds that the
customer does not have
overdraft protection and sets
a flag to the appropriate
value.

MTx

Tx2.1, updates the
audit table.

MTx Scope rolls back MTx,
denying the release of funds.
MT Scope ends.

Tx2.1

MTx

Defining Autonomous Transactions
This section is provided here to round out your general
understanding of autonomous transactions. For a more thorough
understanding of autonomous transactions, refer to Oracle Database
PL/SQL User's Guide and Reference.

Note:

To define autonomous transactions, you use the pragma (compiler directive)
AUTONOMOUS_TRANSACTION. The pragma instructs the PL/SQL compiler to mark the
procedure, function, or PL/SQL block as autonomous (independent).
You can code the pragma anywhere in the declarative section of a procedure, function,
or PL/SQL block. But, for readability, code the pragma at the top of the section. The
syntax follows:
PRAGMA AUTONOMOUS_TRANSACTION;

In the following example, you mark a packaged function as autonomous:
CREATE OR REPLACE PACKAGE Banking AS
FUNCTION Balance (Acct_id INTEGER) RETURN REAL;
-- add additional functions and packages
END Banking;
CREATE OR REPLACE PACKAGE BODY Banking AS
FUNCTION Balance (Acct_id INTEGER) RETURN REAL IS
PRAGMA AUTONOMOUS_TRANSACTION;
My_bal REAL;
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Resuming Execution After a Storage Error Condition

BEGIN
--add appropriate code
END;
-- add additional functions and packages...
END Banking;

Restrictions on Autonomous Transactions
Note the following restrictions on autonomous transactions.
■

You cannot use the pragma to mark all subprograms in a package (or all methods
in an object type) as autonomous. Only individual routines can be marked
autonomous. For example, the following pragma is illegal:
CREATE OR REPLACE PACKAGE Banking AS
PRAGMA AUTONOMOUS_TRANSACTION; -- illegal
FUNCTION Balance (Acct_id INTEGER) RETURN REAL;
END Banking;

■

You cannot execute a PIPE ROW statement in your autonomous routine while
your autonomous transaction is open. You must close the autonomous transaction
before executing the PIPE ROW statement. This is normally accomplished by
committing or rolling back the autonomous transaction before executing the PIPE
ROW statement.
See Also:

Oracle Database PL/SQL User's Guide and Reference

Resuming Execution After a Storage Error Condition
When a long-running transaction is interrupted by an out-of-space error condition,
your application can suspend the statement that encountered the problem and resume
it after the space problem is corrected. This capability is known as resumable storage
allocation. It lets you avoid time-consuming rollbacks, without the need to split the
operation into smaller pieces and write your own code to track its progress.
See Also:
■

Oracle Database Concepts

■

Oracle Database Administrator's Guide

What Operations Can Be Resumed After an Error Condition?
Queries, DML operations, and certain DDL operations can all be resumed if they
encounter an out-of-space error. The capability applies if the operation is performed
directly by a SQL statement, or if it is performed within a stored procedure,
anonymous PL/SQL block, SQL*Loader, or an OCI call such as OCIStmtExecute().
Operations can be resumed after these kinds of error conditions:
■

Out of space errors, such as ORA-01653.

■

Space limit errors, such as ORA-01628.

■

Space quota errors, such as ORA-01536.

Limitations on Resuming Operations After an Error Condition
Certain storage errors cannot be handled using this technique. In dictionary-managed
tablespaces, you cannot resume an operation if you run into the limit for rollback
SQL Processing for Application Developers 2-27

Resuming Execution After a Storage Error Condition

segments, or the maximum number of extents while creating an index or a table. Use
locally managed tablespaces and automatic undo management in combination with
this feature.

Writing an Application to Handle Suspended Storage Allocation
When an operation is suspended, your application does not receive the usual error
code. Instead, perform any logging or notification by coding a trigger to detect the
AFTER SUSPEND event and call the functions in the DBMS_RESUMABLE package to get
information about the problem. Using this package, you can:
■

■

Parse the error message with the DBMS_RESUMABLE.SPACE_ERROR_INFO
function. For details about this function, refer to Oracle Database PL/SQL Packages
and Types Reference.
Set a new timeout value with the SET_TIMEOUT procedure.

Within the body of the trigger, you can perform any notifications, such as sending a
mail message to alert an operator to the space problem.
Alternatively, the DBA can periodically check for suspended statements using the data
dictionary views DBA_RESUMABLE, USER_RESUMABLE, and V$_SESSION_WAIT.
When the space condition is corrected (usually by the DBA), the suspended statement
automatically resumes execution. If it is not corrected before the timeout period
expires, the operation causes a SERVERERROR exception.
To reduce the chance of out-of-space errors within the trigger itself, you must declare it
as an autonomous transaction so that it uses a rollback segment in the SYSTEM
tablespace. If the trigger encounters a deadlock condition because of locks held by the
suspended statement, the trigger is aborted and your application receives the original
error condition, as if it was never suspended. If the trigger encounters an out-of-space
condition, the trigger and the suspended statement are rolled back. You can prevent
the rollback through an exception handler in the trigger, and just wait for the
statement to be resumed.
See Also: Oracle Database Reference for details on the
DBA_RESUMABLE, USER_RESUMABLE, and V$_SESSION_WAIT data
dictionary views

Example of Resumable Storage Allocation
This trigger handles applicable storage errors within the database. For some kinds of
errors, it aborts the statement and alerts the DBA that this has happened through a
mail message. For other errors that might be temporary, it specifies that the statement
should wait for eight hours before resuming, with the expectation that the storage
problem will be fixed by then.
CREATE OR REPLACE TRIGGER suspend_example
AFTER SUSPEND
ON DATABASE
DECLARE
cur_sid NUMBER;
cur_inst NUMBER;
err_type VARCHAR2(64);
object_owner VARCHAR2(64);
object_type VARCHAR2(64);
table_space_name VARCHAR2(64);
object_name VARCHAR2(64);
sub_object_name VARCHAR2(64);

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Resuming Execution After a Storage Error Condition

msg_body VARCHAR2(64);
ret_value boolean;
error_txt varchar2(64);
mail_conn utl_smtp.connection;
BEGIN
SELECT DISTINCT(sid) INTO cur_sid FROM v$mystat;
cur_inst := userenv('instance');
ret_value := dbms_resumable.space_error_info(err_type, object_owner,
object_type, table_space_name, object_name, sub_object_name);
IF object_type = 'ROLLBACK SEGMENT' THEN
INSERT INTO sys.rbs_error ( SELECT sql_text, error_msg, suspend_time
FROM dba_resumable WHERE session_id = cur_sid AND instance_id = cur_inst);
SELECT error_msg into error_txt FROM dba_resumable WHERE session_id = cur_sid
AND instance_id = cur_inst;
msg_body := 'Subject: Space error occurred: Space limit reached for rollback
segment '|| object_name || ' on ' || to_char(SYSDATE, 'Month dd, YYYY, HH:MIam')
|| '. Error message was: ' || error_txt;
mail_conn := utl_smtp.open_connection('localhost', 25);
utl_smtp.helo(mail_conn, 'localhost');
utl_smtp.mail(mail_conn, 'sender@localhost');
utl_smtp.rcpt(mail_conn, 'recipient@localhost');
utl_smtp.data(mail_conn, msg_body);
utl_smtp.quit(mail_conn);
dbms_resumable.abort(cur_sid);
ELSE
dbms_resumable.set_timeout(3600*8);
END IF;
COMMIT;
END;

SQL Processing for Application Developers 2-29

Resuming Execution After a Storage Error Condition

2-30 Oracle Database Application Developer’s Guide - Fundamentals

3
Using SQL Datatypes in Application
Development
This chapter discusses how to use SQL datatypes in database applications. Topics
include the following:
■

Representing Data with SQL Datatypes: Overview

■

Representing Character Data

■

Representing Numeric Data

■

Representing Datetime Data

■

Representing Specialized Data

■

Representing Conditional Expressions as Data

■

Identifying Rows by Address

■

How Oracle Database Converts Datatypes
See Also:
■

■

■

Oracle Database Application Developer's Guide - Object-Relational
Features for information about more complex types, such as object
types, varrays, and nested tables
Oracle Database Application Developer's Guide - Large Objects for
information about LOB datatypes
Oracle Database PL/SQL User's Guide and Reference to learn about
the PL/SQL datatypes. Many SQL datatypes are the same or
similar in PL/SQL.

Representing Data with SQL Datatypes: Overview
A datatype associates a fixed set of properties with the values that can be used in a
column of a table or in an argument of a procedure or function. These properties cause
Oracle Database to treat values of one datatype differently from values of another
datatype. For example, Oracle Database can add values of NUMBER datatype, but not
values of RAW datatype.
Oracle Database provides a number of built-in datatypes as well as several categories
for user-defined types that can be used as datatypes. The datatypes supported by
Oracle Database can be divided into the following categories:

Using SQL Datatypes in Application Development

3-1

Representing Character Data

■

■

■

■

Oracle built-in datatypes, which include datatypes for characters, numbers, dates
and times (known as datetime datatypes), raw data, large objects (LOBs), and row
addresses (ROWIDs).
ANSI datatypes and datatypes from the IBM products SQL/DS and DB2, which
are usable in SQL statements that create tables and clusters
User-defined types, which use Oracle built-in datatypes and other user-defined
datatypes as the building blocks of object types that model the structure and
behavior of data in applications
Oracle-supplied types, which are SQL-based interfaces for defining new types

The Oracle precompilers recognize other datatypes in embedded SQL programs. These
datatypes are called external datatypes and are associated with host variables. You
should not confuse Oracle Database built-in datatypes and user-defined types with
external datatypes.
See Also:
■

■

■

Oracle Database SQL Reference for complete reference information
on the SQL datatypes
Pro*COBOL Programmer's Guide and Pro*C/C++ Programmer's
Guide for information on external datatypes, including how Oracle
converts between them and built-in or user-defined types
Oracle Database Concepts to learn about Oracle built-in datatypes

Representing Character Data
This section contains the following topics:
■

Representing Character Data: Overview

■

Specifying Column Lengths as Bytes or Characters

■

Choosing Between the CHAR and VARCHAR2 Datatypes

■

Using Character Literals in SQL Statements

Representing Character Data: Overview
You can use the following SQL datatypes to store alphanumeric data:
■

CHAR and NCHAR datatypes store fixed-length character literals.

■

VARCHAR2 and NVARCHAR2 datatypes store variable-length character literals.

■

NCHAR and NVARCHAR2 datatypes store Unicode character data only.

■

■

CLOB and NCLOB datatypes store single-byte and multibyte character strings of up
to (4 gigabytes - 1) * (the value obtained from DBMS_LOB.GETCHUNKSIZE).
The LONG datatype stores variable-length character strings containing up to two
gigabytes, but with many restrictions. This datatype is provided only for
backward compatibility with existing applications. In general, new applications
should use CLOB and NCLOB datatypes to store large amounts of character data,
and BLOB and BFILE to store large amounts of binary data.

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Representing Character Data

See Also:
■

■

Oracle Database Application Developer's Guide - Large Objects for
information on LOB datatypes (including CLOB and NCLOB
datatypes) and migration from LONG to LOB datatypes
Oracle Database SQL Reference for restrictions on LONG datatypes

Specifying Column Lengths as Bytes or Characters
You can specify the lengths of CHAR and VARCHAR2 columns as either bytes or
characters. The lengths of NCHAR and NVARCHAR2 columns are always specified in
characters, making them ideal for storing Unicode data, where a character might
consist of multiple bytes.
Consider the following list of column length specifications:
■

id VARCHAR2(32 BYTE)
The id column contains only single-byte data, up to 32 bytes.

■

name VARCHAR2(32 CHAR)
The name column contains data in the database character set. If the database
character set allows multibyte characters, then the 32 characters can be stored as
more than 32 bytes.

■

biography NVARCHAR2(2000)
The biography column can represent 2000 characters in any
Unicode-representable language. The encoding depends on the national character
set, but the column can contain multibyte values even if the database character set
is single-byte.

■

comment VARCHAR2(2000)
The representation of comment as 2000 bytes or characters depends on the
initialization parameter NLS_LENGTH_SEMANTICS.

When using a multibyte database character encoding scheme, consider carefully the
space required for tables with character columns. If the database character encoding
scheme is single-byte, then the number of bytes and the number of characters in a
column is the same. If it is multibyte, however, then there generally is no such
correspondence. A character might consist of one or more bytes, depending upon the
specific multibyte encoding scheme and whether shift-in/shift-out control codes are
present. To avoid overflowing buffers, specify data as NCHAR or NVARCHAR2 if it might
use a Unicode encoding that is different from the database character set.
See Also:
■

Oracle Database Globalization Support Guide

■

Oracle Database SQL Reference

Choosing Between the CHAR and VARCHAR2 Datatypes
When deciding which datatype to use for a column that will store alphanumeric data
in a table, consider the following points of distinction:
■

Space usage
To store data more efficiently, use the VARCHAR2 datatype. The CHAR datatype
blank-pads and stores trailing blanks up to a fixed column length for all column
values, whereas the VARCHAR2 datatype does not add extra blanks.
Using SQL Datatypes in Application Development

3-3

Representing Character Data

■

Comparison semantics
Use the CHAR datatype when you require ANSI compatibility in comparison
semantics (when trailing blanks are not important in string comparisons). Use the
VARCHAR2 when trailing blanks are important in string comparisons.

■

Future compatibility
The CHAR and VARCHAR2 datatypes are fully supported. At this time, the
VARCHAR datatype automatically corresponds to the VARCHAR2 datatype and is
reserved for future use.

When an application interfaces with Oracle Database, there is a character set on the
client and server side. Oracle Database uses the NLS_LANGUAGE parameter to
automatically convert CHAR, VARCHAR2, and LONG data from the database character
set to the character set defined for the user session, if these are different.
In the section "Datatype Comparison Rules," Oracle Database SQL Reference explains the
comparison semantics that Oracle Database uses to compare character data. Because
Oracle Database blank-pads values stored in CHAR columns but not in VARCHAR2
columns, a value stored in a VARCHAR2 column can take up less space than the same
value in a CHAR column. For this reason, a full table scan on a large table containing
VARCHAR2 columns may read fewer data blocks than a full table scan on a table
containing the same data stored in CHAR columns. If your application often performs
full table scans on large tables containing character data, then you may be able to
improve performance by storing data in VARCHAR2 rather than in CHAR columns.
Performance is not the only factor to consider when deciding which datatype to use.
Oracle Database uses different semantics to compare values of each datatype. You
might choose one datatype over the other if your application is sensitive to the
differences between these semantics. For example, if you want Oracle Database to
ignore trailing blanks when comparing character values, then you must store these
values in CHAR columns.
Oracle Database SQL Reference for more information on
comparison semantics for these datatypes

See Also:

Using Character Literals in SQL Statements
Many SQL statements, functions, expressions, and conditions require you to specify
character literal values. You can specify character literals with the following notations:
■

■

Character literals with the 'text' notation, as in the literals 'users01.dbf'
and 'Muthu''s computer'.
National character literals with the N'text' or n'text' notation, where N or n
specifies the literal using the national character set. For example, N'résumé' is a
National character literal.
Oracle Database translates N-quoted text into the national character set by way of
the database character set. If client-side characters do not have corresponding
encoding in the database character set, then Oracle Database converts them into
question marks. To avoid the potential loss of data during the text literal
conversion, set the environment variable $ORA_NCHAR_LITERAL_REPLACE to
TRUE. This setting transparently replaces the N'text' internally and preserves
the text literal for SQL processing.

The UNISTR function provides support for Unicode character literals by enabling you
to specify the Unicode encoding value of characters in the string, as in
UNISTR('\1234'). This technique is useful, for example, when inserting data into

3-4 Oracle Database Application Developer’s Guide - Fundamentals

Representing Numeric Data

NCHAR columns. Because every character has a corresponding Unicode encoding, the
client application can safely send character data to the server without data loss.

Quoting Character Literals
By default you must quote character literals in single-quotes, as in 'Hello'. This
technique can sometimes be inconvenient if the text itself contains single quotes. In
such cases, you can also use the Q-quote mechanism, which enables you to specify q
or Q followed by a single quote and then another character to be used as the quote
delimiter. For example, the literal q'#it's the "final" deadline#' uses the
pound sign (#) as a quote delimiter for the string it's the "final" deadline.
The Q-quote delimiter can be any single- or multibyte character except space, tab, and
return. If the opening quote delimiter is a [, {, <, or ( character, then the closing quote
delimiter must be the corresponding ], }, >, or ) character. In all other cases, the
opening and closing delimiter must be the identical character.
The following character literals use the alternative quoting mechanism:
q'(name LIKE '%DBMS_%%')'
q'<'Data,' he said, 'Make it so.'>'
q'"name like '['"'
nq'ïŸ1234ï'

See Also:
■

■

Oracle Database Globalization Support Guide to learn about national
character sets
Oracle Database SQL Reference to learn about character literals

Representing Numeric Data
This section contains the following topics:
■

What Are the Numeric Datatypes?

■

Using Floating-Point Number Formats

■

Using Comparison Operators for Native Floating-Point Datatypes

■

Performing Arithmetic Operations with Native Floating-Point Datatypes

■

Using Conversion Functions with Native Floating-Point Datatypes

■

Client Interfaces for Native Floating-Point Datatypes

What Are the Numeric Datatypes?
The following SQL datatypes store numeric data:
■

NUMBER

■

BINARY_FLOAT

■

BINARY_DOUBLE

Use the NUMBER datatype to store real numbers in a fixed-point or floating-point
format. Numbers using this datatype are guaranteed to be portable among different
Oracle Database platforms, and offer up to 38 decimal digits of precision. You can store
positive and negative numbers of magnitude 1 x 10-130 through 9.99 x10125, as well as
zero, in a NUMBER column.

Using SQL Datatypes in Application Development

3-5

Representing Numeric Data

The BINARY_FLOAT and BINARY_DOUBLE datatypes store floating-point data in the
32-bit IEEE 754 format and the double precision 64-bit IEEE 754 format respectively.
Compared to the Oracle NUMBER datatype, arithmetic operations on floating-point
data are usually faster for BINARY_FLOAT and BINARY_DOUBLE. Also, high-precision
values require less space when stored as BINARY_FLOAT and BINARY_DOUBLE.
In client interfaces supported by Oracle Database, the native instruction set supplied
by the hardware vendor performs arithmetic operations on BINARY_FLOAT and
BINARY_DOUBLE datatypes. The term native floating-point datatypes refers to
datatypes including BINARY_FLOAT and BINARY_DOUBLE and to all implementations
of these types in supported client interfaces.
The floating-point number system is a common way of representing and manipulating
numeric values in computer systems. A floating-point number is characterized by the
following components:
■

Binary-valued sign

■

Signed exponent

■

Significand

■

Base

A floating-point value is the signed product of its significand and the base raised to
the power of its exponent, as shown in the formula in Example 3–1.
Example 3–1 Components of a Floating-Point Number
(-1)sign

.

significand

.

base

exponent

For example, the number 4.31 can be represented in the following expression:
(-1)0

.

431

.

10

-2

The components of the preceding expression are as follows:
■

0 is the binary-valued sign

■

431 is the significant

■

10 is the base

■

-2 is the exponent
See Also:
■

Oracle Database Concepts for information about the internal format
for the NUMBER datatype
Oracle Database SQL Reference for more information about the
BINARY_FLOAT and BINARY_DOUBLE datatypes formats

Using Floating-Point Number Formats
A floating-point number format specifies how components of a floating-point number
are represented. The choice of representation determines the range and precision of the
values the format can represent. By definition, the range is the interval bounded by the
smallest and the largest values the format can represent and the precision is the
number of digits in the significand.
Formats for floating-point values support neither infinite precision nor infinite range.
There are a finite number of bits to represent a number and only a finite number of

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Representing Numeric Data

values that a format can represent. A floating-point number that uses more precision
than available with a given format is rounded.
A floating-point number can be represented in a binary system (one that uses base 2),
as in the IEEE 754 standard, or in a decimal system (one that uses base 10), such as
Oracle NUMBER. The base affects many properties of the format, including how a
numeric value is rounded.
For a decimal floating-point number format like Oracle NUMBER, rounding is done to
the nearest decimal place (for example. 1000, 10, or 0.01). The IEEE 754 formats use a
binary format for floating-point values and round numbers to the nearest binary place
(for example: 1024, 512, or 1/64).
The native floating-point datatypes supported by the database round to the nearest
binary place, so they are not satisfactory for applications that require decimal
rounding. Use the Oracle NUMBER datatype for applications in which decimal
rounding is required on floating-point data.

Using a Floating-Point Binary Format
The value of a floating-point number that uses a binary format is determined by the
formula in Example 3–2.
Example 3–2 Components of a Floating-Point Number in Binary Format
(-1)s 2E (b0 b1 b2 ... bp-1)

Table 3–1 describes the components of the formula.
Table 3–1

Components of the Binary Format for Floating-Point Numbers

Component

Specifies . . .

s

0 or 1

E

Any integer between Emin and Emax, inclusive (see Table 3–2)

bi

0 or 1, where the sequence of bits represents a number in base 2 (see Table 3–2)

The leading bit of the significand, b0, must be set (1), except for subnormal numbers
(explained later). Consequently, the leading bit is not actually stored, so the formats
provide N bits of precision although only N-1 bits are stored.
The IEEE 754 specification also defines extended
single-precision and extended double-precision formats, which are not
supported by Oracle Database.

Note:

The parameters for these formats are described in Table 3–2.
Table 3–2

Summary of Binary Format Parameters

Parameter

Single-precision (32-bit)

Double-precision (64-bit)

p

24

53

Emin

-126

-1022

Emax

+127

+1023

Using SQL Datatypes in Application Development

3-7

Representing Numeric Data

The storage parameters for the formats are described in Table 3–3. The in-memory
formats for single-precision and double-precision datatypes are specified by IEEE 754.
Table 3–3

Summary of Binary Format Storage Parameters

Datatype

Sign bits

Exponent bits

Significand bits

Total bits

Single-precision

1

8

24 (23 stored)

32

Double-precision

1

11

53 (52 stored)

64

A significand is normalized when the leading bit of the significand is set. IEEE 754
defines denormal or subnormal values as numbers that are too small to be
represented with an implied leading set bit in the significand. The number is too small
because its exponent would be too large if its significand were normalized to have an
implied leading bit set. IEEE 754 formats support subnormal values. Subnormal values
preserve the following property:
if: x - y == 0.0 (using floating-point subtraction)
then: x == y
Table 3–4 shows the range and precision of the required formats in the IEEE 754
standard and those of Oracle NUMBER. Range limits are expressed here in terms of
positive numbers; they also apply to the absolute value of a negative number. (The
notation "number e exponent" used here stands for number multiplied by 10 raised to the
exponent power: number . 10 exponent.)
Table 3–4

Range and Precision of IEEE 754 formats

Range and
Precision

Single-precision
32-bit1

Double-precision
64-bit1

Max positive normal
number

3.40282347e+38

1.7976931348623157e+308 < 1.0e126

Min positive normal
number

1.17549435e-38

2.2250738585072014e-308 1.0e-130

Max positive
subnormal number

1.17549421e-38

2.2250738585072009e-308 not applicable

Min positive
subnormal number

1.40129846e-45

4.9406564584124654e-324 not applicable

Precision (decimal
digits)

6-9

15 - 17

1

Oracle NUMBER
Datatype

38 - 40

These numbers are quoted from the IEEE Numerical Computation Guide.

See Also:
■

■

Oracle Database SQL Reference, section "Numeric Literals", for
information about literal representation of numeric values
Oracle Database SQL Reference for more information about
floating-point formats

Representing Special Values with Native Floating-Point Formats
IEEE 754 allows special values to be represented. These special values are as follows:
■

Positive infinity (+INF)

■

Negative infinity (-INF)

3-8 Oracle Database Application Developer’s Guide - Fundamentals

Representing Numeric Data

■

Not-a-number (NaN)

■

Positive zero (+0)

■

Negative zero (-0)

NaN represent results of operations that are undefined. Many bit patterns in IEEE 754
represent NaN. Bit patterns can represent NaN with and without the sign bit set. IEEE
754 distinguishes between signalling NaNs and quiet NaNs.
IEEE 754 specifies behavior for when exceptions are enabled and disabled. Oracle
Database does not allow exceptions to be enabled; the database behavior is that
specified by IEEE 754 for when exceptions are disabled. In particular, Oracle Database
makes no distinction between signalling and quiet NaNs. Programmers who use OCI
can retrieve NaN values from Oracle Database; whether a retrieved NaN value is
signalling or quiet depends on the client platform and beyond the control of Oracle
Database.
IEEE 754 does not define the bit pattern for either type of NaN. Positive infinity,
negative infinity, positive zero, and negative zero are each represented by a specific bit
pattern.
Ignoring signs, there are the following classes of values, with each of the classes except
for NaN greater than the one preceding it in the list:
■

Zero

■

Subnormal

■

Normal

■

Infinity

■

NaN

In IEEE 754, NaN is unordered with other classes of special values and with itself.
Behavior of Special Values for Native Floating-Point Datatypes When used with the database,
special values of native floating-point datatypes behave as follows:
■

All NaNs are quiet.

■

IEEE 754 exceptions are not raised.

■

NaN is ordered as follows:
All non-NaN < NaN
Any NaN == any other NaN

■

-0 is converted to +0.

■

All NaNs are converted to the same bit pattern.
See Also: "Using Comparison Operators for Native Floating-Point
Datatypes" on page 3-9 for more information on NaN compared to
other values

Using Comparison Operators for Native Floating-Point Datatypes
Oracle Database defines the following comparison operators for operations involving
floating-point datatypes:
■

Equal to

■

Not equal to

Using SQL Datatypes in Application Development

3-9

Representing Numeric Data

■

Greater than

■

Greater than or equal to

■

Less than

■

Less than or equal to

■

Unordered

Note the following special cases:
■
■

Comparisons ignore the sign of zero (-0 is equal to, not less than, +0).
In Oracle Database, NaN is equal to itself. NaN is greater than everything except
itself. That is, NaN == NaN and NaN > x, unless x is NaN.
See Also: "Behavior of Special Values for Native Floating-Point
Datatypes" on page 3-9 for more information on comparison results,
ordering, and other behaviors of special values

Performing Arithmetic Operations with Native Floating-Point Datatypes
Oracle Database defines operators for the following arithmetic operations:
■

Multiplication

■

Division

■

Addition

■

Subtraction

■

Remainder

■

Square root

You can define the mode used to round the result of the operation. Exceptions can be
raised when operations are performed. Exceptions can also be disabled.
Formerly, Java required floating-point arithmetic to be exactly reproducible. IEEE 754
does not require such behavior. The standard allows for the result of operations,
including arithmetic, to be delivered to a destination that uses a range greater than
that used by the operands to the operation.
You can compute the result of a double-precision multiplication at an extended
double-precision destination. When this is done, the result must be rounded as if the
destination were single-precision or double-precision. The range of the result, that is,
the number of bits used for the exponent, can use the range supported by the wider
(extended double-precision) destination. This occurrence may result in a
double-rounding error in which the least significant bit of the result is incorrect.
This state of affairs can only occur for double-precision multiplication and division on
hardware that implements the IA-32 and IA-64 instruction set architecture. Thus, with
the exception of this case, arithmetic for these datatypes is reproducible across
platforms. When the result of a computation is NaN, all platforms produce a value for
which IS NAN is true. However, all platforms do not have to use the same bit pattern.

Using Conversion Functions with Native Floating-Point Datatypes
Oracle Database defines functions that convert between floating-point and other
formats, including string formats that use decimal precision (precision may be lost
during the conversion). For example, you can use the following functions:

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Representing Numeric Data

■

■

TO_BINARY_DOUBLE, which converts float to double, decimal (string) to double,
and float or double to integer-valued double
TO_BINARY_FLOAT, which converts double to float, decimal (string) to float, and
float or double to integer-valued float

■

TO_CHAR, which converts float or double to decimal (string)

■

TO_NUMBER, which converts a float, double, or string to a number

Oracle Database can raise exceptions during conversion. The IEEE 754 specification
defines the following exceptions:
■

Invalid

■

Inexact

■

Divide by zero

■

Underflow

■

Overflow

Oracle Database does not raise these exceptions for native floating-point datatypes.
Generally, situations that would raise an exception produce the values described in
Table 3–5.
Table 3–5

Values Resulting from Exceptions

Exception

Value

Underflow

0

Overflow

-INF, +INF

Invalid Operation

NaN

Divide by Zero

-INF, +INF, NaN

Inexact

Any value – rounding was performed

Client Interfaces for Native Floating-Point Datatypes
Oracle Database has implemented support for native floating-point datatypes in the
following client interfaces:
■

SQL

■

PL/SQL

■

OCI and OCCI

■

Pro*C/C++

■

JDBC

OCI Native Floating-Point Datatypes SQLT_BFLOAT and SQLT_BDOUBLE
The OCI API implements the IEEE 754 single precision and double precision native
floating-point datatypes with the datatypes SQLT_BFLOAT and SQLT_BDOUBLE
respectively. Conversions between these types and the SQL types BINARY_FLOAT and
BINARY_DOUBLE are exact on platforms that implement the IEEE 754 standard for the
C datatypes FLOAT and DOUBLE.
See Also:

Oracle Call Interface Programmer's Guide

Using SQL Datatypes in Application Development 3-11

Representing Datetime Data

Native Floating-Point Datatypes Supported in Oracle OBJECT Types
Oracle Database supports the SQL datatypes BINARY_FLOAT and BINARY_DOUBLE as
attributes of Oracle OBJECT types.

Pro*C/C++ Support for Native Floating-Point Datatypes
Pro*C/C++ supports the native FLOAT and DOUBLE datatypes using the column
datatypes BINARY_FLOAT and BINARY_DOUBLE. You can use these datatypes in the
same way that Oracle NUMBER datatype is used. You can bind the native C/C++
datatypes FLOAT and DOUBLE to BINARY_FLOAT and BINARY_DOUBLE types
respectively by setting the Pro*C/C++ precompiler command line option
NATIVE_TYPES to Y (yes) when you compile your application.

Representing Datetime Data
This section contains the following topics:
■

Representing Datetime Data: Overview

■

Manipulating the Date Format

■

Manipulating the Time Format

■

Performing Date Arithmetic

■

Converting Between Datetime Types

■

Importing and Exporting Datetime Types

Representing Datetime Data: Overview
Oracle Database supports the following datetime datatypes:
■

DATE

■

TIMESTAMP

■

TIMESTAMP WITH TIME ZONE

■

TIMESTAMP WITH LOCAL TIME ZONE

Using the DATE Datatype
Use the DATE datatype to store point-in-time values (dates and times) in a table. The
DATE datatype stores the century, year, month, day, hours, minutes, and seconds.

Using the TIMESTAMP Datatype
Use the TIMESTAMP datatype to store values that are precise to fractional seconds. For
example, an application that must decide which of two events occurred first might use
TIMESTAMP. An application that specifies the time for a job might use DATE.

Using the TIMESTAMP WITH TIME ZONE Datatype
Because TIMESTAMP WITH TIME ZONE can also store time zone information, it is
particularly suited for recording date information that must be gathered or
coordinated across geographic regions.

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Representing Datetime Data

Using the TIMESTAMP WITH LOCAL TIME ZONE Datatype
Use TIMESTAMP WITH LOCAL TIME ZONE when the time zone is not significant. For
example, you might use it in an application that schedules teleconferences, where
participants each see the start and end times for their own time zone.
The TIMESTAMP WITH LOCAL TIME ZONE type is appropriate for two-tier
applications in which you want to display dates and times that use the time zone of
the client system. It is generally inappropriate in three-tier applications because data
displayed in a Web browser is formatted according to the time zone of the Web server,
not the time zone of the browser. The Web server is the database client, so its local time
is used.

Representing the Difference Between Datetime Values
Use the INTERVAL DAY TO SECOND datatype to represent the precise difference
between two datetime values. For example, you might use this value to set a reminder
for a time 36 hours in the future or to record the time between the start and end of a
race. To represent long spans of time with high precision, you can use a large value for
the days portion.
Use the INTERVAL YEAR TO MONTH datatype to represent the difference between
two datetime values, where the only significant portions are the year and the month.
For example, you might use this value to set a reminder for a date 18 months in the
future, or check whether 6 months have elapsed since a particular date.
Oracle Database stores dates in its own internal format. Date data is stored in
fixed-length fields of seven bytes each, corresponding to century, year, month, day,
hour, minute, and second.
See Also: Oracle Call Interface Programmer's Guide for a complete
description of the Oracle Database internal date format

Manipulating the Date Format
For input and output of dates, the standard Oracle Database default date format is
DD-MON-RR. The RR datetime format element enables you store 20th century dates in
the 21st century by specifying only the last two digits of the year.
As explained in Oracle Database SQL Reference, the century of the return value varies
according to the specified two-digit year and the last two digits of the current year. For
example, the following format refers to the year 2004 in a query issued between 1950
and 2049, but to the year 2005 in a query issued between 2050 and 2099:
'13-NOV-04'

Changing the Default Date Format
Use the following techniques to change the default date format:
■

To change on an instance-wide basis, use the NLS_DATE_FORMAT parameter.

■

To change during a session, use the ALTER SESSION statement.

To enter dates that are not in the current default date format, use the TO_DATE
function with a format mask. For example:
SELECT TO_CHAR(TO_DATE('27-OCT-98', 'DD-MON-RR') ,'YYYY') "Year"
FROM DUAL;

Be careful when using a date format such as DD-MON-YY. The YY indicates the year in
the current century. For example, 31-DEC-92 is December 31, 2092, not 1992 as

Using SQL Datatypes in Application Development 3-13

Representing Datetime Data

you might expect. If you want to indicate years in any century other than the current
one, use a different format mask, such as the default RR.
Oracle Database Concepts for information about Julian
dates. Oracle Database Julian dates might not be compatible with
Julian dates generated by other date algorithms.

See Also:

Displaying the Current Date and Time
Use the SQL function SYSDATE to return the system date and time. You can use the
FIXED_DATE initialization parameter to set SYSDATE to a constant, which can be
useful for testing.
By default, SYSDATE is printed without any BC or AD qualifier. You can add BC to the
format string to print the date with BC or AD as appropriate:
SELECT TO_CHAR(SYSDATE, 'DD-MON-YYYY BC')
FROM DUAL;
TO_CHAR(SYSDAT
-------------24-JAN-2004 AD

Manipulating the Time Format
Time is stored in the following 24-hour format:
HH24:MI:SS

By default, the time in a DATE column is 12:00:00 A.M. (midnight) if no time portion is
entered or if the DATE is truncated.
In a time-only entry, the date portion defaults to the first day of the current month. To
enter the time portion of a date, use the TO_DATE function with a format mask
indicating the time portion, as shown in Example 3–3.
Example 3–3 Indicating Time with the TO_DATE Function
-- create test table
CREATE TABLE birthdays
( Bname VARCHAR2(20),
Bday DATE
);
-- insert a row
INSERT INTO birthdays (bname, bday)
VALUES
( 'ANNIE',
TO_DATE('13-NOV-92 10:56 A.M.','DD-MON-YY HH:MI A.M.')
);

Performing Date Arithmetic
Oracle Database provides a number of features to help with date arithmetic, so that
you do not need to perform your own calculations on the number of seconds in a day,
the number of days in each month, and so on. Some useful features include the
following:
■

ADD_MONTHS function, which returns the date plus the specified number of
months.

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Representing Datetime Data

■

■

■

■

■

■

SYSDATE function, which returns the current date and time set for the operating
system on which the database resides.
SYSTIMESTAMP function, which returns the system date, including fractional
seconds and time zone, of the system on which the database resides.
TRUNC function, which when applied to a DATE value, trims off the time portion
so that it represents the very beginning of the day (the stroke of midnight). By
truncating two DATE values and comparing them, you can determine whether
they refer to the same day. You can also use TRUNC along with a GROUP BY clause
to produce daily totals.
Arithmetic operators such as + and -. For example, SYSDATE-7 refers to 7 days
before the current system date.
INTERVAL datatypes, which enable you to represent constants when performing
date arithmetic rather than performing your own calculations. For example, you
can add or subtract INTERVAL constants from DATE values or subtract two DATE
values and compare the result to an INTERVAL.
Comparison operators such as >, <, =, and BETWEEN.

Converting Between Datetime Types
Oracle Database provides several useful functions that enable you to convert to a from
datetime datatypes. Some useful functions include:
■

■

■

EXTRACT, which extracts and returns the value of a specified datetime field from a
datetime or interval value expression
NUMTODSINTERVAL, which converts a NUMBER or expression that can be implicitly
converted to a NUMBER value to an INTERVAL DAY TO SECOND literal
NUMTOYMINTERVAL, which converts a NUMBER or expression that can be implicitly
converted to a NUMBER value to an INTERVAL YEAR TO MONTH literal

■

TO_DATE, which converts character data to a DATE datatype

■

TO_CHAR, which converts DATE data to character data

■

■
■

■

TO_DSINTERVAL, which converts a character string to an INTERVAL DAY TO
SECOND value
TO_TIMESTAMP, which converts character data to a value of TIMESTAMP datatype
TO_TIMESTAMP_TZ, which converts character data to a value of TIMESTAMP
WITH TIME ZONE datatype
TO_YMINTERVAL, which converts a character string to an INTERVAL YEAR TO
MONTH type
See Also:
function

Oracle Database SQL Reference for details about each

Importing and Exporting Datetime Types
TIMESTAMP WITH TIME ZONE and TIMESTAMP WITH LOCAL TIME ZONE values
are always stored in normalized format, so that you can export, import, and compare
them without worrying about time zone offsets. DATE and TIMESTAMP values do not
store an associated time zone, and you must adjust them to account for any time zone
differences between source and target databases.

Using SQL Datatypes in Application Development 3-15

Representing Specialized Data

Representing Specialized Data
This section contains the following topics:
■

Representing Geographic Data

■

Representing Multimedia Data

■

Representing Large Amounts of Data

■

Representing Searchable Text

■

Representing XML

■

Representing Dynamically Typed Data

■

Representing Data with ANSI/ISO, DB2, and SQL/DS Datatypes

Representing Geographic Data
To represent Geographic Information System (GIS) or spatial data in the database, you
can use Oracle Spatial features, including the type MDSYS.SDO_GEOMETRY. You can
store the data in the database by using either an object-relational or a relational model.
You can use a set of PL/SQL packages to query and manipulate the data.
See Also: Oracle Spatial User's Guide and Reference to learn how to use
MDSYS.SDO_GEOMETRY

Representing Multimedia Data
Oracle interMedia enables Oracle Database to store, manage, and retrieve images,
audio, video, or other heterogeneous media data in an integrated fashion with other
enterprise information. Oracle interMedia extends Oracle Database reliability,
availability, and data management to multimedia content in traditional, Internet,
electronic commerce, and media-rich applications.
Whether you store such multimedia data inside the database as BLOB or BFILE
values, or store it externally on a Web server or other kind of server, you can use
interMedia to access the data using either an object-relational or a relational model,
and manipulate and query the data using a set of object types.
Oracle interMedia provides the ORDAudio, ORDDoc, ORDImage,
ORDImageSignature, ORDVideo, and SI_StillImage object types and methods
for the following purposes:
■
■

■

Extracting metadata and attributes from multimedia data
Retrieving and managing multimedia data from Oracle interMedia, Web servers,
file systems, and other servers
Performing manipulation operations on image data
See Also: Oracle interMedia Referenceto learn about the interMedia
types

Representing Large Amounts of Data
Oracle Database provides several datatypes for representing large amounts of data.
These datatypes are grouped under the general category of Large Objects (LOBs).
Table 3–6 describes the different LOBs.

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Table 3–6

Large Object Datatypes

Datatype Name

Description

BLOB

Binary large object

Represents large amounts of binary data such as images,
video, or other multimedia data.

CLOB

Character large object Represents large amounts of character data. CLOB types are
stored by using the database character set. Note that the
database stores a CLOB up to 4,000 bytes inline as a
VARCHAR2. If the CLOB exceeds this length, then the
database moves the CLOB out of line.

NCLOB

National character set Represents large amounts of character data in National
large objects
Character Set format.

BFILE

External large object

Stores objects in the operating system's file system, outside
of the database files or tablespace. Note that the BFILE type
is read-only; other LOB types are read/write. BFILE objects
are also sometimes referred to as external LOBs.

An instance of type BLOB, CLOB, or NCLOB can exist as either a persistent LOB instance
or a temporary LOB instance. Persistent and temporary instances differ as follows:
■

A temporary LOB instance is declared in the scope of your application.

■

A persistent LOB instance is created and stored in the database.

With the exception of declaring, freeing, creating, and committing, operations on
persistent and temporary LOB instances are performed the same way.
See Also: Oracle Database Application Developer's Guide - Large Objects
for more details on using LOBs in applications

Using RAW and LONG RAW Datatypes
The RAW and LONG RAW datatypes store data that is not interpreted by Oracle
Database, that is, it is not converted when moving data between different systems.
These datatypes are intended for binary data and byte strings. For example, LONG RAW
can store graphics, sound, documents, and arrays of binary data; the interpretation is
dependent on the use.
Oracle Net and the Export and Import utilities do not perform character conversion
when transmitting RAW or LONG RAW data. When Oracle Database automatically
converts RAW or LONG RAW data to and from CHAR data, as is the case when entering
RAW data as a literal in an INSERT statement, the database represents the data as one
hexadecimal character representing the bit pattern for every four bits of RAW data. For
example, one byte of RAW data with bits 11001011 is displayed and entered as CB.
You cannot index LONG RAW data, but you can index RAW data. In earlier releases, the
LONG and LONG RAW datatypes were typically used to store large amounts of data.
Use of these types is no longer recommended for new development. If your
application still uses these types, migrate your application to use LOB types. Oracle
recommends that you convert LONG RAW columns to binary LOB (BLOB) columns and
convert LONG columns to character LOB (CLOB or NCLOB) columns. LOB columns are
subject to far fewer restrictions than LONG and LONG RAW columns.

Using SQL Datatypes in Application Development 3-17

Representing Specialized Data

See Also:
■

■

See Oracle Database Application Developer's Guide - Large Objects for
information about the BLOB and BFILE datatypes
See the Oracle Database SQL Reference for restrictions on LONG and
LONG RAW datatypes

Representing Searchable Text
Rather than writing low-level code to do full-text searches, you can use Oracle Text. It
stores the search data in a special kind of index, and lets you query the data with
operators and PL/SQL packages. This technology enables you to create your own
search engine using data from tables, files, or URLs, and combine the search logic with
relational queries. You can also search XML data this way with the XPath notation.
See Also: Oracle Text Application Developer's Guide for more
information

Representing XML
If you have information stored as files in XML format, or if you want to take an object
type and store it as XML, then you can use the XMLType built-in type.
XMLType columns store their data as CLOBs. You can take an existing CLOB,
VARCHAR2, or any object type, and call the XMLType constructor to turn it into an
XML object.
When an XML object is inside the database, you can use queries to traverse it (using
the XML XPath notation) and extract all or part of its data.
You can also produce XML output from existing relational data and split XML
documents across relational tables and columns. You can use the following packages
to transfer XML data into and out of relational tables:
■
■

■

DBMS_XMLQUERY, which provides database-to-XMLType functionality
DBMS_XMLGEN, which converts the results of a SQL query to a canonical XML
format
DBMS_XMLSAVE, which provides XML to database-type functionality

You can use the following SQL functions to process XML:
■

■

■

■

■

■

EXTRACT, which applies a VARCHAR2 XPath string and returns an XMLType
instance containing an XML fragment
SYS_XMLAGG, which aggregates all of the XML documents or fragments
represented by an expression and produces a single XML document
SYS_XMLGEN, which takes an expression that evaluates to a particular row and
column of the database, and returns an instance of type XMLType containing an
XML document
UPDATEXML, which takes as arguments an XMLType instance and an XPath-value
pair and returns an XMLType instance with the updated value
XMLAGG, which takes a collection of XML fragments and returns an aggregated
XML document
XMLCOLATTVAL, which creates an XML fragment and then expands the resulting
XML so that each XML fragment has the name column with the attribute name

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Representing Specialized Data

■

■

■

■

XMLCONCAT, which takes as input a series of XMLType instances, concatenates the
series of elements for each row, and returns the concatenated series
XMLELEMENT, which takes an element name for identifier, an optional collection of
attributes for the element, and arguments that make up the content of the element
XMLFOREST, which converts each of its argument parameters to XML, and then
returns an XML fragment that is the concatenation of these converted arguments
XMLSEQUENCE, which either takes as input an XMLType instance and returns a
varray of the top-level nodes in the XMLType, or takes as input a REFCURSOR
instance, with an optional instance of the XMLFormat object, and returns as an
XMLSequence type an XML document for each row of the cursor
XMLTRANSFORM, which takes as arguments an XMLType instance and an XSL style
sheet, applies the style sheet to the instance, and returns an XMLType
See Also:
■

■

■

Oracle XML DB Developer's Guide for details about the XMLType
datatype
Oracle XML Developer's Kit Programmer's Guide for information
about client-side programming with XML
Oracle Database SQL Reference for information about XML
functions

Representing Dynamically Typed Data
Some languages allow datatypes to change at runtime or let a program check the type
of a variable. For example, C has the union keyword and the void * pointer, while
Java has the typeof operator and wrapper types such as Number. Oracle Database
includes features that enable you to create variables and columns that can hold data of
any type and test such data values to determine their underlying representation. Using
these features, a single table column can represent a numeric value in one row, a string
value in another row, and an object in another row.
You can use the built-in type SYS.ANYDATA to represent values of any scalar or object
type. This type is an object type with methods to bring in a scalar value of any type,
and turn the value back into a scalar or object. In the same way, you can use the
built-in type SYS.ANYDATASET to represent values of any collection type.
To manipulate and check type information, you can use SYS.ANYTYPE in combination
with the DBMS_TYPES package. The program in Example 3–4 represents data of
different underlying types in a table, then interprets the underlying type of each row
and processes each value appropriately.
Example 3–4 Accessing Information in a SYS.ANYDATA Column
-- This example defines and executes a PL/SQL procedure that
-- uses methods built into SYS.ANYDATA to access information about
-- data stored in a SYS.ANYDATA table column.
DROP TYPE Employee_type FORCE;
DROP TABLE mytab;
CREATE OR REPLACE TYPE Employee_type AS OBJECT ( empno NUMBER,
ename VARCHAR2(10) );
/
CREATE TABLE mytab ( id NUMBER, data SYS.ANYDATA );
INSERT INTO mytab VALUES (1, SYS.ANYDATA.ConvertNumber(5));
INSERT INTO mytab VALUES (2,

Using SQL Datatypes in Application Development 3-19

Representing Specialized Data

SYS.ANYDATA.ConvertObject(Employee_type(5555, 'john')));
COMMIT;
CREATE OR REPLACE PROCEDURE p
IS
CURSOR cur IS SELECT id, data FROM mytab;
v_id
mytab.id%TYPE;
v_data
mytab.data%TYPE;
v_type
SYS.ANYTYPE;
v_typecode
PLS_INTEGER;
v_typename
VARCHAR2(60);
v_dummy
PLS_INTEGER;
v_n
NUMBER;
v_employee
Employee_type;
non_null_anytype_for_NUMBER exception;
unknown_typename
exception;
BEGIN
OPEN cur;
LOOP
FETCH cur INTO v_id, v_data;
EXIT WHEN cur%NOTFOUND;
/* The typecode is a number that signifies what type is represented by v_data.
GetType also produces a value of type SYS.AnyType with methods you can call
to find precision and scale of a number, length of a string, and so on. */
v_typecode := v_data.GetType ( v_type /* OUT */ );
/* Now we compare the typecode against constants from DBMS_TYPES to see what
kind of data we have, and decide how to display it. */
CASE v_typecode
WHEN DBMS_TYPES.TYPECODE_NUMBER THEN
IF v_type IS NOT NULL
-- This condition should never happen, but we check just in case.
THEN RAISE non_null_anytype_for_NUMBER; END IF;
-- For each type, there is a Get method.
v_dummy := v_data.GetNUMBER ( v_n /* OUT */ );
DBMS_OUTPUT.PUT_LINE (
TO_CHAR(v_id) || ': NUMBER = ' || To_Char(v_n) );
WHEN DBMS_TYPES.TYPECODE_OBJECT THEN
v_typename := v_data.GetTypeName();
-- An object type's name is qualified with the schema name.
IF v_typename NOT IN ( 'HR.EMPLOYEE_TYPE' )
-- If we encounter any object type besides EMPLOYEE_TYPE, raise an exception.
THEN RAISE unknown_typename; END IF;
v_dummy := v_data.GetObject ( v_employee /* OUT */ );
DBMS_OUTPUT.PUT_LINE (
To_Char(v_id) || ': user-defined type = ' || v_typename ||
' ( ' || v_employee.empno || ', ' || v_employee.ename || ' )' );
END CASE;
END LOOP;
CLOSE cur;
EXCEPTION
WHEN non_null_anytype_for_NUMBER THEN
RAISE_Application_Error ( -20000,
'Paradox: the return AnyType instance FROM GetType ' ||
'should be NULL for all but user-defined types' );
WHEN unknown_typename THEN
RAISE_Application_Error ( -20000, 'Unknown user-defined type ' ||
v_typename || ' - program written to handle only HR.EMPLOYEE_TYPE' );

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END;
/

The query and procedure in Example 3–4 produce output like that shown in
Example 3–5.
Example 3–5 Sample Output for Example 3–4
SQL> SELECT t.data.gettypename() AS "Type Name" FROM mytab t;
Type Name
-------------------------------------------------------------------------------SYS.NUMBER
HR.EMPLOYEE_TYPE
SQL> EXEC p;
1: NUMBER = 5
2: user-defined type = HR.EMPLOYEE_TYPE ( 5555, john )

You can access the same features through the OCI interface by using the OCIType,
OCIAnyData, and OCIAnyDataSet interfaces.
See Also:
■

■

■

Oracle Database PL/SQL Packages and Types Reference for details
about the DBMS_TYPES package
Oracle Database Application Developer's Guide - Object-Relational
Features for information and examples using the ANYDATA,
ANYDATASET, and ANYTYPE types
Oracle Call Interface Programmer's Guide for details about the OCI
interfaces

Representing Data with ANSI/ISO, DB2, and SQL/DS Datatypes
You can define columns of tables in Oracle Database by means of ANSI/ISO, DB2, and
SQL/DS datatypes. Oracle Database internally converts such datatypes to Oracle
datatypes.
The ANSI datatype conversions are shown in Table 3–7. The ANSI/ISO datatypes
NUMERIC, DECIMAL, and DEC can specify only fixed-point numbers. For these
datatypes, s defaults to 0.
Table 3–7

ANSI Datatype Conversions to Oracle Datatypes

ANSI SQL Datatype

Oracle Datatype

CHARACTER (n), CHAR (n)

CHAR (n)

NUMERIC (p,s), DECIMAL (p,s), DEC (p,s)

NUMBER (p,s)

INTEGER, INT, SMALLINT

NUMBER (38)

FLOAT (p)

FLOAT (p)

REAL

FLOAT (63)

DOUBLE PRECISION

FLOAT (126)

CHARACTER VARYING(n), CHAR VARYING(n)

VARCHAR2 (n)

TIMESTAMP

TIMESTAMP

TIMESTAMP WITH TIME ZONE

TIMESTAMP WITH TIME ZONE

Using SQL Datatypes in Application Development 3-21

Representing Conditional Expressions as Data

Table 3–8 shows the DB2 and SQL/DS conversions.
Table 3–8

SQL/DS, DB2 Datatype Conversions to Oracle Datatypes

DB2 or SQL/DS Datatype

Oracle Datatype

CHARACTER (n)

CHAR (n)

VARCHAR (n)

VARCHAR2 (n)

LONG VARCHAR

LONG

DECIMAL (p,s)

NUMBER (p,s)

INTEGER, SMALLINT

NUMBER (38)

FLOAT (p)

FLOAT (p)

DATE

DATE

TIMESTAMP

TIMESTAMP

The datatypes TIME, GRAPHIC, VARGRAPHIC, and LONG VARGRAPHIC of IBM
products SQL/DS and DB2 have no corresponding Oracle datatype, and they cannot
be used.

Representing Conditional Expressions as Data
The Oracle Expression Filter feature enables you to store conditional expressions as
data in the database. The Expression Filter provides a mechanism that you can use to
place a constraint on a VARCHAR2 column to ensure that the values stored are valid
SQL WHERE clause expressions. This mechanism also identifies the set of attributes that
are legal to reference in the conditional expressions.
For example, suppose you create a traders table in which row holds data for a stock
trading account holder. You want to define a column that stores information about
stocks each trader is interested in as a conditional expression. You follow these steps:
1.

Create a table traders holds data for a stock trading account holder:
CREATE TABLE traders
( name
VARCHAR2(50),
email
VARCHAR2(50),
interest VARCHAR2(50)
);

2.

Create the user-defined datatype ticker with attributes for the trading symbol,
limit price, and amount of change in the stock price:
CREATE OR REPLACE TYPE ticker
AS OBJECT
( symbol VARCHAR2(20),
price NUMBER,
change NUMBER
);

3.

Use the following PL/SQL block to create an attribute set ticker based on the
ticker datatype:
BEGIN
DBMS_EXPFIL.CREATE_ATTRIBUTE_SET( attr_set => 'ticker',
from_type => 'YES' );
END;

3-22 Oracle Database Application Developer’s Guide - Fundamentals

Identifying Rows by Address

4.

Associate the attribute set with the expression set stored in the database column
trader.interest as follows:
BEGIN
DBMS_EXPFIL.ASSIGN_ATTRIBUTE_SET (attr_set => 'ticker',
expr_tab => 'traders',
expr_col => 'interest');
END;

The preceding code places a constraint on the interest column that ensures the
column stores valid conditional expressions.
5.

Populate the table with trader names, email addresses and conditional expressions
that represents a stock the trader is interested in at a particular price:
INSERT INTO traders (name, email, interest)
VALUES ('Vishu', 'vishu@abc.com', 'symbol = ''ABC'' AND price > 25');

6.

Use the EVALUATE operator to identify the conditional expressions that evaluate
to TRUE for a given data item. For example, the following query returns traders
who are interested in a given stock quote (symbol='ABC', price=31,
change=5.2):
SELECT Name, Email
FROM Traders
WHERE EVALUATE ( interest,
'symbol=>''ABC'',
price=>31,
change=>5.2'
) = 1;

To speed up this type of query, you can optionally create an Oracle Expression Filter
index on the interest column.
See Also: Oracle Database Application Developer's Guide - Rules
Manager and Expression Filter for details on Oracle Expression Filter

Identifying Rows by Address
Each row in a database table has an address called a rowid. You can examine a row
address by querying the pseudocolumn ROWID, whose values are strings representing
the address of each row. These strings have the datatype ROWID or UROWID. You can
also create tables and clusters that contain actual columns having the ROWID datatype.
Oracle Database does not guarantee that the values of such columns are valid rowids.
Rowid values are important for application development for the following reasons:
■

They are the fastest way to access a single row.

■

They can show you how the rows in a table are stored.

■

They are unique identifiers for rows in a table.
See Also:
■

■

Oracle Database Concepts for general information about the ROWID
pseudocolumn and the ROWID datatype
Oracle Database SQL Reference to learn about the ROWID
pseudocolumn

Using SQL Datatypes in Application Development 3-23

Identifying Rows by Address

Querying the ROWID Pseudocolumn
Each table in Oracle Database has a pseudocolumn named ROWID. If the row is too
large to fit within a single data block, then ROWID identifies the initial row piece.
Although rowids are usually unique, different rows can have the same rowid if they
are in the same data block but in different clustered tables.
The following SQL statements return the ROWID pseudocolumn of the row of the
hr.employees table that satisfies the query, and inserts it into the t_tab table:
CREATE TABLE t_tab (col1 ROWID);
INSERT INTO t_tab
SELECT ROWID
FROM hr.employees
WHERE employee_id = 7499;

Note: Although you can use the ROWID pseudocolumn in the
SELECT and WHERE clause of a query, these pseudocolumn values are
not actually stored in the database. You cannot insert, update, or
delete a value of the ROWID pseudocolumn.

Accessing the ROWID Datatype
In tables that are not index-organized and foreign tables, the values of the ROWID
pseudocolumn have the datatype ROWID. The format of this datatype is either
extended or restricted.

Restricted ROWID
Internally, the ROWID is a structure that holds information that the database server
needs to access a row. The restricted internal ROWID is 6 bytes on most platforms. Each
restricted rowid includes the following data:
■

Datafile identifier

■

Block identifier

■

Row identifier

The restricted ROWID pseudocolumn is returned to client applications in the form of an
18-character string with a hexadecimal encoding of the datablock, row, and datafile
components of the ROWID.

Extended ROWID
The extended ROWID datatype includes the data in the restricted rowid plus a data
object number. The data object number is an identification number assigned to every
database segment. The extended internal ROWID is 10 bytes on most platforms.
Data in an extended ROWID pseudocolumn is returned to the client application in the
form of an 18-character string (for example, "AAAA8mAALAAAAQkAAA"), which
represents a base 64 encoding of the components of the extended ROWID in a
four-piece format, OOOOOOFFFBBBBBBRRR. Extended rowids are not available directly.
You can use a supplied package, DBMS_ROWID, to interpret extended rowid contents.
The package functions extract and provide information that would be available
directly from a restricted rowid as well as information specific to extended rowids.
Oracle Database PL/SQL Packages and Types Reference for
information about the DBMS_ROWID package

See Also:

3-24 Oracle Database Application Developer’s Guide - Fundamentals

How Oracle Database Converts Datatypes

External Binary ROWID
Some client applications use a binary form of the ROWID. For example, OCI and some
precompiler applications can map the ROWID datatype to a 3GL structure on bind or
define calls. The size of the binary ROWID is the same for extended and restricted
ROWIDs. The information for the extended ROWID is included in an unused field of the
restricted ROWID structure.
The format of the extended binary ROWID, expressed as a C struct, is as follows:
struct riddef {
ub4
ridobjnum; /* data obj#--this field is
unused in restricted ROWIDs */
ub2
ridfilenum;
ub1
filler;
ub4
ridblocknum;
ub2
ridslotnum;
}

Accessing the UROWID Datatype
The rows of some tables have addresses that are not physical or permanent or were not
generated by Oracle Database. For example, the row addresses of index-organized
tables are stored in index leaves, which can move. Oracle provides these tables with
logical row identifiers, called logical rowids. Rowids of foreign tables, such as DB2
tables accessed through a gateway, are not standard Oracle Database rowids. Oracle
provides foreign tables with identifiers called foreign rowids.
Oracle Database uses universal rowids (urowids) to store the addresses of
index-organized and foreign tables. Both types of urowid are stored in the ROWID
pseudocolumn, as are the physical rowids of heap-organized tables.
Oracle creates logical rowids based on the primary key of the table. The logical rowids
do not change as long as the primary key does not change. The ROWID pseudocolumn
of an index-organized table has a datatype of UROWID. You can access this
pseudocolumn as you would the ROWID pseudocolumn of a heap-organized table (that
is, using a SELECT ... ROWID statement). If you want to store the rowids of an
index-organized table, then you can define a column of type UROWID for the table and
retrieve the value of the ROWID pseudocolumn into that column.

How Oracle Database Converts Datatypes
In some cases, Oracle Database allows data of one datatype where it expects data of a
different datatype. Generally, an expression cannot contain values with different
datatypes. However, Oracle Database can use various SQL functions to automatically
convert data to the expected datatype.
See Also:

Oracle Database SQL Reference for details about datatype

conversion

Datatype Conversion During Assignments
The datatype conversion for an assignment succeeds if Oracle Database can convert
the datatype of the value used in the assignment to that of the assignment target.
For the examples in the following list, assume a package with a public variable and a
table declared as in the following statements:
CREATE PACKAGE Test_Pack AS var1 CHAR(5); END;
CREATE TABLE Table1_tab (col1 NUMBER);

Using SQL Datatypes in Application Development 3-25

How Oracle Database Converts Datatypes

■

variable := expression
The datatype of expression must be either the same as, or convertible to, the
datatype of variable. For example, Oracle Database automatically converts the
data provided in the following assignment within the body of a stored procedure:
VAR1 := 0;

■

INSERT INTO Table1_tab VALUES (expression1, expression2, ...)
The datatypes of expression1, expression2, and so on, must be either the
same as, or convertible to, the datatypes of the corresponding columns in
Table1_tab. For example, Oracle Database automatically converts the data
provided in the following INSERT statement for Table1_tab:
INSERT INTO Table1_tab VALUES (
'
19
'
);

■

UPDATE Table1_tab SET column = expression
The datatype of expression must be either the same as, or convertible to, the
datatype of column. For example, Oracle Database automatically converts the
data provided in the following UPDATE statement issued against Table1_tab:
UPDATE Table1_tab SET col1 =
'
30
'
;

■

SELECT column INTO variable FROM Table1_tab
The datatype of column must be either the same as, or convertible to, the datatype
of variable. For example, Oracle Database automatically converts data selected
from the table before assigning it to the variable in the following statement:
SELECT Col1 INTO Var1 FROM Table1_tab WHERE Col1 = 30;

Datatype Conversion During Expression Evaluation
For expression evaluation, Oracle Database can automatically perform the same
conversions as for assignments. An expression is converted to a type based on its
context. For example, operands to arithmetic operators are converted to NUMBER, and
operands to string functions are converted to VARCHAR2.
Oracle Database can automatically convert the following:
■

VARCHAR2 or CHAR to NUMBER

■

VARCHAR2 or CHAR to DATE

Character to NUMBER conversions succeed only if the character string represents a
valid number. Character to DATE conversions succeed only if the character string
satisfies the session default format, which is specified by the initialization parameter
NLS_DATE_FORMAT.
Some common types of expressions follow:
■

Simple expressions, such as:

3-26 Oracle Database Application Developer’s Guide - Fundamentals

How Oracle Database Converts Datatypes

commission + '500'
■

Boolean expressions, such as:
bonus > salary / '10'

■

Function and procedure calls, such as:
MOD (counter, '2')

■

WHERE clause conditions, such as:
WHERE hiredate = TO_DATE('1997-01-01','yyyy-mm-dd')

■

WHERE clause conditions, such as:
WHERE rowid = 'AAAAaoAATAAAADAAA'

In general, Oracle Database uses the rule for expression evaluation when a datatype
conversion is needed in places not covered by the rule for assignment conversions.
In assignments of the form:
variable := expression

Oracle Database first evaluates expression using the conversion rules for expressions;
expression can be as simple or complex as desired. If it succeeds, then the evaluation of
expression results in a single value and datatype. Then, Oracle Database tries to assign
this value to the target variable using the conversion rules for assignments.

Using SQL Datatypes in Application Development 3-27

How Oracle Database Converts Datatypes

3-28 Oracle Database Application Developer’s Guide - Fundamentals

4
Using Regular Expressions in Oracle
Database
This chapter introduces regular expression support for Oracle Database. This chapter
covers the following topics:
■

Using Regular Expressions with Oracle Database: Overview

■

Regular Expression Metacharacters in Oracle Database

■

Using Regular Expressions in SQL Statements: Scenarios
See Also:
■

■

■

■

Oracle Database SQL Reference for information about Oracle
Database SQL functions for regular expressions
Oracle Database Globalization Support Guide for details on using
SQL regular expression functions in a multilingual environment
Oracle Regular Expressions Pocket Reference by Jonathan Gennick,
O'Reilly & Associates
Mastering Regular Expressions by Jeffrey E. F. Friedl, O'Reilly &
Associates

Using Regular Expressions with Oracle Database: Overview
This section contains the following topics:
■

What Are Regular Expressions?

■

How Are Oracle Database Regular Expressions Useful?

■

Oracle Database Implementation of Regular Expressions

■

Oracle Database Support for the POSIX Regular Expression Standard

What Are Regular Expressions?
Regular expressions enable you to search for patterns in string data by using
standardized syntax conventions. You specify a regular expression by means of the
following types of characters:
■

Metacharacters, which are operators that specify search algorithms

■

Literals, which are the characters for which you are searching

Using Regular Expressions in Oracle Database 4-1

Using Regular Expressions with Oracle Database: Overview

A regular expression can specify complex patterns of character sequences. For
example, the following regular expression searches for the literals f or ht, the t literal,
the p literal optionally followed by the s literal, and finally the colon (:) literal:
(f|ht)tps?:

The parentheses are metacharacters that group a series of pattern elements to a single
element; the pipe symbol (|) matches one of the alternatives in the group. The
question mark (?) is a metacharacter indicating that the preceding pattern, in this case
the s character, is optional. Thus, the preceding regular expression matches the http:,
https:, ftp:, and ftps: strings.

How Are Oracle Database Regular Expressions Useful?
Regular expressions are a powerful text processing component of programming
languages such as Perl and Java. For example, a Perl script can process each HTML file
in a directory, read its contents into a scalar variable as a single string, and then use
regular expressions to search for URLs in the string. One reason that many developers
write in Perl is for its robust pattern matching functionality.
Oracle's support of regular expressions enables developers to implement complex
match logic in the database. This technique is useful for the following reasons:
■

■

■

By centralizing match logic in Oracle Database, you avoid intensive string
processing of SQL results sets by middle-tier applications. For example, life
science customers often rely on Perl to do pattern analysis on bioinformatics data
stored in huge databases of DNAs and proteins. Previously, finding a match for a
protein sequence such as [AG].{4}GK[ST] would be handled in the middle tier.
The SQL regular expression functions move the processing logic closer to the data,
thereby providing a more efficient solution.
Prior to Oracle Database 10g, developers often coded data validation logic on the
client, requiring the same validation logic to be duplicated for multiple clients.
Using server-side regular expressions to enforce constraints solves this problem.
The built-in SQL and PL/SQL regular expression functions and conditions make
string manipulations more powerful and less cumbersome than in previous
releases of Oracle Database.

Oracle Database Implementation of Regular Expressions
Oracle Database implements regular expression support with a set of Oracle Database
SQL functions and conditions that enable you to search and manipulate string data.
You can use these functions in any environment that supports Oracle Database SQL.
You can use these functions on a text literal, bind variable, or any column that holds
character data such as CHAR, NCHAR, CLOB, NCLOB, NVARCHAR2, and VARCHAR2 (but
not LONG).
Table 4–1 gives a brief description of the regular expression functions and conditions.
Table 4–1

SQL Regular Expression Functions and Conditions

SQL Element

Category Description

REGEXP_LIKE

Condition Searches a character column for a pattern. Use this function in
the WHERE clause of a query to return rows matching a regular
expression. The condition is also valid in a constraint or as a
PL/SQL function returning a boolean. The following WHERE
clause filters employees with a first name of Steven or Stephen:
WHERE REGEXP_LIKE(first_name, '^Ste(v|ph)en$')

4-2 Oracle Database Application Developer’s Guide - Fundamentals

Using Regular Expressions with Oracle Database: Overview

Table 4–1 (Cont.) SQL Regular Expression Functions and Conditions
SQL Element

Category Description

REGEXP_REPLACE Function

Searches for a pattern in a character column and replaces each
occurrence of that pattern with the specified string. The
following function puts a space after each character in the
country_name column:
REGEXP_REPLACE(country_name, '(.)', '\1 ')

REGEXP_INSTR

Function

Searches a string for a given occurrence of a regular expression
pattern and returns an integer indicating the position in the
string where the match is found. You specify which occurrence
you want to find and the start position. For example, the
following performs a boolean test for a valid email address in
the email column:
REGEXP_INSTR(email, '\w+@\w+(\.\w+)+') > 0

REGEXP_SUBSTR

Function

Returns the substring matching the regular expression pattern
that you specify. The following function uses the x flag to match
the first string by ignoring spaces in the regular expression:
REGEXP_SUBSTR('oracle', 'o r a c l e', 1, 1, 'x')

A string literal in a REGEXP function or condition conforms to the rules of SQL text
literals. By default, regular expressions must be enclosed in single quotes. If your
regular expression includes the single quote character, then enter two single quotation
marks to represent one single quotation mark within the expression. This technique
ensures that the entire expression is interpreted by the SQL function and improves the
readability of your code. You can also use the q-quote syntax to define your own
character to terminate a text literal. For example, you could delimit your regular
expression with the pound sign (#) and then use a single quote within the expression.
If your expression comes from a column or a bind variable,
then the same rules for quoting do not apply.

Note:

See Also:
■

■

Oracle Database SQL Reference for syntax, descriptions, and
examples of the REGEXP functions and conditions
Oracle Database SQL Reference for a discussion of character literals

Oracle Database Support for the POSIX Regular Expression Standard
Oracle's implementation of regular expressions conforms to the following standards:
■

IEEE Portable Operating System Interface (POSIX) standard draft 1003.2/D11.2

■

Unicode Regular Expression Guidelines of the Unicode Consortium

Oracle Database follows the exact syntax and matching semantics for these operators
as defined in the POSIX standard for matching ASCII (English language) data. You can
find the POSIX standard draft at the following URL:
http://www.opengroup.org/onlinepubs/007908799/xbd/re.html

Oracle Database enhances regular expression support in the following ways:
■

Extends the matching capabilities for multilingual data beyond what is specified
in the POSIX standard.

Using Regular Expressions in Oracle Database 4-3

Regular Expression Metacharacters in Oracle Database

■

Adds support for the common Perl regular expression extensions that are not
included in the POSIX standard but do not conflict with it. Oracle Database
provides built-in support for some of the most heavily used Perl regular
expression operators, for example, character class shortcuts, the non-greedy
modifier, and so on.

Oracle Database supports a set of common metacharacters used in regular expressions.
The behavior of supported metacharacters and related features is described in
"Regular Expression Metacharacters in Oracle Database" on page 4-4.
The interpretation of metacharacters differs between tools that
support regular expressions. If you are porting regular expressions
from another environment to Oracle Database, ensure that the regular
expression syntax is supported and the behavior is what you expect.

Note:

Regular Expression Metacharacters in Oracle Database
This section contains the following topics:
■

POSIX Metacharacters in Oracle Database Regular Expressions

■

Regular Expression Operator Multilingual Enhancements

■

Perl-Influenced Extensions in Oracle Regular Expressions

POSIX Metacharacters in Oracle Database Regular Expressions
Table 4–2 lists the list of metacharacters supported for use in regular expressions
passed to SQL regular expression functions and conditions. These metacharacters
conform to the POSIX standard; any differences in behavior from the standard are
noted in the "Description" column.
Table 4–2

POSIX Metacharacters in Oracle Database Regular Expressions

Syntax

Operator Name

Description

Example

.

Any Character —
Dot

Matches any character in the database character set. The expression a.b matches the
If the n flag is set, it matches the newline character. strings abb, acb, and adb, but does
The newline is recognized as the linefeed character not match acc.
(\x0a) on UNIX and Windows or the carriage
return character (\x0d) on Macintosh platforms.
Note: In the POSIX standard, this operator matches
any English character except NULL and the
newline character.

+

One or More —
Plus Quantifier

Matches one or more occurrences of the preceding
subexpression.

The expression a+ matches the
strings a, aa, and aaa, but does not
match bbb.

?

Zero or One —
Question Mark
Quantifier

Matches zero or one occurrence of the preceding
subexpression.

The expression ab?c matches the
strings abc and ac, but does not
match abbc.

*

Zero or More —
Star Quantifier

Matches zero or more occurrences of the preceding
subexpression. By default, a quantifier match is
greedy because it matches as many times as
possible while still allowing the rest of the match to
succeed.

The expression ab*c matches the
strings ac, abc, and abbc, but does
not match abb.

{m}

Interval—Exact
Count

Matches exactly m occurrences of the preceding
subexpression.

The expression a{3} matches the
strings aaa, but does not match aa.

{m,}

Interval—At
Least Count

Matches at least m occurrences of the preceding
subexpression.

The expression a{3,} matches the
strings aaa and aaaa, but does not
match aa.

4-4 Oracle Database Application Developer’s Guide - Fundamentals

Regular Expression Metacharacters in Oracle Database

Table 4–2 (Cont.) POSIX Metacharacters in Oracle Database Regular Expressions
Syntax

Operator Name

Description

Example

{m,n}

Interval—Betwee
n Count

Matches at least m, but not more than n occurrences
of the preceding subexpression.

The expression a{3,5} matches
the strings aaa, aaaa, and aaaaa,
but does not match aa.

[ ... ]

Matching
Character List

Matches any single character in the list within the
brackets. The following operators are allowed
within the list, but other metacharacters included
are treated as literals:

The expression [abc] matches the
first character in the strings all,
bill, and cold, but does not
match any characters in doll.

■

Range operator: -

■

POSIX character class: [: :]

■

POSIX collation element: [. .]

■

POSIX character equivalence class: [= =]

A dash (-) is a literal when it occurs first or last in
the list, or as an ending range point in a range
expression, as in [#--]. A right bracket (]) is
treated as a literal if it occurs first in the list.
Note: In the POSIX standard, a range includes all
collation elements between the start and end of the
range in the linguistic definition of the current
locale. Thus, ranges are linguistic rather than byte
values ranges; the semantics of the range
expression are independent of character set. In
Oracle Database, the linguistic range is determined
by the NLS_SORT initialization parameter.
[^ ... ]

Non-Matching
Character List

Matches any single character not in the list within
the brackets. Characters not in the non-matching
character list are returned as a match. Refer to the
description of the Matching Character List operator
for an account of metacharacters allowed in the
character list.

The expression [^abc] matches
the character d in the string
abcdef, but not the character a, b,
or c. The expression [^abc]+
matches the sequence def in the
string abcdef, but not a, b, or c.
The expression [^a-i] excludes
any character between a and i
from the search result. This
expression matches the character j
in the string hij, but does not
match any characters in the string
abcdefghi.

|

Or

Matches one of the alternatives.

The expression a|b matches
character a or character b.

( ... )

Subexpression or
Grouping

Treats the expression within parentheses as a unit.
The subexpression can be a string of literals or a
complex expression containing operators.

The expression (abc)?def
matches the optional string abc,
followed by def. Thus, the
expression matches abcdefghi
and def, but does not match ghi.

\n

Backreference

Matches the nth preceding subexpression, that is,
whatever is grouped within parentheses, where n is
an integer from 1 to 9. The parentheses cause an
expression to be remembered; a backreference
refers to it. A backreference counts subexpressions
from left to right, starting with the opening
parenthesis of each preceding subexpression. The
expression is invalid if the source string contains
fewer than n subexpressions preceding the \n.

The expression (abc|def)xy\1
matches the strings abcxyabc and
defxydef, but does not match
abcxydef or abcxy.

Treats the subsequent metacharacter in the
expression as a literal. Use a backslash (\) to search
for a character that is normally treated as a
metacharacter. Use consecutive backslashes (\\) to
match the backslash literal itself.

The expression \+ searches for the
plus character (+). It matches the
plus character in the string
abc+def, but does not match
abcdef.

A backreference enables you to
search for a repeated string without
knowing the actual string ahead of
time. For example, the expression
^(.*)\1$ matches a line
Oracle supports the backreference expression in the consisting of two adjacent instances
of the same string.
regular expression pattern and the replacement
string of the REGEXP_REPLACE function.

\

Escape Character

Using Regular Expressions in Oracle Database 4-5

Regular Expression Metacharacters in Oracle Database

Table 4–2 (Cont.) POSIX Metacharacters in Oracle Database Regular Expressions
Syntax

Operator Name

Description

Example

^

Beginning of Line Matches the beginning of a string (default). In
Anchor
multiline mode, it matches the beginning of any
line within the source string.

The expression ^def matches def
in the string defghi but does not
match def in abcdef.

$

End of Line
Anchor

Matches the end of a string (default). In multiline
mode, it matches the beginning of any line within
the source string.

The expression def$ matches def
in the string abcdef but does not
match def in the string defghi.

[:class:]

POSIX Character
Class

Matches any character belonging to the specified
POSIX character class. You can use this operator
to search for characters with specific formatting
such as uppercase characters, or you can search for
special characters such as digits or punctuation
characters. The full set of POSIX character classes is
supported.

The expression [[:upper:]]+
searches for one or more
consecutive uppercase characters.
This expression matches DEF in the
string abcDEFghi but does not
match the string abcdefghi.

Note: In English regular expressions, range
expressions often indicate a character class. For
example, [a-z] indicates any lowercase character.
This convention is not useful in multilingual
environments, where the first and last character of a
given character class may not be the same in all
languages. Oracle supports the character classes in
Table 4–3 based on character class definitions in
Globalization classification data.
[.element.]

POSIX Collating Specifies a collating element to use in the regular
Element Operator expression. The element must be a defined
collating element in the current locale. Use any
collating element defined in the locale, including
single-character and multicharacter elements. The
NLS_SORT initialization parameter determines
supported collation elements.

The expression [[.ch.]] searches
for the collating element ch and
matches ch in string chabc, but
does not match cdefg. The
expression [a-[.ch.]] specifies
the range a to ch.

This operator lets you use a multicharacter collating
element in cases where only one character would
otherwise be allowed. For example, you can ensure
that the collating element ch, when defined in a
locale such as Traditional Spanish, is treated as one
character in operations that depend on the ordering
of characters.
[=character=] POSIX Character
Equivalence
Class

Matches all characters that are members of the same The expression [[=n=]] searches
for characters equivalent to n in a
character equivalence class in the current locale as
Spanish locale. It matches both N
the specified character.
and ñ in the string El Niño.
The character equivalence class must occur within a
character list, so the character equivalence class is
always nested within the brackets for the character
list in the regular expression.
Usage of character equivalents depends on how
canonical rules are defined for your database locale.
Refer to the Oracle Database Globalization Support
Guide for more information on linguistic sorting
and string searching.

Oracle Database SQL Reference for syntax, descriptions, and
examples of the REGEXP functions and conditions

See Also:

Regular Expression Operator Multilingual Enhancements
When applied to multilingual data, Oracle's implementation of the POSIX operators
extends beyond the matching capabilities specified in the POSIX standard. Table 4–3
shows the relationship of the operators in the context of the POSIX standard.
■
■

The first column lists the supported operators.
The second column indicates whether the POSIX standard for Basic Regular
Expression (BRE) defines the operator.

4-6 Oracle Database Application Developer’s Guide - Fundamentals

Regular Expression Metacharacters in Oracle Database

■

■

The third column indicates whether the POSIX standard for Extended Regular
Expression (ERE) defines the operator.
The fourth column indicates whether the Oracle Database implementation extends
the operator's semantics for handling multilingual data.

Oracle Database lets you enter multibyte characters directly, if you have a direct input
method, or use functions to compose the multibyte characters. You cannot use the
Unicode hexadecimal encoding value of the form \xxxx. Oracle evaluates the
characters based on the byte values used to encode the character, not the graphical
representation of the character.
Table 4–3

POSIX and Multilingual Operator Relationships

Operator

POSIX BRE syntax

POSIX ERE Syntax

Multilingual
Enhancement

\

Yes

Yes

--

*

Yes

Yes

--

+

--

Yes

--

?

--

Yes

--

|

--

Yes

--

^

Yes

Yes

Yes

$

Yes

Yes

Yes

.

Yes

Yes

Yes

[ ]

Yes

Yes

Yes

( )

Yes

Yes

--

{m}

Yes

Yes

--

{m,}

Yes

Yes

--

{m,n}

Yes

Yes

--

\n

Yes

Yes

Yes

[..]

Yes

Yes

Yes

[::]

Yes

Yes

Yes

[==]

Yes

Yes

Yes

Perl-Influenced Extensions in Oracle Regular Expressions
Table 4–4 describes Perl-influenced metacharacters supported in Oracle Database
regular expression functions and conditions. These metacharacters are not in the
POSIX standard, but are common at least partly due to the popularity of Perl. Note
that Perl character class matching is based on the locale model of the operating system,
whereas Oracle Database regular expressions are based on the language-specific data
of the database. In general, a regular expression involving locale data cannot be
expected to produce the same results between Perl and Oracle Database.

Using Regular Expressions in Oracle Database 4-7

Regular Expression Metacharacters in Oracle Database

Table 4–4

Perl-Influenced Extensions in Oracle Regular Expressions

Reg. Exp. Matches . . .

Example

\d

A digit character. It is equivalent to the
POSIX class [[:digit:]].

The expression ^\(\d{3}\) \d{3}-\d{4}$ matches
(650) 555-1212 but does not match
650-555-1212.

\D

A non-digit character. It is equivalent to the
POSIX class [^[:digit:]].

The expression \w\d\D matches b2b and b2_ but does
not match b22.

\w

A word character, which is defined as an
alphanumeric or underscore (_) character. It
is equivalent to the POSIX class
[[:alnum:]_]. Note that if you do not
want to include the underscore character,
you can use the POSIX class [[:alnum:]].

The expression \w+@\w+(\.\w+)+ matches the string
jdoe@company.co.uk but not the string
jdoe@company.

\W

A non-word character. It is equivalent to the
POSIX class [^[:alnum:]_].

The expression \w+\W\s\w+ matches the string to:
bill but not the string to bill.

\s

A whitespace character. It is equivalent to
the POSIX class [[:space:]].

The expression \(\w\s\w\s\) matches the string (a
b ) but not the string (ab).

\S

A non-whitespace character. It is equivalent
to the POSIX class [^[:space:]].

The expression \(\w\S\w\S\) matches the string
(abde) but not the string (a b d e).

\A

Only at the beginning of a string. In
multi-line mode, that is, when embedded
newline characters in a string are considered
the termination of a line, \A does not match
the beginning of each line.

The expression \AL matches only the first L character
in the string Line1\nLine2\n, regardless of whether
the search is in single-line or multi-line mode.

\Z

Only at the end of string or before a newline
ending a string. In multi-line mode, that is,
when embedded newline characters in a
string are considered the termination of a
line, \Z does not match the end of each line.

In the expression \s\Z, the \s matches the last space
in the string L i n e \n, regardless of whether the
search is in single-line or multi-line mode.

\z

Only at the end of a string.

In the expression \s\z, the \s matches the newline in
the string L i n e \n, regardless of whether the
search is in single-line or multi-line mode.

*?

The expression \w*?x\w is "non-greedy" and so
The preceding pattern element 0 or more
times (non-greedy). Note that this quantifier matches abxc in the string abxcxd. The expression
matches the empty string whenever possible. \w*x\w is "greedy" and so matches abxcxd in the
string abxcxd. The expression \w*?x\w also
matches the string xa.

+?

The preceding pattern element 1 or more
times (non-greedy).

??

The expression a??aa is "non-greedy" and matches aa
The preceding pattern element 0 or 1 time
in the string aaaa. The expression a?aa is "greedy"
(non-greedy). Note that this quantifier
matches the empty string whenever possible. and so matches aaa in the string aaaa.

{n}?

The preceding pattern element exactly n
times (non-greedy). In this case {n}? is
equivalent to {n}.

The expression (a|aa){2}? matches aa in the string
aaaa.

{n,}?

The preceding pattern element at least n
times (non-greedy).

The expression a{2,}? is "non-greedy" and matches
aa in the string aaaaa. The expression a{2,} is
"greedy" and so matches aaaaa.

{n,m}?

At least n but not more than m times
The expression a{2,4}? is "non-greedy" and matches
(non-greedy). Note that {0,m}? matches the aa in the string aaaaa. The expression a{2,4} is
empty string whenever possible.
"greedy" and so matches aaaa.

The expression \w+?x\w is "non-greedy" and so
matches abxc in the string abxcxd. The expression
\w+x\w is "greedy" and so matches abxcxd in the
string abxcxd. The expression \w+?x\w does not
match the string xa, but does match the string axa.

4-8 Oracle Database Application Developer’s Guide - Fundamentals

Using Regular Expressions in SQL Statements: Scenarios

The Oracle Database regular expression functions and conditions support the pattern
matching modifiers described in Table 4–5.
Table 4–5

Pattern Matching Modifiers

Mod. Description
i

Example

Specifies case-insensitive matching.

The following regular expression returns AbCd:
REGEXP_SUBSTR('AbCd', 'abcd', 1, 1, 'i')

c

Specifies case-sensitive matching.

The following regular expression fails to match:
REGEXP_SUBSTR('AbCd', 'abcd', 1, 1, 'c')

n

m

x

Allows the period (.), which by default does
not match newlines, to match the newline
character.
Performs the search in multi-line mode. The
metacharacter ^ and $ signify the start and
end, respectively, of any line anywhere in
the source string, rather than only at the
start or end of the entire source string.
Ignores whitespace characters in the regular
expression. By default, whitespace
characters match themselves.

The following regular expression matches the string, but
would not match if the n flag were not specified:
REGEXP_SUBSTR('a'||CHR(10)||'d', 'a.d', 1, 1, 'n')
The following regular expression returns ac:
REGEXP_SUBSTR('ab'||CHR(10)||'ac', '^a.', 1, 2, 'm')

The following regular expression returns abcd:
REGEXP_SUBSTR('abcd', 'a b c d', 1, 1, 'x')

Using Regular Expressions in SQL Statements: Scenarios
This section contains the following scenarios:
■

Using an Integrity Constraint to Enforce a Phone Number Format

■

Using Back References to Reposition Characters

Using an Integrity Constraint to Enforce a Phone Number Format
Regular expressions are a useful way to enforce integrity constraints. For example,
suppose that you want to ensure that phone numbers are entered into the database in
a standard format. Example 4–1 creates a contacts table and adds a check constraint
to the p_number column to enforce the following format mask:
(XXX) XXX-XXXX
Example 4–1 Enforcing a Phone Number Format with Regular Expressions
CREATE TABLE contacts
(
l_name
VARCHAR2(30),
p_number VARCHAR2(30)
CONSTRAINT p_number_format
CHECK ( REGEXP_LIKE ( p_number, '^\(\d{3}\) \d{3}-\d{4}$' ) )
);

Table 4–6 explains the elements of the regular expression.
Table 4–6

Explanation of the Regular Expression Elements in Example 4–1

Regular Expression
Element

Matches . . .

^

The beginning of the string.

Using Regular Expressions in Oracle Database 4-9

Using Regular Expressions in SQL Statements: Scenarios

Table 4–6 (Cont.) Explanation of the Regular Expression Elements in Example 4–1
Regular Expression
Element

Matches . . .

\(

A left parenthesis. The backward slash (\) is an escape character that
indicates that the left parenthesis following it is a literal rather than a
grouping expression.

\d{3}

Exactly three digits.

\)

A right parenthesis. The backward slash (\) is an escape character that
indicates that the right parenthesis following it is a literal rather than a
grouping expression.

(space character)

A space character.

\d{3}

Exactly three digits.

-

A hyphen.

\d{4}

Exactly four digits.

$

The end of the string.

Example 4–2 shows a SQL script that attempts to insert seven phone numbers into the
contacts table. Only the first two INSERT statements use a format that conforms to
the p_number_format constraint; the remaining statements generate check
constraint errors.
Example 4–2 insert_contacts.sql
-- first two statements use valid phone number format
INSERT INTO contacts (p_number)
VALUES( '(650) 555-5555'
);
INSERT INTO contacts (p_number)
VALUES( '(215) 555-3427'
);
-- remaining statements generate check contraint errors
INSERT INTO contacts (p_number)
VALUES( '650 555-5555'
);
INSERT INTO contacts (p_number)
VALUES( '650 555 5555'
);
INSERT INTO contacts (p_number)
VALUES( '650-555-5555'
);
INSERT INTO contacts (p_number)
VALUES( '(650)555-5555'
);
INSERT INTO contacts (p_number)
VALUES( ' (650) 555-5555' );
/

Using Back References to Reposition Characters
As explained in Table 4–2, back references store matched subexpressions in a
temporary buffer, thereby enabling you to reposition characters. You access buffers
with the \n notation, where \n is a number between 1 and 9. Each subexpression is
contained in parentheses and is numbered from left to right.
Example 4–3 creates a famous_people table and populates the
famous_people.names column with names in different formats.
Example 4–3 Using Back References to Reposition Characters
CREATE TABLE famous_people
( names VARCHAR2(30) );

4-10 Oracle Database Application Developer’s Guide - Fundamentals

Using Regular Expressions in SQL Statements: Scenarios

-- populate table with data
INSERT INTO famous_people
VALUES ('John Quincy Adams');
INSERT INTO famous_people
VALUES ('Harry S. Truman');
INSERT INTO famous_people
VALUES ('John Adams');
INSERT INTO famous_people
VALUES (' John Quincy Adams');
INSERT INTO famous_people
VALUES ('John_Quincy_Adams');
COMMIT;

Example 4–4 shows a query that repositions names in the format "first middle last" to
the format "last, first middle". It ignores names not in the format "first middle last".
Example 4–4 Using Back References to Reposition Characters
SELECT names "names",
REGEXP_REPLACE(names,
'^(\S+)\s(\S+)\s(\S+)$',
'\3, \1 \2')
AS "names after regexp"
FROM famous_people;

Table 4–7 explains the elements of the regular expression.
Table 4–7

Explanation of the Regular Expression Elements in Example 4–4

Regular Expression
Element

Description

^

Matches the beginning of the string.

$

Matches the end of the string.

(\S+)

Matches one or more non-space characters. The parentheses are not
escaped so they function as a grouping expression.

\s

Matches a whitespace character.

\1

Substitutes the first subexpression, that is, the first group of
parentheses in the matching pattern.

\2

Substitutes the second subexpression, that is, the second group of
parentheses in the matching pattern.

\3

Substitutes the third subexpression, that is, the third group of
parentheses in the matching pattern.

,

Inserts a comma character.

Example 4–5 shows the result set of the query in Example 4–4. The regular expression
matched only the first two rows.
Example 4–5 Result Set of Regular Expression Query
names
-----------------------------names after regexp
-----------------------------John Quincy Adams
Adams, John Quincy

Using Regular Expressions in Oracle Database 4-11

Using Regular Expressions in SQL Statements: Scenarios

Harry S. Truman
Truman, Harry S.
John Adams
John Adams
John Quincy Adams
John Quincy Adams
John_Quincy_Adams
John_Quincy_Adams

4-12 Oracle Database Application Developer’s Guide - Fundamentals

5
Using Indexes in Application Development
This chapter discusses considerations for using the different types of indexes in an
application. The topics include:
■

Guidelines for Application-Specific Indexes

■

Creating Indexes: Basic Examples

■

When to Use Domain Indexes

■

When to Use Function-Based Indexes
See Also:
■

■

■

■

Oracle Database Administrator's Guide for information about
creating and managing indexes
Oracle Database Performance Tuning Guide for detailed information
about using indexes
Oracle Database SQL Reference for the syntax of commands to work
with indexes
Oracle Database Administrator's Guide for information on creating
hash clusters to improve performance, as an alternative to
indexing

Guidelines for Application-Specific Indexes
You can create indexes on columns to speed up queries. Indexes provide faster access
to data for operations that return a small portion of a table's rows.
In general, you should create an index on a column in any of the following situations:
■

The column is queried frequently.

■

A referential integrity constraint exists on the column.

■

A UNIQUE key integrity constraint exists on the column.

You can create an index on any column; however, if the column is not used in any of
these situations, creating an index on the column does not increase performance and
the index takes up resources unnecessarily.
Although the database creates an index for you on a column with an integrity
constraint, explicitly creating an index on such a column is recommended.
You can use the following techniques to determine which columns are best candidates
for indexing:

Using Indexes in Application Development

5-1

Guidelines for Application-Specific Indexes

■

■

Use the EXPLAIN PLAN feature to show a theoretical execution plan of a given
query statement.
Use the V$SQL_PLAN view to determine the actual execution plan used for a given
query statement.

Sometimes, if an index is not being used by default and it would be most efficient to
use that index, you can use a query hint so that the index is used.
The following sections explain how to create, alter, and drop indexes using SQL
commands, and give guidelines for managing indexes.
See Also: Oracle Database Performance Tuning Guide for information
on using the V$SQL_PLAN view, the EXPLAIN PLAN statement, query
hints, and measuring the performance benefits of indexes

Create Indexes After Inserting Table Data
Typically, you insert or load data into a table (using SQL*Loader or Import) before
creating indexes. Otherwise, the overhead of updating the index slows down the insert
or load operation. The exception to this rule is that you must create an index for a
cluster before you insert any data into the cluster.

Switch Your Temporary Tablespace to Avoid Space Problems Creating Indexes
When you create an index on a table that already has data, Oracle Database must use
sort space to create the index. The database uses the sort space in memory allocated for
the creator of the index (the amount for each user is determined by the initialization
parameter SORT_AREA_SIZE), but the database must also swap sort information to
and from temporary segments allocated on behalf of the index creation. If the index is
extremely large, it can be beneficial to complete the following steps:
1.

Create a new temporary tablespace using the CREATE TABLESPACE command.

2.

Use the TEMPORARY TABLESPACE option of the ALTER USER command to make
this your new temporary tablespace.

3.

Create the index using the CREATE INDEX command.

4.

Drop this tablespace using the DROP TABLESPACE command. Then use the
ALTER USER command to reset your temporary tablespace to your original
temporary tablespace.

Under certain conditions, you can load data into a table with the SQL*Loader "direct
path load", and an index can be created as data is loaded.
See Also:

Oracle Database Utilities for information on direct path load

Index the Correct Tables and Columns
Use the following guidelines for determining when to create an index:
■

■

Create an index if you frequently want to retrieve less than about 15% of the rows
in a large table. This threshold percentage varies greatly, however, according to the
relative speed of a table scan and how clustered the row data is about the index
key. The faster the table scan, the lower the percentage; the more clustered the row
data, the higher the percentage.
Index columns that are used for joins to improve join performance.

5-2 Oracle Database Application Developer’s Guide - Fundamentals

Guidelines for Application-Specific Indexes

■

■

Primary and unique keys automatically have indexes, but you might want to
create an index on a foreign key; see Chapter 6, "Maintaining Data Integrity in
Application Development" for more information.
Small tables do not require indexes; if a query is taking too long, then the table
might have grown from small to large.

Some columns are strong candidates for indexing. Columns with one or more of the
following characteristics are good candidates for indexing:
■

Values are unique in the column, or there are few duplicates.

■

There is a wide range of values (good for regular indexes).

■

There is a small range of values (good for bitmap indexes).

■

The column contains many nulls, but queries often select all rows having a value.
In this case, a comparison that matches all the non-null values, such as:
WHERE COL_X >= -9.99 *power(10,125)

is preferable to
WHERE COL_X IS NOT NULL

This is because the first uses an index on COL_X (assuming that COL_X is a
numeric column).
Columns with the following characteristics are less suitable for indexing:
■

There are many nulls in the column and you do not search on the non-null values.

LONG and LONG RAW columns cannot be indexed.
The size of a single index entry cannot exceed roughly one-half (minus some
overhead) of the available space in the data block. Consult with the database
administrator for assistance in determining the space required by an index.

Limit the Number of Indexes for Each Table
The more indexes, the more overhead is incurred as the table is altered. When rows are
inserted or deleted, all indexes on the table must be updated. When a column is
updated, all indexes on the column must be updated.
You must weigh the performance benefit of indexes for queries against the
performance overhead of updates. For example, if a table is primarily read-only, you
might use more indexes; but, if a table is heavily updated, you might use fewer
indexes.

Choose the Order of Columns in Composite Indexes
Although you can specify columns in any order in the CREATE INDEX command, the
order of columns in the CREATE INDEX statement can affect query performance. In
general, you should put the column expected to be used most often first in the index.
You can create a composite index (using several columns), and the same index can be
used for queries that reference all of these columns, or just some of them.
For example, assume the columns of the VENDOR_PARTS table are as shown in
Figure 5–1.

Using Indexes in Application Development

5-3

Guidelines for Application-Specific Indexes

Figure 5–1 The VENDOR_PARTS Table

Table VENDOR_PARTS
VEND ID
1012
1012
1012
1010
1010
1220
1012
1292

PART NO

UNIT COST

10–440
10–441
457
10–440
457
08–300
08–300
457

.25
.39
4.95
.27
5.10
1.33
1.19
5.28

Assume that there are five vendors, and each vendor has about 1000 parts.
Suppose that the VENDOR_PARTS table is commonly queried by SQL statements such
as the following:
SELECT * FROM vendor_parts
WHERE part_no = 457 AND vendor_id = 1012;

To increase the performance of such queries, you might create a composite index
putting the most selective column first; that is, the column with the most values:
CREATE INDEX ind_vendor_id
ON vendor_parts (part_no, vendor_id);

Composite indexes speed up queries that use the leading portion of the index. So in this
example, queries with WHERE clauses using only the PART_NO column also note a
performance gain. Because there are only five distinct values, placing a separate index
on VENDOR_ID would serve no purpose.

Gather Statistics to Make Index Usage More Accurate
The database can use indexes more effectively when it has statistical information about
the tables involved in the queries. You can gather statistics when the indexes are
created by including the keywords COMPUTE STATISTICS in the CREATE INDEX
statement. As data is updated and the distribution of values changes, you or the DBA
can periodically refresh the statistics by calling procedures like DBMS_
STATS.GATHER_TABLE_STATISTICS and DBMS_STATS.GATHER_SCHEMA_
STATISTICS.

Drop Indexes That Are No Longer Required
You might drop an index if:
■

It does not speed up queries. The table might be very small, or there might be
many rows in the table but very few index entries.

■

The queries in your applications do not use the index.

■

The index must be dropped before being rebuilt.

When you drop an index, all extents of the index's segment are returned to the
containing tablespace and become available for other objects in the tablespace.
Use the SQL command DROP INDEX to drop an index. For example, the following
statement drops a specific named index:
DROP INDEX Emp_ename;

5-4 Oracle Database Application Developer’s Guide - Fundamentals

Creating Indexes: Basic Examples

If you drop a table, then all associated indexes are dropped.
To drop an index, the index must be contained in your schema or you must have the
DROP ANY INDEX system privilege.

Privileges Required to Create an Index
When using indexes in an application, you might need to request that the DBA grant
privileges or make changes to initialization parameters.
To create a new index, you must own, or have the INDEX object privilege for, the
corresponding table. The schema that contains the index must also have a quota for
the tablespace intended to contain the index, or the UNLIMITED TABLESPACE system
privilege. To create an index in another user's schema, you must have the CREATE ANY
INDEX system privilege.

Creating Indexes: Basic Examples
You can create an index for a table to improve the performance of queries issued
against the corresponding table. You can also create an index for a cluster. You can
create a composite index on multiple columns up to a maximum of 32 columns. A
composite index key cannot exceed roughly one-half (minus some overhead) of the
available space in the data block.
Oracle Database automatically creates an index to enforce a UNIQUE or PRIMARY KEY
integrity constraint. In general, it is better to create such constraints to enforce
uniqueness, instead of using the obsolete CREATE UNIQUE INDEX syntax.
Use the SQL command CREATE INDEX to create an index.
In this example, an index is created for a single column, to speed up queries that test
that column:
CREATE INDEX emp_ename ON emp_tab(ename);

In this example, several storage settings are explicitly specified for the index:
CREATE INDEX emp_ename ON emp_tab(ename)
TABLESPACE users
STORAGE (INITIAL
20K
NEXT
20k
PCTINCREASE 75)
PCTFREE
0
COMPUTE STATISTICS;

In this example, the index applies to two columns, to speed up queries that test either
the first column or both columns:
CREATE INDEX emp_ename ON emp_tab(ename, empno) COMPUTE STATISTICS;

In this example, the query is going to sort on the function UPPER(ENAME). An index
on the ENAME column itself would not speed up this operation, and it might be slow to
call the function for each result row. A function-based index precomputes the result of
the function for each column value, speeding up queries that use the function for
searching or sorting:
CREATE INDEX emp_upper_ename ON emp_tab(UPPER(ename)) COMPUTE STATISTICS;

Using Indexes in Application Development

5-5

When to Use Domain Indexes

When to Use Domain Indexes
Domain indexes are appropriate for special-purpose applications implemented using
data cartridges. The domain index helps to manipulate complex data, such as spatial,
audio, or video data. If you need to develop such an application, refer to Oracle
Database Data Cartridge Developer's Guide.
Oracle Database supplies a number of specialized data cartridges to help manage
these kinds of complex data. So, if you need to create a search engine, or a geographic
information system, you can do much of the work simply by creating the right kind of
index.

When to Use Function-Based Indexes
A function-based index is an index built on an expression. It extends your indexing
capabilities beyond indexing on a column. A function-based index increases the
variety of ways in which you can access data.
Note:
■

■

The index is more effective if you gather statistics for the table or
schema, using the procedures in the DBMS_STATS package.
The index cannot contain any null values. Either make sure the
appropriate columns contain no null values, or use the NVL
function in the index expression to substitute some other value for
nulls.

The expression indexed by a function-based index can be an arithmetic expression or
an expression that contains a PL/SQL function, package function, C callout, or SQL
function. Function-based indexes also support linguistic sorts based on collation keys,
efficient linguistic collation of SQL statements, and case-insensitive sorts.
Like other indexes, function-based indexes improve query performance. For example,
if you need to access a computationally complex expression often, then you can store it
in an index. Then when you need to access the expression, it is already computed. You
can find a detailed description of the advantages of function-based indexes in
"Advantages of Function-Based Indexes" on page 5-6.
Function-based indexes have all of the same properties as indexes on columns. Unlike
indexes on columns which can be used by both cost-based and rule-based
optimization, however, function-based indexes can be used by only by cost-based
optimization. Other restrictions on function-based indexes are described in
"Restrictions for Function-Based Indexes" on page 5-8.
See Also:
■
■

Oracle Database Concepts
Oracle Database Administrator's Guide for information on creating
function-based indexes

Advantages of Function-Based Indexes
Function-based indexes:
■

Increase the number of situations where the optimizer can perform a range scan instead of
a full table scan. For example, consider the expression in this WHERE clause:

5-6 Oracle Database Application Developer’s Guide - Fundamentals

When to Use Function-Based Indexes

CREATE INDEX Idx ON Example_tab(Column_a + Column_b);
SELECT * FROM Example_tab WHERE Column_a + Column_b < 10;

The optimizer can use a range scan for this query because the index is built on
(column_a + column_b). Range scans typically produce fast response times if the
predicate selects less than 15% of the rows of a large table. The optimizer can
estimate how many rows are selected by expressions more accurately if the
expressions are materialized in a function-based index. (Expressions of
function-based indexes are represented as virtual columns and ANALYZE can build
histograms on such columns.)
■

■

■

Precompute the value of a computationally intensive function and store it in the index. An
index can store computationally intensive expression that you access often. When
you need to access a value, it is already computed, greatly improving query
execution performance.
Create indexes on object columns and REF columns. Methods that describe objects can
be used as functions on which to build indexes. For example, you can use the MAP
method to build indexes on an object type column.
Create more powerful sorts. You can perform case-insensitive sorts with the UPPER
and LOWER functions, descending order sorts with the DESC keyword, and
linguistic-based sorts with the NLSSORT function.
Note: Oracle Database sorts columns with the DESC keyword in
descending order. Such indexes are treated as function-based indexes.
Descending indexes cannot be bitmapped or reverse, and cannot be
used in bitmapped optimizations. To get the DESC functionality prior
to Oracle Database version 8, remove the DESC keyword from the
CREATE INDEX statement.

Another function-based index calls the object method distance_from_equator for
each city in the table. The method is applied to the object column Reg_Obj. A query
could use this index to quickly find cities that are more than 1000 miles from the
equator:
CREATE INDEX Distance_index
ON Weatherdata_tab (Distance_from_equator (Reg_obj));
SELECT * FROM Weatherdata_tab
WHERE (Distance_from_equator (Reg_Obj)) > '1000';

Another index stores the temperature delta and the maximum temperature. The result
of the delta is sorted in descending order. A query could use this index to quickly find
table rows where the temperature delta is less than 20 and the maximum temperature
is greater than 75.
CREATE INDEX compare_index
ON Weatherdata_tab ((Maxtemp - Mintemp) DESC, Maxtemp);
SELECT * FROM Weatherdata_tab
WHERE ((Maxtemp - Mintemp) < '20' AND Maxtemp > '75');

Examples of Function-Based Indexes
This section presents several examples of function-based indexes.

Using Indexes in Application Development

5-7

When to Use Function-Based Indexes

Example: Function-Based Index for Case-Insensitive Searches
The following command allows faster case-insensitive searches in table EMP_TAB.
CREATE INDEX Idx ON Emp_tab (UPPER(Ename));

The SELECT command uses the function-based index on UPPER(e_name) to return all
of the employees with name like :KEYCOL.
SELECT * FROM Emp_tab WHERE UPPER(Ename) like :KEYCOL;

Example: Precomputing Arithmetic Expressions with a Function-Based Index
The following command computes a value for each row using columns A, B, and C,
and stores the results in the index.
CREATE INDEX Idx ON Fbi_tab (A + B * (C - 1), A, B);

The SELECT statement can either use index range scan (since the expression is a prefix
of index IDX) or index fast full scan (which may be preferable if the index has
specified a high parallel degree).
SELECT a FROM Fbi_tab WHERE A + B * (C - 1) < 100;

Example: Function-Based Index for Language-Dependent Sorting
This example demonstrates how a function-based index can be used to sort based on
the collation order for a national language. The NLSSORT function returns a sort key
for each name, using the collation sequence GERMAN.
CREATE INDEX Nls_index
ON Nls_tab (NLSSORT(Name, 'NLS_SORT = German'));

The SELECT statement selects all of the contents of the table and orders it by NAME.
The rows are ordered using the German collation sequence. The Globalization Support
parameters are not needed in the SELECT statement, because in a German session,
NLS_SORT is set to German and NLS_COMP is set to ANSI.
SELECT * FROM Nls_tab WHERE Name IS NOT NULL
ORDER BY Name;

Restrictions for Function-Based Indexes
Note the following restrictions for function-based indexes:
■

■

Only cost-based optimization can use function-based indexes. Remember to call
DBMS_STATS.GATHER_TABLE_STATISTICS or DBMS_STATS.GATHER_
SCHEMA_STATISTICS, for the function-based index to be effective.
Any top-level or package-level PL/SQL functions that are used in the index
expression must be declared as DETERMINISTIC. That is, they always return the
same result given the same input, for example, the UPPER function. You must
ensure that the subprogram really is deterministic, because Oracle Database does
not check that the assertion is true.
The following semantic rules demonstrate how to use the keyword
DETERMINISTIC:
■
■

You can declare a top level subprogram as DETERMINISTIC.
You can declare a PACKAGE level subprogram as DETERMINISTIC in the
PACKAGE specification but not in the PACKAGE BODY. Errors are raised if
DETERMINISTIC is used inside a PACKAGE BODY.

5-8 Oracle Database Application Developer’s Guide - Fundamentals

When to Use Function-Based Indexes

■

■

■

■

You can declare a private subprogram (declared inside another subprogram or
a PACKAGE BODY) as DETERMINISTIC.
A DETERMINISTIC subprogram can call another subprogram whether the
called program is declared as DETERMINISTIC or not.

If you change the semantics of a DETERMINISTIC function and recompile it, then
existing function-based indexes and materialized views report results for the prior
version of the function. Thus, if you change the semantics of a function, you must
manually rebuild any dependent function-based indexes and materialized views.
Expressions in a function-based index cannot contain any aggregate functions. The
expressions should reference only columns in a row in the table.

■

You must analyze the table or index before the index is used.

■

Bitmap optimizations cannot use descending indexes.

■

Function-based indexes are not used when OR-expansion is done.

■

■

The index function cannot be marked NOT NULL. To avoid a full table scan, you
must ensure that the query cannot fetch null values.
Function-based indexes cannot use expressions that return VARCHAR2 or RAW data
types of unknown length from PL/SQL functions. A workaround is to limit the
size of the function's output by indexing a substring of known length:
-- INITIALS() might return 1 letter, 2 letters, 3 letters, and so on.
-- We limit the return value to 10 characters for purposes of the index.
CREATE INDEX func_substr_index ON
emp_tab(substr(initials(ename),1,10);
-- Call SUBSTR both when creating the index and when referencing
-- the function in queries.
SELECT SUBSTR(initials(ename),1,10) FROM emp_tab;

See Also: Oracle Database SQL Reference for an account of CREATE
FUNCTION restrictions

Using Indexes in Application Development

5-9

When to Use Function-Based Indexes

5-10 Oracle Database Application Developer’s Guide - Fundamentals

6
Maintaining Data Integrity in Application
Development
This chapter explains how to enforce the business rules associated with your database
and prevent the entry of invalid information into tables by using integrity constraints.
Topics include the following:
■

Overview of Integrity Constraints

■

Enforcing Referential Integrity with Constraints

■

Managing Constraints That Have Associated Indexes

■

Guidelines for Indexing Foreign Keys

■

About Referential Integrity in a Distributed Database

■

When to Use CHECK Integrity Constraints

■

Examples of Defining Integrity Constraints

■

Enabling and Disabling Integrity Constraints

■

Altering Integrity Constraints

■

Dropping Integrity Constraints

■

Managing FOREIGN KEY Integrity Constraints

■

Viewing Definitions of Integrity Constraints

Overview of Integrity Constraints
You can define integrity constraints to enforce business rules on data in your tables.
Business rules specify conditions and relationships that must always be true, or must
always be false. Because each company defines its own policies about things like
salaries, employee numbers, inventory tracking, and so on, you can specify a different
set of rules for each database table.
When an integrity constraint applies to a table, all data in the table must conform to
the corresponding rule. When you issue a SQL statement that modifies data in the
table, Oracle Database ensures that the new data satisfies the integrity constraint,
without the need to do any checking within your program.

When to Enforce Business Rules with Integrity Constraints
You can enforce rules by defining integrity constraints more reliably than by adding
logic to your application. Oracle Database can check that all the data in a table obeys
an integrity constraint faster than an application can.

Maintaining Data Integrity in Application Development

6-1

Overview of Integrity Constraints

Example of an Integrity Constraint for a Business Rule
To ensure that each employee works for a valid department, first create a rule that all
values in the department table are unique:
ALTER TABLE Dept_tab
ADD PRIMARY KEY (Deptno);

Then, create a rule that every department listed in the employee table must match one
of the values in the department table:
ALTER TABLE Emp_tab
ADD FOREIGN KEY (Deptno) REFERENCES Dept_tab(Deptno);

When you add a new employee record to the table, Oracle Database automatically
checks that its department number appears in the department table.
To enforce this rule without integrity constraints, you can use a trigger to query the
department table and test that each new employee's department is valid. But this
method is less reliable than the integrity constraint. SELECT in Oracle Database uses
"consistent read", so the query might miss uncommitted changes from other
transactions.

When to Enforce Business Rules in Applications
You might enforce business rules through application logic as well as through integrity
constraints, if you can filter out bad data before attempting an insert or update. This
might let you provide instant feedback to the user, and reduce the load on the
database. This technique is appropriate when you can determine that data values are
wrong or out of range without checking against any data already in the table.

Creating Indexes for Use with Constraints
All enabled unique and primary keys require corresponding indexes. You should
create these indexes by hand, rather than letting the database create them for you.
Note that:
■
■

Constraints use existing indexes where possible, rather than creating new ones.
Unique and primary keys can use non-unique as well as unique indexes. They can
even use just the first few columns of non-unique indexes.

■

At most one unique or primary key can use each non-unique index.

■

The column orders in the index and the constraint do not need to match.

■

If you need to check whether an index is used by a constraint, for example when
you want to drop the index, the object number of the index used by a unique or
primary key constraint is stored in CDEF$.ENABLED for that constraint. It is not
shown in any catalog view.

You should almost always index foreign keys; the database does not do this for you
automatically.

When to Use NOT NULL Integrity Constraints
By default, all columns can contain nulls. Only define NOT NULL constraints for
columns of a table that absolutely require values at all times.
For example, a new employee's manager or hire date might be temporarily omitted.
Some employees might not have a commission. Columns like these should not have

6-2 Oracle Database Application Developer’s Guide - Fundamentals

Overview of Integrity Constraints

NOT NULL integrity constraints. However, an employee name might be required from
the very beginning, and you can enforce this rule with a NOT NULL integrity constraint.
NOT NULL constraints are often combined with other types of integrity constraints to
further restrict the values that can exist in specific columns of a table. Use the
combination of NOT NULL and UNIQUE key integrity constraints to force the input of
values in the UNIQUE key; this combination of data integrity rules eliminates the
possibility that any new row' data will ever attempt to conflict with an existing row's
data.
Because Oracle Database indexes do not store keys that are all null, if you want to
allow index-only scans of the table or some other operation that requires indexing all
rows, you must put a NOT NULL constraint on at least one indexed column.
See Also:

"Defining Relationships Between Parent and Child Tables"

on page 6-8
A NOT NULL constraint is specified like this:
ALTER TABLE emp MODIFY ename NOT NULL;

Figure 6–1 shows an example of a table with NOT NULL integrity constraints.
Figure 6–1 Table with NOT NULL Integrity Constraints

Table EMPLOYEES
ID

LNAME

JOB

MGR

HIREDATE

100
101
102
103

King
Kochhar
De Hann
Hunold

AD_PRES
AD_VP
AD_VP
IT_PROG

100
100
102

17–JUN–87
21–SEP–89
13–JAN–93
03–JAN–90

SAL
24000
17000
17000
9000

COMM

DEPTNO
90
90
90
60

Absence of NOT NULL Constraint
(any row can contain a null
for this column)

NOT NULL Constraint
(no row may contain a null
value for this column)

When to Use Default Column Values
Assign default values to columns that contain a typical value. For example, in the
DEPT_TAB table, if most departments are located at one site, then the default value for
the LOC column can be set to this value (such as NEW YORK).
Default values can help avoid errors where there is a number, such as zero, that
applies to a column that has no entry. For example, a default value of zero can simplify
testing, by changing a test like this:
IF sal IS NOT NULL AND sal < 50000

to the simpler form:
IF sal < 50000

Depending upon your business rules, you might use default values to represent zero
or false, or leave the default values as NULL to signify an unknown value.
Defaults are also useful when you use a view to make a subset of a table's columns
visible. For example, you might allow users to insert rows through a view. The base
table might also have a column named INSERTER, not included in the definition of the

Maintaining Data Integrity in Application Development

6-3

Overview of Integrity Constraints

view, to log the user that inserts each row. To record the user name automatically,
define a default value that calls the USER function:
CREATE TABLE
(
value1
value2
inserter
);

audit_trail
NUMBER,
VARCHAR2(32),
VARCHAR2(30) DEFAULT USER

Setting Default Column Values
Default values can be defined using any literal, or almost any expression, including
calls to the following:
■

SYSDATE

■

SYS_CONTEXT

■

USER

■

USERENV

■

UID

Default values cannot include expressions that refer to a sequence, PL/SQL function,
column, LEVEL, ROWNUM, or PRIOR. The datatype of a default literal or expression
must match or be convertible to the column datatype.
Sometimes the default value is the result of a SQL function. For example, a call to
SYS_CONTEXT can set a different default value depending on conditions such as the
user name. To be used as a default value, a SQL function must have parameters that
are all literals, cannot reference any columns, and cannot call any other functions.
If you do not explicitly define a default value for a column, the default for the column
is implicitly set to NULL.
You can use the keyword DEFAULT within an INSERT statement instead of a literal
value, and the corresponding default value is inserted.

Choosing a Table's Primary Key
Each table can have one primary key, which uniquely identifies each row in a table
and ensures that no duplicate rows exist. Use the following guidelines when selecting
a primary key:
■

■

■

■

■

Whenever practical, use a column containing a sequence number. It is a simple
way to satisfy all the other guidelines.
Choose a column whose data values are unique, because the purpose of a primary
key is to uniquely identify each row of the table.
Choose a column whose data values are never changed. A primary key value is
only used to identify a row in the table, and its data should never be used for any
other purpose. Therefore, primary key values should rarely or never be changed.
Choose a column that does not contain any nulls. A PRIMARY KEY constraint, by
definition, does not allow any row to contain a null in any column that is part of
the primary key.
Choose a column that is short and numeric. Short primary keys are easy to type.
You can use sequence numbers to easily generate numeric primary keys.

6-4 Oracle Database Application Developer’s Guide - Fundamentals

Overview of Integrity Constraints

Minimize your use of composite primary keys. Although composite primary keys
are allowed, they do not satisfy all of the other recommendations. For example,
composite primary key values are long and cannot be assigned by sequence
numbers.

■

When to Use UNIQUE Key Integrity Constraints
Choose columns for unique keys carefully. The purpose of these constraints is different
from that of primary keys. Unique key constraints are appropriate for any column
where duplicate values are not allowed. Primary keys identify each row of the table
uniquely, and typically contain values that have no significance other than being
unique. Figure 6–2 shows an example of a table with a unique key constraint.
Figure 6–2 Table with a UNIQUE Key Constraint

Table DEPARTMENTS
DEPID

DNAME

LOC

10
20
30
40

Administration
Marketing
Purchasing
Human Resources

1700
1800
1700
2400

UNIQUE Key Constraint
(no row may duplicate a
value in the constraint's
column)

INSERT
INTO
50

MARKETING

60

1700

This row violates the UNIQUE key constraint,
because "MARKETING" is already present in another
row; therefore, it is not allowed in the table.

2400

This row is allowed because a null value is
entered for the DNAME column; however, if a
NOT NULL constraint is also defined on the
DNAME column, this row is not allowed.

You cannot have identical values in the non-null columns of a
composite UNIQUE key constraint (UNIQUE key constraints allow
NULL values).

Note:

Some examples of good unique keys include:
■

■
■

■

An employee social security number (the primary key might be the employee
number)
A truck license plate number (the primary key might be the truck number)
A customer phone number, consisting of the two columns AREA_CODE and
LOCAL_PHONE (the primary key might be the customer number)
A department name and location (the primary key might be the department
number)

Constraints On Views: for Performance, Not Data Integrity
The constraints discussed throughout this chapter apply to tables, not views.
Although you can declare constraints on views, such constraints do not help maintain
data integrity. Instead, they are used to enable query rewrites on queries involving
Maintaining Data Integrity in Application Development

6-5

Enforcing Referential Integrity with Constraints

views, which helps performance with materialized views and other data warehousing
features. Such constraints are always declared with the DISABLE keyword, and you
cannot use the VALIDATE keyword. The constraints are never enforced, and there is no
associated index.
See Also: Oracle Database Data Warehousing Guide for information on
query rewrite, materialized views, and the performance reasons for
declaring constraints on views

Enforcing Referential Integrity with Constraints
Whenever two tables contain one or more common columns, Oracle Database can
enforce the relationship between the two tables through a referential integrity
constraint. Define a PRIMARY or UNIQUE key constraint on the column in the parent
table (the one that has the complete set of column values). Define a FOREIGN KEY
constraint on the column in the child table (the one whose values must refer to existing
values in the other table).
"Defining Relationships Between Parent and Child Tables"
on page 6-8 for information on defining additional integrity
constraints, including the foreign key

See Also:

Figure 6–3 shows a foreign key defined on the department number. It guarantees that
every value in this column must match a value in the primary key of the department
table. This constraint prevents erroneous department numbers from getting into the
employee table.
Foreign keys can be comprised of multiple columns. Such a composite foreign key
must reference a composite primary or unique key of the exact same structure, with
the same number of columns and the same datatypes. Because composite primary and
unique keys are limited to 32 columns, a composite foreign key is also limited to 32
columns.

6-6 Oracle Database Application Developer’s Guide - Fundamentals

Enforcing Referential Integrity with Constraints

Figure 6–3 Tables with Referential Integrity Constraints
Parent Key
Primary key of
referenced table

Table DEPARTMENTS
DEPID

DNAME

LOC

10
20
30
40

Administration
Marketing
Purchasing
Human Resources

1700
1800
1700
2400

Foreign Key
(values in dependent
table must match a value
in unique key or primary
key of referenced table)

Referenced or
Parent Table

Table EMPLOYEES
ID

LNAME

JOB

MGR

HIREDATE

100
101
102
103

King
Kochhar
De Hann
Hunold

AD_PRES
AD_VP
AD_VP
IT_PROG

100
100
102

17–JUN–87
21–SEP–89
13–JAN–93
03–JAN–90

SAL

COMM

DEPTNO

24000
17000
17000
9000

90
90
90
60

25

Dependent or Child Table

INSERT
INTO
556

CRICKET

PU_CLERK

31–OCT–96

5000

556

CRICKET

PU_CLERK

31–OCT–96

5000

This row violates
the referential
constraint
because "50"
is not present
in the referenced
table's primary
key; therefore,
the row is not
allowed in
the table.

This row is
allowed in the
table because a
null value is
entered in the
DEPTNO column;
however, if a not
null constraint is
also defined for
this column, this
row is not allowed.

About Nulls and Foreign Keys
Foreign keys allow key values that are all NULL, even if there are no matching
PRIMARY or UNIQUE keys.
■

■

By default (without any NOT NULL or CHECK clauses), the FOREIGN KEY constraint
enforces the "match none" rule for composite foreign keys in the ANSI/ISO
standard.
To enforce the match full rule for NULL values in composite foreign keys, which
requires that all components of the key be NULL or all be non-NULL, define a
CHECK constraint that allows only all nulls or all non-nulls in the composite
foreign key. For example, with a composite key comprised of columns A, B, and C:
CHECK ((A IS NULL AND B IS NULL AND C IS NULL) OR
(A IS NOT NULL AND B IS NOT NULL AND C IS NOT NULL))

■

In general, it is not possible to use declarative referential integrity to enforce the
match partial rule for NULL values in composite foreign keys, which requires the
Maintaining Data Integrity in Application Development

6-7

Enforcing Referential Integrity with Constraints

non-NULL portions of the key to appear in the corresponding portions in the
primary or unique key of a single row in the referenced table. You can often use
triggers to handle this case, as described in Chapter 9, "Coding Triggers".

Defining Relationships Between Parent and Child Tables
Several relationships between parent and child tables can be determined by the other
types of integrity constraints defined on the foreign key in the child table.
No Constraints on the Foreign Key When no other constraints are defined on the
foreign key, any number of rows in the child table can reference the same parent key
value. This model allows nulls in the foreign key.

This model establishes a one-to-many relationship between the parent and foreign
keys that allows undetermined values (nulls) in the foreign key. An example of such a
relationship is shown in Figure 6–3, "Tables with Referential Integrity Constraints"
between the employee and department tables. Each department (parent key) has
many employees (foreign key), and some employees might not be in a department
(nulls in the foreign key).
NOT NULL Constraint on the Foreign Key When nulls are not allowed in a foreign
key, each row in the child table must explicitly reference a value in the parent key
because nulls are not allowed in the foreign key.

Any number of rows in the child table can reference the same parent key value, so this
model establishes a one-to-many relationship between the parent and foreign keys.
However, each row in the child table must have a reference to a parent key value; the
absence of a value (a null) in the foreign key is not allowed. The same example in the
previous section can be used to illustrate such a relationship. However, in this case,
employees must have a reference to a specific department.
When a UNIQUE constraint is defined on the
foreign key, only one row in the child table can reference a given parent key value.
This model allows nulls in the foreign key.

UNIQUE Constraint on the Foreign Key

This model establishes a one-to-one relationship between the parent and foreign keys
that allows undetermined values (nulls) in the foreign key. For example, assume that
the employee table had a column named MEMBERNO, referring to an employee
membership number in the company insurance plan. Also, a table named INSURANCE
has a primary key named MEMBERNO, and other columns of the table keep respective
information relating to an employee insurance policy. The MEMBERNO in the employee
table should be both a foreign key and a unique key:
■

■

To enforce referential integrity rules between the EMP_TAB and INSURANCE tables
(the FOREIGN KEY constraint)
To guarantee that each employee has a unique membership number (the UNIQUE
key constraint)

When both UNIQUE and
NOT NULL constraints are defined on the foreign key, only one row in the child table
can reference a given parent key value, and because NULL values are not allowed in
the foreign key, each row in the child table must explicitly reference a value in the
parent key.
UNIQUE and NOT NULL Constraints on the Foreign Key

This model establishes a one-to-one relationship between the parent and foreign keys
that does not allow undetermined values (nulls) in the foreign key. If you expand the
previous example by adding a NOT NULL constraint on the MEMBERNO column of the

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Enforcing Referential Integrity with Constraints

employee table, in addition to guaranteeing that each employee has a unique
membership number, you also ensure that no undetermined values (nulls) are allowed
in the MEMBERNO column of the employee table.

Rules for Multiple FOREIGN KEY Constraints
Oracle Database allows a column to be referenced by multiple FOREIGN KEY
constraints; there is no limit on the number of dependent keys. This situation might be
present if a single column is part of two different composite foreign keys.

Deferring Constraint Checks
When Oracle Database checks a constraint, it signals an error if the constraint is not
satisfied. You can use the SET CONSTRAINTS statement to defer checking the validity
of constraints until the end of a transaction.
Note: You cannot issue a SET CONSTRAINTS statement inside a
trigger.

The SET CONSTRAINTS setting lasts for the duration of the transaction, or until
another SET CONSTRAINTS statement resets the mode.
See Also:

Oracle Database SQL Reference

Guidelines for Deferring Constraint Checks
Consider the following guidelines when deferring constraint checks.
Select Appropriate Data You may wish to defer constraint checks on UNIQUE and
FOREIGN keys if the data you are working with has any of the following
characteristics:
■
■

■

Tables are snapshots.
Some tables contain a large amount of data being manipulated by another
application, which may or may not return the data in the same order.
Update cascade operations on foreign keys.

Ensure Constraints Are Created Deferrable After you have identified and selected the
appropriate tables, make sure their FOREIGN, UNIQUE and PRIMARY key constraints
are created deferrable. You can do so by issuing statements similar to the following:
CREATE TABLE dept (
deptno NUMBER PRIMARY KEY,
dname VARCHAR2 (30)
);
CREATE TABLE emp (
empno NUMBER,
ename VARCHAR2 (30),
deptno NUMBER REFERENCES (dept),
CONSTRAINT epk PRIMARY KEY (empno) DEFERRABLE,
CONSTRAINT efk FOREIGN KEY (deptno)
REFERENCES (dept.deptno) DEFERRABLE);
INSERT INTO dept VALUES (10, 'Accounting');
INSERT INTO dept VALUES (20, 'SALES');
INSERT INTO emp VALUES (1, 'Corleone', 10);
INSERT INTO emp VALUES (2, 'Costanza', 20);
COMMIT;

Maintaining Data Integrity in Application Development

6-9

Managing Constraints That Have Associated Indexes

SET CONSTRAINT efk DEFERRED;
UPDATE dept SET deptno = deptno + 10
WHERE deptno = 20;
SELECT * from emp ORDER BY deptno;
EMPNO
ENAME
DEPTNO
------------------ ------1
Corleone
10
2
Costanza
20
UPDATE emp SET deptno = deptno + 10
WHERE deptno = 20;
SELECT * FROM emp ORDER BY deptno;
EMPNO
----1
2
COMMIT;

ENAME
-------------Corleone
Costanza

DEPTNO
------10
30

Set All Constraints Deferred Within the application that manipulates the data, you must
set all constraints deferred before you begin processing any data. Use the following
DML statement to set all constraints deferred:
SET CONSTRAINTS ALL DEFERRED;

Note: The SET CONSTRAINTS statement applies only to the current
transaction. The defaults specified when you create a constraint
remain as long as the constraint exists. The ALTER SESSION SET
CONSTRAINTS statement applies for the current session only.

Check the Commit (Optional) You can check for constraint violations before committing
by issuing the SET CONSTRAINTS ALL IMMEDIATE statement just before issuing the
COMMIT. If there are any problems with a constraint, this statement will fail and the
constraint causing the error will be identified. If you commit while constraints are
violated, the transaction will be rolled back and you will receive an error message.

Managing Constraints That Have Associated Indexes
When you create a UNIQUE or PRIMARY key, Oracle Database checks to see if an
existing index can be used to enforce uniqueness for the constraint. If there is no such
index, the database creates one.

Minimizing Space and Time Overhead for Indexes Associated with Constraints
When Oracle Database uses a unique index to enforce a constraint, and constraints
associated with the unique index are dropped or disabled, the index is dropped. To
preserve the statistics associated with the index (because, for example, it would take a
long time to re-create it), you can specify the KEEP INDEX clause on the DROP
command for the constraint.
While enabled foreign keys reference a PRIMARY or UNIQUE key, you cannot disable or
drop the PRIMARY or UNIQUE key constraint or the index.
UNIQUE and PRIMARY keys with deferrable constraints must
all use non-unique indexes.

Note:

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When to Use CHECK Integrity Constraints

To reuse existing indexes when creating unique and primary key constraints, you can
include USING INDEX in the constraint clause. For example:
CREATE TABLE b
(
b1 INTEGER,
b2 INTEGER,
CONSTRAINT unique1 (b1, b2) USING INDEX (CREATE UNIQUE INDEX b_index on b(b1,
b2),
CONSTRAINT unique2 (b1, b2) USING INDEX b_index
);

Guidelines for Indexing Foreign Keys
You should almost always index foreign keys. The only exception is when the
matching unique or primary key is never updated or deleted.
Oracle Database Concepts for information on locking
mechanisms involving indexes and keys

See Also:

About Referential Integrity in a Distributed Database
The declaration of a referential integrity constraint cannot specify a foreign key that
references a primary or unique key of a remote table.
However, you can maintain parent/child table relationships across nodes using
triggers.
See Also: Chapter 9, "Coding Triggers" for more information about
triggers that enforce referential integrity

If you decide to define referential integrity across the nodes of
a distributed database using triggers, be aware that network failures
can make both the parent table and the child table inaccessible.

Note:

For example, assume that the child table is in the SALES database, and
the parent table is in the HQ database.
If the network connection between the two databases fails, then some
DML statements against the child table (those that insert rows or
update a foreign key value) cannot proceed, because the referential
integrity triggers must have access to the parent table in the HQ
database.

When to Use CHECK Integrity Constraints
Use CHECK constraints when you need to enforce integrity rules based on logical
expressions, such as comparisons. Never use CHECK constraints when any of the other
types of integrity constraints can provide the necessary checking.
See Also: "Choosing Between CHECK and NOT NULL Integrity
Constraints" on page 6-13

Examples of CHECK constraints include the following:
■

A CHECK constraint on employee salaries so that no salary value is greater than
10000.
Maintaining Data Integrity in Application Development 6-11

When to Use CHECK Integrity Constraints

■

■

A CHECK constraint on department locations so that only the locations "BOSTON",
"NEW YORK", and "DALLAS" are allowed.
A CHECK constraint on the salary and commissions columns to prevent the
commission from being larger than the salary.

Restrictions on CHECK Constraints
A CHECK integrity constraint requires that a condition be true or unknown for every
row of the table. If a statement causes the condition to evaluate to false, then the
statement is rolled back. The condition of a CHECK constraint has the following
limitations:
■

■
■

■

The condition must be a boolean expression that can be evaluated using the values
in the row being inserted or updated.
The condition cannot contain subqueries or sequences.
The condition cannot include the SYSDATE, UID, USER, or USERENV SQL
functions.
The condition cannot contain the pseudocolumns LEVEL, PRIOR, or ROWNUM.
Oracle Database SQL Reference for an explanation of these
pseudocolumns

See Also:

■

The condition cannot contain a user-defined SQL function.

Designing CHECK Constraints
When using CHECK constraints, remember that a CHECK constraint is violated only if
the condition evaluates to false; true and unknown values (such as comparisons with
nulls) do not violate a check condition. Make sure that any CHECK constraint that you
define is specific enough to enforce the rule.
For example, consider the following CHECK constraint:
CHECK (Sal > 0 OR Comm >= 0)

At first glance, this rule may be interpreted as "do not allow a row in the employee
table unless the employee salary is greater than zero or the employee commission is
greater than or equal to zero." But if a row is inserted with a null salary, that row does
not violate the CHECK constraint, regardless of whether or not the commission value is
valid, because the entire check condition is evaluated as unknown. In this case, you
can prevent such violations by placing NOT NULL integrity constraints on both the SAL
and COMM columns.
Note: If you are not sure when unknown values result in NULL
conditions, review the truth tables for the logical operators AND and
OR in Oracle Database SQL Reference

Rules for Multiple CHECK Constraints
A single column can have multiple CHECK constraints that reference the column in its
definition. There is no limit to the number of CHECK constraints that can be defined
that reference a column.
The order in which the constraints are evaluated is not defined, so be careful not to
rely on the order or to define multiple constraints that conflict with each other.
6-12 Oracle Database Application Developer’s Guide - Fundamentals

Examples of Defining Integrity Constraints

Choosing Between CHECK and NOT NULL Integrity Constraints
According to the ANSI/ISO standard, a NOT NULL integrity constraint is an example of
a CHECK integrity constraint, where the condition is the following:
CHECK (Column_name IS NOT NULL)

Therefore, NOT NULL integrity constraints for a single column can, in practice, be
written in two forms: using the NOT NULL constraint or a CHECK constraint. For ease of
use, you should always choose to define NOT NULL integrity constraints, instead of
CHECK constraints with the IS NOT NULL condition.
In the case where a composite key can allow only all nulls or all values, you must use a
CHECK integrity constraint. For example, the following expression of a CHECK integrity
constraint allows a key value in the composite key made up of columns C1 and C2 to
contain either all nulls or all values:
CHECK ((C1 IS NULL AND C2 IS NULL) OR
(C1 IS NOT NULL AND C2 IS NOT NULL))

Examples of Defining Integrity Constraints
Here are some examples showing how to create simple constraints during the
prototype phase of your database design.
Each constraint is given a name in these examples. Naming the constraints prevents
the database from creating multiple copies of the same constraint, with different
system-generated names, if the DDL is run multiple times.
See Also: Oracle Database Administrator's Guide for information on
creating and maintaining constraints for a large production database

Example: Defining Integrity Constraints with the CREATE TABLE Command
The following examples of CREATE TABLE statements show the definition of several
integrity constraints:
CREATE TABLE Dept_tab (
Deptno NUMBER(3) CONSTRAINT Dept_pkey PRIMARY KEY,
Dname
VARCHAR2(15),
Loc
VARCHAR2(15),
CONSTRAINT Dname_ukey UNIQUE (Dname, Loc),
CONSTRAINT Loc_check1
CHECK (loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')));
CREATE TABLE
Empno
Ename
Job
Mgr
Hiredate
Sal
Comm
Deptno

Emp_tab (
NUMBER(5) CONSTRAINT Emp_pkey PRIMARY KEY,
VARCHAR2(15) NOT NULL,
VARCHAR2(10),
NUMBER(5) CONSTRAINT Mgr_fkey REFERENCES Emp_tab,
DATE,
NUMBER(7,2),
NUMBER(5,2),
NUMBER(3) NOT NULL
CONSTRAINT dept_fkey REFERENCES Dept_tab ON DELETE CASCADE);

Maintaining Data Integrity in Application Development 6-13

Enabling and Disabling Integrity Constraints

Example: Defining Constraints with the ALTER TABLE Command
You can also define integrity constraints using the constraint clause of the ALTER
TABLE command. For example:
CREATE UNIQUE INDEX I_dept ON Dept_tab(deptno);
ALTER TABLE Dept_tab
ADD CONSTRAINT Dept_pkey PRIMARY KEY (deptno);
ALTER TABLE Emp_tab
ADD CONSTRAINT Dept_fkey FOREIGN KEY (Deptno) REFERENCES Dept_tab;
ALTER TABLE Emp_tab MODIFY (Ename VARCHAR2(15) NOT NULL);

You cannot create a validated constraint on a table if the table already contains any
rows that would violate the constraint.

Privileges Required to Create Constraints
The creator of a constraint must have the ability to create tables (the CREATE TABLE or
CREATE ANY TABLE system privilege), or the ability to alter the table (the ALTER object
privilege for the table or the ALTER ANY TABLE system privilege) with the constraint.
Additionally, UNIQUE and PRIMARY KEY integrity constraints require that the owner
of the table have either a quota for the tablespace that contains the associated index or
the UNLIMITED TABLESPACE system privilege. FOREIGN KEY integrity constraints
also require some additional privileges.
See Also: "Privileges Required to Create FOREIGN KEY Integrity
Constraints" on page 6-20

Naming Integrity Constraints
Assign names to constraints NOT NULL, UNIQUE KEY, PRIMARY KEY, FOREIGN KEY,
and CHECK using the CONSTRAINT option of the constraint clause. This name must be
unique with respect to other constraints that you own. If you do not specify a
constraint name, one is assigned automatically by Oracle Database.
Picking your own name makes error messages for constraint violations more
understandable, and prevents the creation of duplicate constraints with different
names if the SQL statements are run more than once.
See the previous examples of the CREATE TABLE and ALTER TABLE statements for
examples of the CONSTRAINT option of the constraint clause. Note that the name
of each constraint is included with other information about the constraint in the data
dictionary.
"Viewing Definitions of Integrity Constraints" on
page 6-21 for examples of data dictionary views

See Also:

Enabling and Disabling Integrity Constraints
This section explains the mechanisms and procedures for manually enabling and
disabling integrity constraints.
enabled constraint. When a constraint is enabled, the corresponding rule is enforced
on the data values in the associated columns. The definition of the constraint is stored
in the data dictionary.
disabled constraint. When a constraint is disabled, the corresponding rule is not
enforced. The definition of the constraint is still stored in the data dictionary.

6-14 Oracle Database Application Developer’s Guide - Fundamentals

Enabling and Disabling Integrity Constraints

An integrity constraint represents an assertion about the data in a database. This
assertion is always true when the constraint is enabled. The assertion may or may not
be true when the constraint is disabled, because data that violates the integrity
constraint can be in the database.

Why Disable Constraints?
During day-to-day operations, constraints should always be enabled. In certain
situations, temporarily disabling the integrity constraints of a table makes sense for
performance reasons. For example:
■
■

■

When loading large amounts of data into a table using SQL*Loader
When performing batch operations that make massive changes to a table (such as
changing each employee number by adding 1000 to the existing number)
When importing or exporting one table at a time

Temporarily turning off integrity constraints can speed up these operations.

About Exceptions to Integrity Constraints
If a row of a table disobeys an integrity constraint, then this row is in violation of the
constraint and is called an exception to the constraint. If any exceptions exist, then the
constraint cannot be enabled. The rows that violate the constraint must be updated or
deleted before the constraint can be enabled.
You can identify exceptions for a specific integrity constraint as you try to enable the
constraint.
See Also: "Fixing Constraint Exceptions" on page 6-17 for more
information on this procedure

Enabling Constraints
When you define an integrity constraint in a CREATE TABLE or ALTER TABLE
statement, Oracle Database automatically enables the constraint by default. For code
clarity, you can explicitly enable the constraint by including the ENABLE clause in its
definition.
Use this technique when creating tables that start off empty, and are populated a row
at a time by individual transactions. In such cases, you want to ensure that data is
consistent at all times, and the performance overhead of each DML operation is small.
The following CREATE TABLE and ALTER TABLE statements both define and enable
integrity constraints:
CREATE TABLE Emp_tab (
Empno NUMBER(5) PRIMARY KEY);
ALTER TABLE Emp_tab
ADD PRIMARY KEY (Empno);

An ALTER TABLE statement that tries to enable an integrity constraint will fail if any
existing rows of the table violate the integrity constraint. The statement is rolled back
and the constraint definition is not stored and not enabled.
See Also: "Fixing Constraint Exceptions" on page 6-17 for more
information about rows that violate integrity constraints

Maintaining Data Integrity in Application Development 6-15

Enabling and Disabling Integrity Constraints

Creating Disabled Constraints
The following CREATE TABLE and ALTER TABLE statements both define and disable
integrity constraints:
CREATE TABLE Emp_tab (
Empno NUMBER(5) PRIMARY KEY DISABLE);
ALTER TABLE Emp_tab
ADD PRIMARY KEY (Empno) DISABLE;

Use this technique when creating tables that will be loaded with large amounts of data
before anybody else accesses them, particularly if you need to cleanse data after
loading it, or need to fill empty columns with sequence numbers or parent/child
relationships.
An ALTER TABLE statement that defines and disables an integrity constraints never
fails, because its rule is not enforced.

Enabling and Disabling Existing Integrity Constraints
Use the ALTER TABLE command to:
■

Enable a disabled constraint, using the ENABLE clause.

■

Disable an enabled constraint, using the DISABLE clause.

Enabling Existing Constraints
Once you have finished cleansing data and filling empty columns, you can enable
constraints that were disabled during data loading.
The following statements are examples of statements that enable disabled integrity
constraints:
ALTER TABLE Dept_tab
ENABLE CONSTRAINT Dname_ukey;
ALTER TABLE Dept_tab
ENABLE PRIMARY KEY
ENABLE UNIQUE (Dname)
ENABLE UNIQUE (Loc);

An ALTER TABLE statement that attempts to enable an integrity constraint fails if any
of the table rows violate the integrity constraint. The statement is rolled back and the
constraint is not enabled.
See Also: "Fixing Constraint Exceptions" on page 6-17 for more
information about rows that violate integrity constraints

Disabling Existing Constraints
If you need to perform a large load or update when a table already contains data, you
can temporarily disable constraints to improve performance of the bulk operation.
The following statements are examples of statements that disable enabled integrity
constraints:
ALTER TABLE Dept_tab
DISABLE CONSTRAINT Dname_ukey;
ALTER TABLE Dept_tab

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Altering Integrity Constraints

DISABLE PRIMARY KEY
DISABLE UNIQUE (Dname)
DISABLE UNIQUE (Loc);

Tip: Using the Data Dictionary to Find Constraints
The preceding examples require that you know the relevant constraint names and
which columns they affect. To find this information, you can query one of the data
dictionary views defined for constraints, USER_CONSTRAINTS or USER_CONS_
COLUMNS. For more information about these views, refer to "Viewing Definitions of
Integrity Constraints" on page 6-21 and Oracle Database Reference.

Guidelines for Enabling and Disabling Key Integrity Constraints
When enabling or disabling UNIQUE, PRIMARY KEY, and FOREIGN KEY integrity
constraints, you should be aware of several important issues and prerequisites.
UNIQUE key and PRIMARY KEY constraints are usually managed by the database
administrator.
See Also: Oracle Database Administrator's Guide and "Managing
FOREIGN KEY Integrity Constraints" on page 6-19

Fixing Constraint Exceptions
When you try to create or enable a constraint, and the statement fails because integrity
constraint exceptions exist, the statement is rolled back. You cannot enable the
constraint until all exceptions are either updated or deleted. To determine which rows
violate the integrity constraint, include the EXCEPTIONS option in the ENABLE clause
of a CREATE TABLE or ALTER TABLE statement.
See Also: Oracle Database Administrator's Guide for more information
about responding to constraint exceptions

Altering Integrity Constraints
Starting with Oracle8i, you can alter the state of an existing constraint with the
MODIFY CONSTRAINT clause.
Oracle Database SQL Reference for information on the
parameters you can modify

See Also:

MODIFY CONSTRAINT Example #1
The following commands show several alternatives for whether the CHECK constraint
is enforced, and when the constraint checking is done:
CREATE TABLE X1_tab (a1 NUMBER CONSTRAINT y CHECK (a1>3) DEFERRABLE DISABLE);
ALTER
ALTER
ALTER
ALTER

TABLE
TABLE
TABLE
TABLE

X1_tab
X1_tab
X1_tab
X1_tab

MODIFY
MODIFY
MODIFY
MODIFY

CONSTRAINT
CONSTRAINT
CONSTRAINT
CONSTRAINT

Y_cnstrt
Y_cnstrt
Y_cnstrt
Y_cnstrt

ENABLE;
RELY;
INITIALLY DEFERRED;
ENABLE NOVALIDATE;

MODIFY CONSTRAINT Example #2
The following commands show several alternatives for whether the NOT NULL
constraint is enforced, and when the checking is done:
Maintaining Data Integrity in Application Development 6-17

Altering Integrity Constraints

CREATE TABLE X1_tab (A1 NUMBER CONSTRAINT Y_cnstrt
NOT NULL DEFERRABLE INITIALLY DEFERRED NORELY DISABLE);
ALTER TABLE X1_tab ADD CONSTRAINT One_cnstrt UNIQUE(A1)
DEFERRABLE INITIALLY IMMEDIATE RELY USING INDEX PCTFREE = 30
ENABLE VALIDATE;
ALTER TABLE X1_tab MODIFY UNIQUE(A1)
INITIALLY DEFERRED NORELY USING INDEX PCTFREE = 40
ENABLE NOVALIDATE;
ALTER TABLE X1_tab MODIFY CONSTRAINT One_cnstrt
INITIALLY IMMEDIATE RELY;

Modify Constraint Example #3
The following commands show several alternatives for whether the primary key
constraint is enforced, and when the checking is done:
CREATE TABLE T1_tab (A1 INT, B1 INT);
ALTER TABLE T1_tab add CONSTRAINT P1_cnstrt PRIMARY KEY(a1) DISABLE;
ALTER TABLE T1_tab MODIFY PRIMARY KEY INITIALLY IMMEDIATE
USING INDEX PCTFREE = 30 ENABLE NOVALIDATE;
ALTER TABLE T1_tab MODIFY PRIMARY KEY
USING INDEX PCTFREE = 35 ENABLE;
ALTER TABLE T1_tab MODIFY PRIMARY KEY ENABLE NOVALIDATE;

Renaming Integrity Constraints
Because constraint names must be unique, even across multiple schemas, you can
encounter problems when you want to clone a table and all its constraints, because the
constraint name for the new table conflicts with the one for the original table. Or, you
might create a constraint with a default system-generated name, and later realize that
you want to give the constraint a name that is easy to remember, so that you can easily
enable and disable it.
One of the properties you can alter for a constraint is its name. The following SQL*Plus
script finds the system-generated name for a constraint and changes it:
prompt Enter table name to find its primary key:
accept table_name
select constraint_name from user_constraints
where table_name = upper('&table_name.')
and constraint_type = 'P';
prompt Enter new name for its primary key:
accept new_constraint
set serveroutput on
declare
-- USER_CONSTRAINTS.CONSTRAINT_NAME is declared as VARCHAR2(30).
-- Using %TYPE here protects us if the length changes in a future release.
constraint_name user_constraints.constraint_name%type;
begin
select constraint_name into constraint_name from user_constraints
where table_name = upper('&table_name.')
and constraint_type = 'P';

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Managing FOREIGN KEY Integrity Constraints

dbms_output.put_line('The primary key for ' || upper('&table_name.') || ' is: '
|| constraint_name);
execute immediate
'alter table &table_name. rename constraint ' || constraint_name ||
' to &new_constraint.';
end;
/

Dropping Integrity Constraints
Drop an integrity constraint if the rule that it enforces is no longer true or if the
constraint is no longer needed. Drop an integrity constraint using the ALTER TABLE
command and the DROP clause. For example, the following statements drop integrity
constraints:
ALTER TABLE Dept_tab
DROP UNIQUE (Dname);
ALTER TABLE Dept_tab
DROP UNIQUE (Loc);
ALTER TABLE Emp_tab
DROP PRIMARY KEY,
DROP CONSTRAINT Dept_fkey;
DROP TABLE Emp_tab CASCADE CONSTRAINTS;

When dropping UNIQUE, PRIMARY KEY, and FOREIGN KEY integrity constraints, you
should be aware of several important issues and prerequisites. UNIQUE and PRIMARY
KEY constraints are usually managed by the database administrator.
See Also: Oracle Database Administrator's Guide and "Managing
FOREIGN KEY Integrity Constraints" on page 6-19

Managing FOREIGN KEY Integrity Constraints
General information about defining, enabling, disabling, and dropping all types of
integrity constraints is given in section "Dropping Integrity Constraints". The present
section supplements this information, focusing specifically on issues regarding
FOREIGN KEY integrity constraints, which enforce relationships between columns in
different tables.
FOREIGN KEY integrity constraints cannot be enabled if the
constraint of the referenced primary or unique key is not present or
not enabled.

Note:

Datatypes and Names for Foreign Key Columns
You must use the same datatype for corresponding columns in the dependent and
referenced tables. The column names do not need to match.

Limit on Columns in Composite Foreign Keys
Because foreign keys reference primary and unique keys of the parent table, and
PRIMARY KEY and UNIQUE key constraints are enforced using indexes, composite
foreign keys are limited to 32 columns.

Maintaining Data Integrity in Application Development 6-19

Managing FOREIGN KEY Integrity Constraints

Foreign Key References Primary Key by Default
If the column list is not included in the REFERENCES option when defining a FOREIGN
KEY constraint (single column or composite), then Oracle Database assumes that you
intend to reference the primary key of the specified table. Alternatively, you can
explicitly specify the column(s) to reference in the parent table within parentheses.
Oracle Database automatically checks to verify that this column list references a
primary or unique key of the parent table. If it does not, then an informative error is
returned.

Privileges Required to Create FOREIGN KEY Integrity Constraints
To create a FOREIGN KEY constraint, the creator of the constraint must have privileged
access to the parent and child tables.
■

■

The Parent Table The creator of the referential integrity constraint must own the
parent table or have REFERENCES object privileges on the columns that constitute
the parent key of the parent table.
The Child Table The creator of the referential integrity constraint must have the
ability to create tables (that is, the CREATE TABLE or CREATE ANY TABLE system
privilege) or the ability to alter the child table (that is, the ALTER object privilege
for the child table or the ALTER ANY TABLE system privilege).

In both cases, necessary privileges cannot be obtained through a role; they must be
explicitly granted to the creator of the constraint.
These restrictions allow:
■

■

The owner of the child table to explicitly decide which constraints are enforced
and which other users can create constraints
The owner of the parent table to explicitly decide if foreign keys can depend on the
primary and unique keys in her tables

Choosing How Foreign Keys Enforce Referential Integrity
Oracle Database allows different types of referential integrity actions to be enforced, as
specified with the definition of a FOREIGN KEY constraint:
■

Prevent Delete or Update of Parent Key The default setting prevents the deletion
or update of a parent key if there is a row in the child table that references the key.
For example:
CREATE TABLE Emp_tab (
FOREIGN KEY (Deptno) REFERENCES Dept_tab);

■

Delete Child Rows When Parent Key Deleted The ON DELETE CASCADE action
allows parent key data that is referenced from the child table to be deleted, but not
updated. When data in the parent key is deleted, all rows in the child table that
depend on the deleted parent key values are also deleted. To specify this
referential action, include the ON DELETE CASCADE option in the definition of the
FOREIGN KEY constraint. For example:
CREATE TABLE Emp_tab (
FOREIGN KEY (Deptno) REFERENCES Dept_tab
ON DELETE CASCADE);

■

Set Foreign Keys to Null When Parent Key Deleted The ON DELETE SET NULL
action allows data that references the parent key to be deleted, but not updated.
When referenced data in the parent key is deleted, all rows in the child table that

6-20 Oracle Database Application Developer’s Guide - Fundamentals

Viewing Definitions of Integrity Constraints

depend on those parent key values have their foreign keys set to null. To specify
this referential action, include the ON DELETE SET NULL option in the definition
of the FOREIGN KEY constraint. For example:
CREATE TABLE Emp_tab (
FOREIGN KEY (Deptno) REFERENCES Dept_tab
ON DELETE SET NULL);

Viewing Definitions of Integrity Constraints
The data dictionary contains the following views that relate to integrity constraints:
■

ALL_CONSTRAINTS

■

ALL_CONS_COLUMNS

■

USER_CONSTRAINTS

■

USER_CONS_COLUMNS

■

DBA_CONSTRAINTS

■

DBA_CONS_COLUMNS

You can query these views to find the names of constraints, what columns they affect,
and other information to help you manage constraints.
See Also:

Oracle Database Reference for information on each view

Examples of Defining Integrity Constraints
The following CREATE TABLE statements define a number of integrity constraints:
CREATE TABLE
Dept_tab
(
Deptno
NUMBER(3) PRIMARY KEY,
Dname
VARCHAR2(15),
Loc
VARCHAR2(15),
CONSTRAINT Dname_ukey UNIQUE (Dname, Loc),
CONSTRAINT LOC_CHECK1
CHECK (Loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')));
CREATE TABLE
Emp_tab
(
Empno
NUMBER(5) PRIMARY KEY,
Ename
VARCHAR2(15) NOT NULL,
Job
VARCHAR2(10),
Mgr
NUMBER(5) CONSTRAINT Mgr_fkey
REFERENCES Emp_tab ON DELETE CASCADE,
Hiredate DATE,
Sal
NUMBER(7,2),
Comm
NUMBER(5,2),
Deptno
NUMBER(3) NOT NULL
CONSTRAINT Dept_fkey REFERENCES Dept_tab);

Example 1: Listing All of Your Accessible Constraints

The following query lists all

constraints defined on all tables accessible to the user:
SELECT Constraint_name, Constraint_type, Table_name,
R_constraint_name

Maintaining Data Integrity in Application Development 6-21

Viewing Definitions of Integrity Constraints

FROM User_constraints;

Considering the example statements at the beginning of this section, a list similar to
this is returned:
CONSTRAINT_NAME
--------------SYS_C00275
DNAME_UKEY
LOC_CHECK1
SYS_C00278
SYS_C00279
SYS_C00280
MGR_FKEY
DEPT_FKEY

C
P
U
C
C
C
P
R
R

TABLE_NAME
---------DEPT_TAB
DEPT_TAB
DEPT_TAB
EMP_TAB
EMP_TAB
EMP_TAB
EMP_TAB
EMP_TAB

R_CONSTRAINT_NAME
-----------------

SYS_C00280
SYS_C00275

Notice the following:
■

■

Some constraint names are user specified (such as DNAME_UKEY), while others are
system specified (such as SYS_C00275).
Each constraint type is denoted with a different character in the CONSTRAINT_
TYPE column. The following table summarizes the characters used for each
constraint type.

Constraint Type

Character

PRIMARY KEY

P

UNIQUE KEY

U

FOREIGN KEY

R

CHECK, NOT NULL C

An additional constraint type is indicated by the character "V"
in the CONSTRAINT_TYPE column. This constraint type corresponds
to constraints created using the WITH CHECK OPTION for views.

Note:

Example 2: Distinguishing NOT NULL Constraints from CHECK Constraints In the
previous example, several constraints are listed with a constraint type of C. To
distinguish which constraints are NOT NULL constraints and which are CHECK
constraints in the EMP_TAB and DEPT_TAB tables, submit the following query:
SELECT Constraint_name, Search_condition
FROM User_constraints
WHERE (Table_name = 'DEPT_TAB' OR Table_name = 'EMP_TAB') AND
Constraint_type = 'C';

Considering the example CREATE TABLE statements at the beginning of this section, a
list similar to this is returned:
CONSTRAINT_NAME
--------------LOC_CHECK1
SYS_C00278
SYS_C00279

SEARCH_CONDITION
---------------------------------------loc IN ('NEW YORK', 'BOSTON', 'CHICAGO')
ENAME IS NOT NULL
DEPTNO IS NOT NULL

Notice that the following are explicitly listed in the SEARCH_CONDITION column:

6-22 Oracle Database Application Developer’s Guide - Fundamentals

Viewing Definitions of Integrity Constraints

■

NOT NULL constraints

■

The conditions for user-defined CHECK constraints

The
following query lists all columns that constitute the constraints defined on all tables
accessible to you, the user:

Example 3: Listing Column Names that Constitute an Integrity Constraint

SELECT Constraint_name, Table_name, Column_name
FROM User_cons_columns;

Considering the example statements at the beginning of this section, a list similar to
this is returned:
CONSTRAINT_NAME
--------------DEPT_FKEY
DNAME_UKEY
DNAME_UKEY
LOC_CHECK1
MGR_FKEY
SYS_C00275
SYS_C00278
SYS_C00279
SYS_C00280

TABLE_NAME
----------EMP_TAB
DEPT_TAB
DEPT_TAB
DEPT_TAB
EMP_TAB
DEPT_TAB
EMP_TAB
EMP_TAB
EMP_TAB

COLUMN_NAME
--------------DEPTNO
DNAME
LOC
LOC
MGR
DEPTNO
ENAME
DEPTNO
EMPNO

Maintaining Data Integrity in Application Development 6-23

Viewing Definitions of Integrity Constraints

6-24 Oracle Database Application Developer’s Guide - Fundamentals

Part II
PL/SQL for Application Developers
This part contains the following chapters:
■

Chapter 7, "Coding PL/SQL Procedures and Packages"

■

Chapter 8, "Coding Dynamic SQL"

■

Chapter 9, "Coding Triggers"

■

Chapter 10, "Developing Flashback Applications"

■

Chapter 11, "Developing Applications with the PL/SQL Web Toolkit"

■

Chapter 12, "Developing PL/SQL Server Pages"

■

Chapter 13, "Developing Applications with Database Change Notification"

7
Coding PL/SQL Procedures and Packages
This chapter describes some of the procedural capabilities of Oracle Database for
application development, including:
■

Overview of PL/SQL Program Units

■

Compiling PL/SQL Procedures for Native Execution

■

Remote Dependencies

■

Cursor Variables

■

Handling PL/SQL Compile-Time Errors

■

Handling Run-Time PL/SQL Errors

■

Debugging Stored Procedures

■

Calling Stored Procedures

■

Calling Remote Procedures

■

Calling Stored Functions from SQL Expressions

■

Returning Large Amounts of Data from a Function

■

Coding Your Own Aggregate Functions

Overview of PL/SQL Program Units
PL/SQL is a modern, block-structured programming language. It provides several
features that make developing powerful database applications very convenient. For
example, PL/SQL provides procedural constructs, such as loops and conditional
statements, that are not available in standard SQL.
You can directly enter SQL data manipulation language (DML) statements inside
PL/SQL blocks, and you can use procedures supplied by Oracle to perform data
definition language (DDL) statements.
PL/SQL code runs on the server, so using PL/SQL lets you centralize significant parts
of your database applications for increased maintainability and security. It also enables
you to achieve a significant reduction of network overhead in client/server
applications.
Some Oracle tools, such as Oracle Forms, contain a PL/SQL
engine that lets you run PL/SQL locally.

Note:

Coding PL/SQL Procedures and Packages 7-1

Overview of PL/SQL Program Units

You can even use PL/SQL for some database applications in place of 3GL programs
that use embedded SQL or Oracle Call Interface (OCI).
PL/SQL program units include:
■

Anonymous Blocks

■

Stored Program Units (Procedures, Functions, and Packages)

■

Triggers
See Also:
■

■

Oracle Database PL/SQL User's Guide and Reference for syntax and
examples of operations on PL/SQL packages
Oracle Database PL/SQL Packages and Types Reference for
information about the PL/SQL packages that come with Oracle
Database

Anonymous Blocks
An anonymous block is a PL/SQL program unit that has no name. An anonymous
block consists of an optional declarative part, an executable part, and one or more
optional exception handlers.
The declarative part declares PL/SQL variables, exceptions, and cursors. The
executable part contains PL/SQL code and SQL statements, and can contain nested
blocks. Exception handlers contain code that is called when the exception is raised,
either as a predefined PL/SQL exception (such as NO_DATA_FOUND or ZERO_DIVIDE)
or as an exception that you define.
The following short example of a PL/SQL anonymous block prints the names of all
employees in department 20 in the hr.employees table by using the DBMS_OUTPUT
package:
DECLARE
Last_name
Cursor

VARCHAR2(10);
c1 IS SELECT last_name
FROM employees
WHERE department_id = 20;

BEGIN
OPEN c1;
LOOP
FETCH c1 INTO Last_name;
EXIT WHEN c1%NOTFOUND;
DBMS_OUTPUT.PUT_LINE(Last_name);
END LOOP;
END;
/

If you test this block using SQL*Plus, then enter the statement
SET SERVEROUTPUT ON so that output using the DBMS_OUTPUT
procedures (for example, PUT_LINE) is activated. Also, end the
example with a slash (/) to activate it.

Note:

Exceptions let you handle Oracle Database error conditions within PL/SQL program
logic. This allows your application to prevent the server from issuing an error that
could cause the client application to end. The following anonymous block handles the

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Overview of PL/SQL Program Units

predefined Oracle Database exception NO_DATA_FOUND (which would result in an
ORA-01403 error if not handled):
DECLARE
Emp_number
INTEGER := 9999;
Emp_name
VARCHAR2(10);
BEGIN
SELECT Ename INTO Emp_name FROM Emp_tab
WHERE Empno = Emp_number;
-- no such number
DBMS_OUTPUT.PUT_LINE('Employee name is ' || Emp_name);
EXCEPTION
WHEN NO_DATA_FOUND THEN
DBMS_OUTPUT.PUT_LINE('No such employee: ' || Emp_number);
END;

You can also define your own exceptions, declare them in the declaration part of a
block, and define them in the exception part of the block. An example follows:
DECLARE
Emp_name
VARCHAR2(10);
Emp_number
INTEGER;
Empno_out_of_range EXCEPTION;
BEGIN
Emp_number := 10001;
IF Emp_number > 9999 OR Emp_number < 1000 THEN
RAISE Empno_out_of_range;
ELSE
SELECT Ename INTO Emp_name FROM Emp_tab
WHERE Empno = Emp_number;
DBMS_OUTPUT.PUT_LINE('Employee name is ' || Emp_name);
END IF;
EXCEPTION
WHEN Empno_out_of_range THEN
DBMS_OUTPUT.PUT_LINE('Employee number ' || Emp_number ||
' is out of range.');
END;

Anonymous blocks are usually used interactively from a tool, such as SQL*Plus, or in
a precompiler, OCI, or SQL*Module application. They are usually used to call stored
procedures or to open cursor variables.
See Also:
■

■

■

Oracle Database PL/SQL Packages and Types Reference for complete
information about the DBMS_OUTPUT package
Oracle Database PL/SQL User's Guide and Reference and "Handling
Run-Time PL/SQL Errors" on page 7-26
"Cursor Variables" on page 7-22

Stored Program Units (Procedures, Functions, and Packages)
A stored procedure, function, or package is a PL/SQL program unit that:
■

Has a name.

■

Can take parameters, and can return values.

■

Is stored in the data dictionary.

■

Can be called by many users.

Coding PL/SQL Procedures and Packages 7-3

Overview of PL/SQL Program Units

The term stored procedure is sometimes used generically for
both stored procedures and stored functions. The only difference
between procedures and functions is that functions always return a
single value to the caller, while procedures do not return a value to the
caller.

Note:

Naming Procedures and Functions
Because a procedure or function is stored in the database, it must be named. This
distinguishes it from other stored procedures and makes it possible for applications to
call it. Each publicly-visible procedure or function in a schema must have a unique
name, and the name must be a legal PL/SQL identifier.
If you plan to call a stored procedure using a stub generated
by SQL*Module, then the stored procedure name must also be a legal
identifier in the calling host 3GL language, such as Ada or C.

Note:

Parameters for Procedures and Functions
Stored procedures and functions can take parameters. The following example shows a
stored procedure that is similar to the anonymous block in "Anonymous Blocks" on
page 7-2.
To execute the following, use CREATE OR REPLACE
PROCEDURE...

Caution:

PROCEDURE Get_emp_names (Dept_num IN NUMBER) IS
Emp_name
VARCHAR2(10);
CURSOR
c1 (Depno NUMBER) IS
SELECT Ename FROM Emp_tab
WHERE deptno = Depno;
BEGIN
OPEN c1(Dept_num);
LOOP
FETCH c1 INTO Emp_name;
EXIT WHEN C1%NOTFOUND;
DBMS_OUTPUT.PUT_LINE(Emp_name);
END LOOP;
CLOSE c1;
END;

In this stored procedure example, the department number is an input parameter which
is used when the parameterized cursor c1 is opened.
The formal parameters of a procedure have three major attributes, described in
Table 7–1.
Table 7–1

Attributes of Procedure Parameters

Parameter Attribute Description
Name

This must be a legal PL/SQL identifier.

Mode

This indicates whether the parameter is an input-only parameter (IN),
an output-only parameter (OUT), or is both an input and an output
parameter (IN OUT). If the mode is not specified, then IN is assumed.

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Table 7–1 (Cont.) Attributes of Procedure Parameters
Parameter Attribute Description
Datatype

This is a standard PL/SQL datatype.

Parameter Modes Parameter modes define the behavior of formal parameters. The three
parameter modes, IN (the default), OUT, and IN OUT, can be used with any
subprogram. However, avoid using the OUT and IN OUT modes with functions. The
purpose of a function is to take no arguments and return a single value. It is poor
programming practice to have a function return multiple values. Also, functions
should be free from side effects, which change the values of variables not local to the
subprogram.
Table 7–2 summarizes the information about parameter modes.
Table 7–2

Parameter Modes

IN

OUT

IN OUT

The default.

Must be specified.

Must be specified.

Passes values to a
subprogram.

Returns values to the caller.

Passes initial values to a
subprogram; returns updated
values to the caller.

Formal parameter acts like a Formal parameter acts like an
constant.
uninitialized variable.

Formal parameter acts like an
initialized variable.

Formal parameter cannot be Formal parameter cannot be
assigned a value.
used in an expression; must be
assigned a value.

Formal parameter should be
assigned a value.

Actual parameter can be a
Actual parameter must be a
constant, initialized variable, variable.
literal, or expression.

Actual parameter must be a
variable.

See Also: Oracle Database PL/SQL User's Guide and Reference for
details about parameter modes

Parameter Datatypes The datatype of a formal parameter consists of one of the
following:
■

An unconstrained type name, such as NUMBER or VARCHAR2.

■

A type that is constrained using the %TYPE or %ROWTYPE attributes.
Note: Numerically constrained types such as NUMBER(2) or
VARCHAR2(20) are not allowed in a parameter list.

%TYPE and %ROWTYPE Attributes Use the type attributes %TYPE and %ROWTYPE to
constrain the parameter. For example, the Get_emp_names procedure specification in
"Parameters for Procedures and Functions" on page 7-4 could be written as the
following:
PROCEDURE Get_emp_names(Dept_num IN Emp_tab.Deptno%TYPE)

This has the Dept_num parameter take the same datatype as the Deptno column in
the Emp_tab table. The column and table must be available when a declaration using
%TYPE (or %ROWTYPE) is elaborated.

Coding PL/SQL Procedures and Packages 7-5

Overview of PL/SQL Program Units

Using %TYPE is recommended, because if the type of the column in the table changes,
then it is not necessary to change the application code.
If the Get_emp_names procedure is part of a package, then you can use
previously-declared public (package) variables to constrain a parameter datatype. For
example:
Dept_number
number(2);
...
PROCEDURE Get_emp_names(Dept_num IN Dept_number%TYPE);

Use the %ROWTYPE attribute to create a record that contains all the columns of the
specified table. The following example defines the Get_emp_rec procedure, which
returns all the columns of the Emp_tab table in a PL/SQL record for the given empno:
To execute the following, use CREATE OR REPLACE
PROCEDURE...

Caution:

PROCEDURE Get_emp_rec (Emp_number IN Emp_tab.Empno%TYPE,
Emp_ret
OUT Emp_tab%ROWTYPE) IS
BEGIN
SELECT Empno, Ename, Job, Mgr, Hiredate, Sal, Comm, Deptno
INTO Emp_ret
FROM Emp_tab
WHERE Empno = Emp_number;
END;

You could call this procedure from a PL/SQL block as follows:
DECLARE
Emp_row

Emp_tab%ROWTYPE;

BEGIN
Get_emp_rec(7499, Emp_row);
DBMS_OUTPUT.PUT(Emp_row.Ename
DBMS_OUTPUT.PUT(' '
DBMS_OUTPUT.PUT(' '
DBMS_OUTPUT.PUT(' '
DBMS_OUTPUT.NEW_LINE;
END;

-- declare a record matching a
-- row in the Emp_tab table
-||
||
||
||

call for Emp_tab# 7499
' '
|| Emp_row.Empno);
Emp_row.Job || ' ' || Emp_row.Mgr);
Emp_row.Hiredate
|| ' ' || Emp_row.Sal);
Emp_row.Comm || ' '|| Emp_row.Deptno);

Stored functions can also return values that are declared using %ROWTYPE. For
example:
FUNCTION Get_emp_rec (Dept_num IN Emp_tab.Deptno%TYPE)
RETURN Emp_tab%ROWTYPE IS ...

Tables and Records You can pass PL/SQL tables as parameters to stored procedures
and functions. You can also pass tables of records as parameters.
When passing a user defined type, such as a PL/SQL table or
record to a remote procedure, to make PL/SQL use the same
definition so that the type checker can verify the source, you must
create a redundant loop back DBLINK so that when the PL/SQL
compiles, both sources pull from the same location.

Note:

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Default Parameter Values Parameters can take default values. Use the DEFAULT keyword
or the assignment operator to give a parameter a default value. For example, the
specification for the Get_emp_names procedure could be written as the following:
PROCEDURE Get_emp_names (Dept_num IN NUMBER DEFAULT 20) IS ...

or
PROCEDURE Get_emp_names (Dept_num IN NUMBER := 20) IS ...

When a parameter takes a default value, it can be omitted from the actual parameter
list when you call the procedure. When you do specify the parameter value on the call,
it overrides the default value.
Unlike in an anonymous PL/SQL block, you do not use the
keyword DECLARE before the declarations of variables, cursors, and
exceptions in a stored procedure. In fact, it is an error to use it.

Note:

Creating Stored Procedures and Functions
Use a text editor to write the procedure or function. At the beginning of the procedure,
place the following statement:
CREATE PROCEDURE Procedure_name AS

...

For example, to use the example in "%TYPE and %ROWTYPE Attributes" on page 7-5,
create a text (source) file called get_emp.sql containing the following code:
CREATE PROCEDURE Get_emp_rec (Emp_number IN Emp_tab.Empno%TYPE,
Emp_ret
OUT Emp_tab%ROWTYPE) AS
BEGIN
SELECT Empno, Ename, Job, Mgr, Hiredate, Sal, Comm, Deptno
INTO Emp_ret
FROM Emp_tab
WHERE Empno = Emp_number;
END;
/

Then, using an interactive tool such as SQL*Plus, load the text file containing the
procedure by entering the following statement:
SQL> @get_emp

This loads the procedure into the current schema from the get_emp.sql file (.sql is
the default file extension). Note the slash (/) at the end of the code. This is not part of
the code; it just activates the loading of the procedure.
Use the CREATE [OR REPLACE] FUNCTION... statement to store functions.
Caution: When developing a new procedure, it is usually much
more convenient to use the CREATE OR REPLACE PROCEDURE
statement. This replaces any previous version of that procedure in the
same schema with the newer version, but note that this is done
without warning.

You can use either the keyword IS or AS after the procedure parameter list.

Coding PL/SQL Procedures and Packages 7-7

Overview of PL/SQL Program Units

Oracle Database Reference for the complete syntax of the
CREATE PROCEDURE and CREATE FUNCTION statements

See Also:

Privileges to Create Procedures and Functions To create a standalone procedure or
function, or package specification or body, you must meet the following prerequisites:
■

You must have the CREATE PROCEDURE system privilege to create a procedure or
package in your schema, or the CREATE ANY PROCEDURE system privilege to
create a procedure or package in another user's schema.
To create without errors (to compile the procedure or package
successfully) requires the following additional privileges:

Note:
■

■

The owner of the procedure or package must be explicitly granted
the necessary object privileges for all objects referenced within the
body of the code.
The owner cannot obtain required privileges through roles.

If the privileges of the owner of a procedure or package change, then the procedure
must be reauthenticated before it is run. If a necessary privilege to a referenced object
is revoked from the owner of the procedure or package, then the procedure cannot be
run.
The EXECUTE privilege on a procedure gives a user the right to run a procedure
owned by another user. Privileged users run the procedure under the security domain
of the owner of the procedure. Therefore, users never need to be granted the privileges
to the objects referenced by a procedure. This allows for more disciplined and efficient
security strategies with database applications and their users. Furthermore, all
procedures and packages are stored in the data dictionary (in the SYSTEM tablespace).
No quota controls the amount of space available to a user who creates procedures and
packages.
Package creation requires a sort. So the user creating the
package should be able to create a sort segment in the temporary
tablespace with which the user is associated.

Note:

See Also:

"Privileges Required to Execute a Procedure" on page 7-33

Altering Stored Procedures and Functions
To alter a stored procedure or function, you must first drop it using the DROP
PROCEDURE or DROP FUNCTION statement, then re-create it using the CREATE
PROCEDURE or CREATE FUNCTION statement. Alternatively, use the CREATE OR
REPLACE PROCEDURE or CREATE OR REPLACE FUNCTION statement, which first
drops the procedure or function if it exists, then re-creates it as specified.
Caution: The procedure or function is dropped without any
warning.

Dropping Procedures and Functions
A standalone procedure, a standalone function, a package body, or an entire package
can be dropped using the SQL statements DROP PROCEDURE, DROP FUNCTION, DROP

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Overview of PL/SQL Program Units

PACKAGE BODY, and DROP PACKAGE, respectively. A DROP PACKAGE statement drops
both the specification and body of a package.
The following statement drops the Old_sal_raise procedure in your schema:
DROP PROCEDURE Old_sal_raise;

Privileges to Drop Procedures and Functions To drop a procedure, function, or package,
the procedure or package must be in your schema, or you must have the DROP ANY
PROCEDURE privilege. An individual procedure within a package cannot be dropped;
the containing package specification and body must be re-created without the
procedures to be dropped.

External Procedures
A PL/SQL procedure executing on an Oracle Database instance can call an external
procedure written in a 3GL. The 3GL procedure runs in a separate address space from
that of the database.
Chapter 14, "Calling External Procedures" for information
about external procedures

See Also:

PL/SQL Packages
A package is an encapsulated collection of related program objects (for example,
procedures, functions, variables, constants, cursors, and exceptions) stored together in
the database.
Using packages is an alternative to creating procedures and functions as standalone
schema objects. Packages have many advantages over standalone procedures and
functions. For example, they:
■

Let you organize your application development more efficiently.

■

Let you grant privileges more efficiently.

■

Let you modify package objects without recompiling dependent schema objects.

■

Enable Oracle Database to read multiple package objects into memory at once.

■

■

Can contain global variables and cursors that are available to all procedures and
functions in the package.
Let you overload procedures or functions. Overloading a procedure means
creating multiple procedures with the same name in the same package, each
taking arguments of different number or datatype.
Oracle Database PL/SQL User's Guide and Reference for more
information about subprogram name overloading

See Also:

The specification part of a package declares the public types, variables, constants, and
subprograms that are visible outside the immediate scope of the package. The body of
a package defines the objects declared in the specification, as well as private objects that
are not visible to applications outside the package.
Example of a PL/SQL Package Specification and Body The following example shows a
package specification for a package named Employee_management. The package
contains one stored function and two stored procedures. The body for this package
defines the function and the procedures:
CREATE PACKAGE BODY Employee_management AS
FUNCTION Hire_emp (Name VARCHAR2, Job VARCHAR2,
Coding PL/SQL Procedures and Packages 7-9

Overview of PL/SQL Program Units

Mgr NUMBER, Hiredate DATE, Sal NUMBER, Comm NUMBER,
Deptno NUMBER) RETURN NUMBER IS
New_empno
NUMBER(10);
------

This function accepts all arguments for the fields in
the employee table except for the employee number.
A value for this field is supplied by a sequence.
The function returns the sequence number generated
by the call to this function.
BEGIN
SELECT Emp_sequence.NEXTVAL INTO New_empno FROM dual;
INSERT INTO Emp_tab VALUES (New_empno, Name, Job, Mgr,
Hiredate, Sal, Comm, Deptno);
RETURN (New_empno);
END Hire_emp;
PROCEDURE fire_emp(emp_id IN NUMBER) AS

-- This procedure deletes the employee with an employee
-- number that corresponds to the argument Emp_id. If
-- no employee is found, then an exception is raised.
BEGIN
DELETE FROM Emp_tab WHERE Empno = Emp_id;
IF SQL%NOTFOUND THEN
Raise_application_error(-20011, 'Invalid Employee
Number: ' || TO_CHAR(Emp_id));
END IF;
END fire_emp;
PROCEDURE Sal_raise (Emp_id IN NUMBER, Sal_incr IN NUMBER) AS
------

This procedure accepts two arguments. Emp_id is a
number that corresponds to an employee number.
SAL_INCR is the amount by which to increase the
employee's salary. If employee exists, then update
salary with increase.

BEGIN
UPDATE Emp_tab
SET Sal = Sal + Sal_incr
WHERE Empno = Emp_id;
IF SQL%NOTFOUND THEN
Raise_application_error(-20011, 'Invalid Employee
Number: ' || TO_CHAR(Emp_id));
END IF;
END Sal_raise;
END Employee_management;

If you want to try this example, then first create the sequence
number Emp_sequence. Do this with the following SQL*Plus
statement:

Note:

SQL> CREATE SEQUENCE Emp_sequence
> START WITH 8000 INCREMENT BY 10;

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PL/SQL Object Size Limitation
The size limitation for PL/SQL stored database objects such as procedures, functions,
triggers, and packages is the size of the DIANA (Descriptive Intermediate Attributed
Notation for Ada) code in the shared pool in bytes. The UNIX limit on the size of the
flattened DIANA/pcode size is 64K but the limit may be 32K on desktop platforms.
The most closely related number that a user can access is the PARSED_SIZE in the
data dictionary view USER_OBJECT_SIZE. That gives the size of the DIANA in bytes
as stored in the SYS.IDL_xxx$ tables. This is not the size in the shared pool. The size
of the DIANA part of PL/SQL code (used during compilation) is significantly larger in
the shared pool than it is in the system table.

Creating Packages
Each part of a package is created with a different statement. Create the package
specification using the CREATE PACKAGE statement. The CREATE PACKAGE statement
declares public package objects.
To create a package body, use the CREATE PACKAGE BODY statement. The CREATE
PACKAGE BODY statement defines the procedural code of the public procedures and
functions declared in the package specification.
You can also define private, or local, package procedures, functions, and variables in a
package body. These objects can only be accessed by other procedures and functions in
the body of the same package. They are not visible to external users, regardless of the
privileges they hold.
It is often more convenient to add the OR REPLACE clause in the CREATE PACKAGE or
CREATE PACKAGE BODY statements when you are first developing your application.
The effect of this option is to drop the package or the package body without warning.
The CREATE statements would then be the following:
CREATE OR REPLACE PACKAGE Package_name AS ...

and
CREATE OR REPLACE PACKAGE BODY Package_name AS ...

Creating Packaged Objects The body of a package can contain:
■

Procedures and functions declared in the package specification.

■

Definitions of cursors declared in the package specification.

■

Local procedures and functions, not declared in the package specification.

■

Local variables.

Procedures, functions, cursors, and variables that are declared in the package
specification are global. They can be called, or used, by external users that have
EXECUTE permission for the package or that have EXECUTE ANY PROCEDURE
privileges.
When you create the package body, make sure that each procedure that you define in
the body has the same parameters, by name, datatype, and mode, as the declaration in the
package specification. For functions in the package body, the parameters and the return
type must agree in name and type.
Privileges to Create or Drop Packages The privileges required to create or drop a package
specification or package body are the same as those required to create or drop a
standalone procedure or function.
Coding PL/SQL Procedures and Packages 7-11

Overview of PL/SQL Program Units

See Also:
■

"Privileges to Create Procedures and Functions" on page 7-8

■

"Privileges to Drop Procedures and Functions" on page 7-9

Naming Packages and Package Objects
The names of a package and all public objects in the package must be unique within a
given schema. The package specification and its body must have the same name. All
package constructs must have unique names within the scope of the package, unless
overloading of procedure names is desired.

Package Invalidations and Session State
Each session that references a package object has its own instance of the corresponding
package, including persistent state for any public and private variables, cursors, and
constants. If any of the session's instantiated packages (specification or body) are
invalidated, then all package instances in the session are invalidated and recompiled.
As a result, the session state is lost for all package instances in the session.
When a package in a given session is invalidated, the session receives the following
error the first time it attempts to use any object of the invalid package instance:
ORA-04068: existing state of packages has been discarded

The second time a session makes such a package call, the package is reinstantiated for
the session without error.
For optimal performance, Oracle Database returns this error
message only once—each time the package state is discarded.

Note:

If you handle this error in your application, ensure that your error
handling strategy can accurately handle this error. For example, when
a procedure in one package calls a procedure in another package, your
application should be aware that the session state is lost for both
packages.
In most production environments, DDL operations that can cause invalidations are
usually performed during inactive working hours; therefore, this situation might not
be a problem for end-user applications. However, if package invalidations are
common in your system during working hours, then you might want to code your
applications to handle this error when package calls are made.

Packages Supplied With Oracle Database
There are many packages provided with Oracle Database, either to extend the
functionality of the database or to give PL/SQL access to SQL features. You can call
these packages from your application.
See Also: Oracle Database PL/SQL Packages and Types Reference for an
overview of these Oracle Database packages

Overview of Bulk Binds
Oracle Database uses two engines to run PL/SQL blocks and subprograms. The
PL/SQL engine runs procedural statements, while the SQL engine runs SQL
statements. During execution, every SQL statement causes a context switch between
the two engines, resulting in performance overhead.
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Performance can be improved substantially by minimizing the number of context
switches required to run a particular block or subprogram. When a SQL statement
runs inside a loop that uses collection elements as bind variables, the large number of
context switches required by the block can cause poor performance. Collections
include the following:
■

Varrays

■

Nested tables

■

Index-by tables

■

Host arrays

Binding is the assignment of values to PL/SQL variables in SQL statements. Bulk
binding is binding an entire collection at once. Bulk binds pass the entire collection
back and forth between the two engines in a single operation.
Typically, using bulk binds improves performance for SQL statements that affect four
or more database rows. The more rows affected by a SQL statement, the greater the
performance gain from bulk binds.
This section provides an overview of bulk binds to help you
decide if you should use them in your PL/SQL applications. For
detailed information about using bulk binds, including ways to
handle exceptions that occur in the middle of a bulk bind operation,
refer to the Oracle Database PL/SQL User's Guide and Reference.

Note:

When to Use Bulk Binds
If you have scenarios like these in your applications, consider using bulk binds to
improve performance.
DML Statements that Reference Collections The FORALL keyword can improve the
performance of INSERT, UPDATE, or DELETE statements that reference collection
elements.
For example, the following PL/SQL block increases the salary for employees whose
manager's ID number is 7902, 7698, or 7839, both with and without using bulk binds:
DECLARE
TYPE Numlist IS VARRAY (100) OF NUMBER;
Id NUMLIST := NUMLIST(7902, 7698, 7839);
BEGIN
-- Efficient method, using a bulk bind
FORALL i IN Id.FIRST..Id.LAST
-- bulk-bind the VARRAY
UPDATE Emp_tab SET Sal = 1.1 * Sal
WHERE Mgr = Id(i);
-- Slower method, running the UPDATE statements within a regular loop
FOR i IN Id.FIRST..Id.LAST LOOP
UPDATE Emp_tab SET Sal = 1.1 * Sal
WHERE Mgr = Id(i);
END LOOP;
END;

Without the bulk bind, PL/SQL sends a SQL statement to the SQL engine for each
employee that is updated, leading to context switches that hurt performance.

Coding PL/SQL Procedures and Packages 7-13

Overview of PL/SQL Program Units

If you have a set of rows prepared in a PL/SQL table, you can bulk-insert or
bulk-update the data using a loop like:
FORALL i in Emp_Data.FIRST..Emp_Data.LAST
INSERT INTO Emp_tab VALUES(Emp_Data(i));

SELECT Statements that Reference Collections The BULK COLLECT INTO clause can
improve the performance of queries that reference collections.
For example, the following PL/SQL block queries multiple values into PL/SQL tables,
both with and without bulk binds:
-- Find all employees whose manager's ID number is 7698.
DECLARE
TYPE Var_tab IS TABLE OF VARCHAR2(20) INDEX BY BINARY_INTEGER;
Empno VAR_TAB;
Ename VAR_TAB;
Counter NUMBER;
CURSOR C IS
SELECT Empno, Ename FROM Emp_tab WHERE Mgr = 7698;
BEGIN
-- Efficient method, using a bulk bind
SELECT Empno, Ename BULK COLLECT INTO Empno, Ename
FROM Emp_Tab WHERE Mgr = 7698;
-- Slower method, assigning each collection element within a loop.
counter := 1;
FOR rec IN C LOOP
Empno(Counter) := rec.Empno;
Ename(Counter) := rec.Ename;
Counter := Counter + 1;
END LOOP;
END;

You can use BULK COLLECT INTO with tables of scalar values, or tables of %TYPE
values.
Without the bulk bind, PL/SQL sends a SQL statement to the SQL engine for each
employee that is selected, leading to context switches that hurt performance.
FOR Loops that Reference Collections and the Returning Into Clause You can use the FORALL
keyword along with the BULK COLLECT INTO keywords to improve the performance
of FOR loops that reference collections and return DML.
For example, the following PL/SQL block updates the Emp_tab table by computing
bonuses for a collection of employees; then it returns the bonuses in a column called
Bonlist. The actions are performed both with and without using bulk binds:
DECLARE
TYPE Emplist IS VARRAY(100) OF NUMBER;
Empids EMPLIST := EMPLIST(7369, 7499, 7521, 7566, 7654, 7698);
TYPE Bonlist IS TABLE OF Emp_tab.sal%TYPE;
Bonlist_inst BONLIST;
BEGIN
Bonlist_inst := BONLIST(1,2,3,4,5);
FORALL i IN Empids.FIRST..empIDs.LAST

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Remote Dependencies

UPDATE Emp_tab SET Bonus = 0.1 * Sal
WHERE Empno = Empids(i)
RETURNING Sal BULK COLLECT INTO Bonlist;
FOR i IN Empids.FIRST..Empids.LAST LOOP
UPDATE Emp_tab Set Bonus = 0.1 * sal
WHERE Empno = Empids(i)
RETURNING Sal INTO BONLIST(i);
END LOOP;
END;

Without the bulk bind, PL/SQL sends a SQL statement to the SQL engine for each
employee that is updated, leading to context switches that hurt performance.

Triggers
A trigger is a special kind of PL/SQL anonymous block. You can define triggers to fire
before or after SQL statements, either on a statement level or for each row that is
affected. You can also define INSTEAD OF triggers or system triggers (triggers on
DATABASE and SCHEMA).
See Also:

Chapter 9, "Coding Triggers"

Compiling PL/SQL Procedures for Native Execution
You can speed up PL/SQL procedures by compiling them into native code residing in
shared libraries. The procedures are translated into C code, then compiled with your
usual C compiler and linked into the Oracle Database process.
You can use this technique with both the supplied Oracle Database PL/SQL packages,
and procedures you write yourself. You can use the ALTER SYSTEM or
ALTER SESSION command, or update your initialization file, to set the parameter
PLSQL_CODE_TYPE to the value NATIVE (the default setting is the value
INTERPRETED).
Because this technique cannot do much to speed up SQL statements called from these
procedures, it is most effective for compute-intensive procedures that do not spend
much time executing SQL.
With Java, you can use the ncomp tool to compile your own packages and classes.
See Also:
■

■

Oracle Database PL/SQL User's Guide and Reference for details on
PL/SQL native compilation
Oracle Database Java Developer's Guide for details on Java native
compilation

Remote Dependencies
Dependencies among PL/SQL program units can be handled in two ways:
■

Timestamps

■

Signatures

Coding PL/SQL Procedures and Packages 7-15

Remote Dependencies

Timestamps
If timestamps are used to handle dependencies among PL/SQL program units, then
whenever you alter a program unit or a relevant schema object, all of its dependent
units are marked as invalid and must be recompiled before they can be run.
Each program unit carries a timestamp that is set by the server when the unit is
created or recompiled. Figure 7–1 demonstrates this graphically. Procedures P1 and P2
call stored procedure P3. Stored procedure P3 references table T1. In this example,
each of the procedures is dependent on table T1. P3 depends upon T1 directly, while
P1 and P2 depend upon T1 indirectly.
Figure 7–1
P1

Dependency Relationships
P3

T1

P2

If P3 is altered, then P1 and P2 are marked as invalid immediately, if they are on the
same server as P3. The compiled states of P1 and P2 contain records of the timestamp
of P3. Therefore, if the procedure P3 is altered and recompiled, then the timestamp on
P3 no longer matches the value that was recorded for P3 during the compilation of P1
and P2.
If P1 and P2 are on a client system, or on another Oracle Database instance in a
distributed environment, then the timestamp information is used to mark them as
invalid at runtime.

Disadvantages of the Timestamp Model
The disadvantage of this dependency model is that it is unnecessarily restrictive.
Recompilation of dependent objects across the network are often performed when not
strictly necessary, leading to performance degradation.
Furthermore, on the client side, the timestamp model can lead to situations that block
an application from running at all, if the client-side application is built using PL/SQL
version 2. Earlier releases of tools, such as Oracle Forms, that used PL/SQL version 1
on the client side did not use this dependency model, because PL/SQL version 1 had
no support for stored procedures.
For releases of Oracle Forms that are integrated with PL/SQL version 2 on the client
side, the timestamp model can present problems. For example, during the installation
of the application, the application is rendered invalid unless the client-side PL/SQL
procedures that it uses are recompiled at the client site. Also, if a client-side procedure
depends on a server procedure, and if the server procedure is changed or
automatically recompiled, then the client-side PL/SQL procedure must then be
recompiled. Yet in many application environments (such as Forms runtime
applications), there is no PL/SQL compiler available on the client. This blocks the
application from running at all. The client application developer must then
redistribute new versions of the application to all customers.

Signatures
To alleviate some of the problems with the timestamp-only dependency model, Oracle
Database provides the additional capability of remote dependencies using signatures.

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Remote Dependencies

The signature capability affects only remote dependencies. Local (same server)
dependencies are not affected, as recompilation is always possible in this environment.
A signature is associated with each compiled stored program unit. It identifies the unit
using the following criteria:
■

The name of the unit (the package, procedure, or function name).

■

The types of each of the parameters of the subprogram.

■

The modes of the parameters (IN, OUT, IN OUT).

■

The number of parameters.

■

The type of the return value for a function.

The user has control over whether signatures or timestamps govern remote
dependencies.
See Also:

"Controlling Remote Dependencies" on page 7-20

When the signature dependency model is used, a dependency on a remote program
unit causes an invalidation of the dependent unit if the dependent unit contains a call
to a subprogram in the parent unit, and if the signature of this subprogram has been
changed in an incompatible manner.
For example, consider a procedure get_emp_name stored on a server in Boston
(BOSTON_SERVER). The procedure is defined as the following:
You may need to set up data structures, similar to the
following, for certain examples to work:

Note:

CONNECT system/manager
CREATE PUBLIC DATABASE LINK boston_server USING 'inst1_alias';
CONNECT scott/tiger

CREATE OR REPLACE PROCEDURE get_emp_name (
emp_number
IN NUMBER,
hire_date
OUT VARCHAR2,
emp_name
OUT VARCHAR2) AS
BEGIN
SELECT ename, to_char(hiredate, 'DD-MON-YY')
INTO emp_name, hire_date
FROM emp
WHERE empno = emp_number;
END;

When get_emp_name is compiled on BOSTON_SERVER, its signature, as well as its
timestamp, is recorded.
Suppose that on another server in California, some PL/SQL code calls get_emp_name
identifying it using a DBlink called BOSTON_SERVER, as follows:
CREATE OR REPLACE PROCEDURE print_ename (emp_number IN NUMBER) AS
hire_date
VARCHAR2(12);
ename
VARCHAR2(10);
BEGIN
get_emp_name@BOSTON_SERVER(emp_number, hire_date, ename);
dbms_output.put_line(ename);
dbms_output.put_line(hire_date);
END;

Coding PL/SQL Procedures and Packages 7-17

Remote Dependencies

When this California server code is compiled, the following actions take place:
■

A connection is made to the Boston server.

■

The signature of get_emp_name is transferred to the California server.

■

The signature is recorded in the compiled state of print_ename.

At runtime, during the remote procedure call from the California server to the Boston
server, the recorded signature of get_emp_name that was saved in the compiled state
of print_ename gets sent to the Boston server, regardless of whether or not there
were any changes.
If the timestamp dependency mode is in effect, then a mismatch in timestamps causes
an error status to be returned to the calling procedure.
However, if the signature mode is in effect, then any mismatch in timestamps is
ignored, and the recorded signature of get_emp_name in the compiled state of
Print_ename on the California server is compared with the current signature of
get_emp_name on the Boston server. If they match, then the call succeeds. If they do
not match, then an error status is returned to the print_name procedure.
Note that the get_emp_name procedure on the Boston server could have been
changed. Or, its timestamp could be different from that recorded in the print_name
procedure on the California server, possibly due to the installation of a new release of
the server. As long as the signature remote dependency mode is in effect on the
California server, a timestamp mismatch does not cause an error when
get_emp_name is called.
Note: DETERMINISTIC, PARALLEL_ENABLE, and purity
information do not show in the signature mode. Optimizations based
on these settings are not automatically reconsidered if a function on a
remote system is redefined with different settings. This may lead to
incorrect query results when calls to the remote function occur, even
indirectly, in a SQL statement, or if the remote function is used, even
indirectly, in a function-based index.

When Does a Signature Change?
Here is information on when a signature changes.

Switching Datatype Classes
A signature changes when you switch from one class of datatype to another. Within
each datatype class, there can be several types. Changing a parameter datatype from
one type to another within a class does not cause the signature to change. Datatypes
that are not listed in the following table, such as NCHAR or TIMESTAMP, are not part of
any class; changing their type always causes a signature mismatch.
VARCHAR types: VARCHAR2, VARCHAR, STRING, LONG, ROWID
Character types: CHARACTER, CHAR
Raw types: RAW, LONG RAW
Integer types: BINARY_INTEGER, PLS_INTEGER, BOOLEAN, NATURAL,
POSITIVE, POSITIVEN, NATURALN
Number types: NUMBER, INTEGER, INT, SMALLINT, DECIMAL, DEC, REAL,
FLOAT, NUMERIC, DOUBLE PRECISION, DOUBLE PRECISION, NUMERIC

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Remote Dependencies

Date types: DATE, TIMESTAMP, TIMESTAMP WITH TIME ZONE, TIMESTAMP
WITH LOCAL TIME ZONE, INTERVAL YEAR TO MONTH, INTERVAL DAY TO
SECOND
Modes Changing to or from an explicit specification of the default parameter mode IN
does not change the signature of a subprogram. For example, changing between:
PROCEDURE P1 (Param1 NUMBER);
PROCEDURE P1 (Param1 IN NUMBER);

does not change the signature. Any other change of parameter mode does change the
signature.
Default Parameter Values Changing the specification of a default parameter value does
not change the signature. For example, procedure P1 has the same signature in the
following two examples:
PROCEDURE P1 (Param1 IN NUMBER := 100);
PROCEDURE P1 (Param1 IN NUMBER := 200);

An application developer who requires that callers get the new default value must
recompile the called procedure, but no signature-based invalidation occurs when a
default parameter value assignment is changed.

Examples of Changing Procedure Signatures
Using the Get_emp_names procedure defined in "Parameters for Procedures and
Functions" on page 7-4, if the procedure body is changed to the following:
DECLARE
Emp_number NUMBER;
Hire_date
DATE;
BEGIN
-- date format model changes
SELECT Ename, To_char(Hiredate, 'DD/MON/YYYY')
INTO Emp_name, Hire_date
FROM Emp_tab
WHERE Empno = Emp_number;
END;

The specification of the procedure has not changed, so its signature has not changed.
But if the procedure specification is changed to the following:
CREATE OR REPLACE PROCEDURE Get_emp_name (
Emp_number IN NUMBER,
Hire_date
OUT DATE,
Emp_name
OUT VARCHAR2) AS

And if the body is changed accordingly, then the signature changes, because the
parameter Hire_date has a different datatype.
However, if the name of that parameter changes to When_hired, and the datatype
remains VARCHAR2, and the mode remains OUT, the signature does not change.
Changing the name of a formal parameter does not change the signature of the unit.
Consider the following example:
CREATE OR REPLACE PACKAGE Emp_package AS
TYPE Emp_data_type IS RECORD (
Emp_number NUMBER,

Coding PL/SQL Procedures and Packages 7-19

Remote Dependencies

Hire_date VARCHAR2(12),
Emp_name
VARCHAR2(10));
PROCEDURE Get_emp_data
(Emp_data IN OUT Emp_data_type);
END;
CREATE OR REPLACE PACKAGE BODY Emp_package AS
PROCEDURE Get_emp_data
(Emp_data IN OUT Emp_data_type) IS
BEGIN
SELECT Empno, Ename, TO_CHAR(Hiredate, 'DD/MON/YY')
INTO Emp_data
FROM Emp_tab
WHERE Empno = Emp_data.Emp_number;
END;
END;

If the package specification is changed so that the record's field names are changed,
but the types remain the same, then this does not affect the signature. For example, the
following package specification has the same signature as the previous package
specification example:
CREATE OR REPLACE PACKAGE Emp_package AS
TYPE Emp_data_type IS RECORD (
Emp_num
NUMBER,
-- was Emp_number
Hire_dat
VARCHAR2(12),
-- was Hire_date
Empname
VARCHAR2(10)); -- was Emp_name
PROCEDURE Get_emp_data
(Emp_data IN OUT Emp_data_type);
END;

Changing the name of the type of a parameter does not cause a change in the signature
if the type remains the same as before. For example, the following package
specification for Emp_package is the same as the first one:
CREATE OR REPLACE PACKAGE Emp_package AS
TYPE Emp_data_record_type IS RECORD (
Emp_number NUMBER,
Hire_date VARCHAR2(12),
Emp_name
VARCHAR2(10));
PROCEDURE Get_emp_data
(Emp_data IN OUT Emp_data_record_type);
END;

Controlling Remote Dependencies
The dynamic initialization parameter REMOTE_DEPENDENCIES_MODE controls
whether the timestamp or the signature dependency model is in effect.
■

If the initialization parameter file contains the following specification:
REMOTE_DEPENDENCIES_MODE = TIMESTAMP

Then only timestamps are used to resolve dependencies (if this is not explicitly
overridden dynamically).
■

If the initialization parameter file contains the following parameter specification:
REMOTE_DEPENDENCIES_MODE = SIGNATURE

Then signatures are used to resolve dependencies (if this not explicitly overridden
dynamically).

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Remote Dependencies

■

You can alter the mode dynamically by using the DDL statements. For example,
this example alters the dependency model for the current session:
ALTER SESSION SET REMOTE_DEPENDENCIES_MODE =
{SIGNATURE | TIMESTAMP}
Thise example alters the dependency model systemwide after startup:
ALTER SYSTEM SET REMOTE_DEPENDENCIES_MODE =
{SIGNATURE | TIMESTAMP}

If the REMOTE_DEPENDENCIES_MODE parameter is not specified, either in the
init.ora parameter file or using the ALTER SESSION or ALTER SYSTEM DDL
statements, then timestamp is the default value. Therefore, unless you explicitly use
the REMOTE_DEPENDENCIES_MODE parameter, or the appropriate DDL statement,
your server is operating using the timestamp dependency model.
When you use REMOTE_DEPENDENCIES_MODE=SIGNATURE:
■

■

■

If you change the default value of a parameter of a remote procedure, then the
local procedure calling the remote procedure is not invalidated. If the call to the
remote procedure does not supply the parameter, then the default value is used. In
this case, because invalidation/recompilation does not automatically occur, the old
default value is used. If you want to see the new default values, then you must
recompile the calling procedure manually.
If you add a new overloaded procedure in a package (a new procedure with the
same name as an existing one), then local procedures that call the remote
procedure are not invalidated. If it turns out that this overloading results in a
rebinding of existing calls from the local procedure under the timestamp mode,
then this rebinding does not happen under the signature mode, because the local
procedure does not get invalidated. You must recompile the local procedure
manually to achieve the new rebinding.
If the types of parameters of an existing packaged procedure are changed so that
the new types have the same shape as the old ones, then the local calling
procedure is not invalidated or recompiled automatically. You must recompile the
calling procedure manually to get the semantics of the new type.

Dependency Resolution
When REMOTE_DEPENDENCIES_MODE = TIMESTAMP (the default value),
dependencies among program units are handled by comparing timestamps at runtime.
If the timestamp of a called remote procedure does not match the timestamp of the
called procedure, then the calling (dependent) unit is invalidated and must be
recompiled. In this case, if there is no local PL/SQL compiler, then the calling
application cannot proceed.
In the timestamp dependency mode, signatures are not compared. If there is a local
PL/SQL compiler, then recompilation happens automatically when the calling
procedure is run.
When REMOTE_DEPENDENCIES_MODE = SIGNATURE, the recorded timestamp in the
calling unit is first compared to the current timestamp in the called remote unit. If they
match, then the call proceeds. If the timestamps do not match, then the signature of the
called remote subprogram, as recorded in the calling subprogram, is compared with
the current signature of the called subprogram. If they do not match (using the criteria
described in the section "When Does a Signature Change?" on page 7-18), then an error
is returned to the calling session.

Coding PL/SQL Procedures and Packages 7-21

Cursor Variables

Suggestions for Managing Dependencies
Follow these guidelines for setting the REMOTE_DEPENDENCIES_MODE parameter:
■

■

■

■

Server-side PL/SQL users can set the parameter to TIMESTAMP (or let it default to
that) to get the timestamp dependency mode.
Server-side PL/SQL users can choose to use the signature dependency mode if
they have a distributed system and they want to avoid possible unnecessary
recompilations.
Client-side PL/SQL users should set the parameter to SIGNATURE. This allows:
–

Installation of new applications at client sites, without the need to recompile
procedures.

–

Ability to upgrade the server, without encountering timestamp mismatches.

When using signature mode on the server side, add new procedures to the end of
the procedure (or function) declarations in a package specification. Adding a new
procedure in the middle of the list of declarations can cause unnecessary
invalidation and recompilation of dependent procedures.

Cursor Variables
A cursor is a static object; a cursor variable is a pointer to a cursor. Because cursor
variables are pointers, they can be passed and returned as parameters to procedures
and functions. A cursor variable can also refer to different cursors in its lifetime.
Some additional advantages of cursor variables include:
■

■

■

Encapsulation Queries are centralized in the stored procedure that opens the cursor
variable.
Ease of maintenance If you need to change the cursor, then you only need to make
the change in one place: the stored procedure. There is no need to change each
application.
Convenient security The user of the application is the username used when the
application connects to the server. The user must have EXECUTE permission on the
stored procedure that opens the cursor. But, the user does not need to have READ
permission on the tables used in the query. This capability can be used to limit
access to the columns in the table, as well as access to other stored procedures.
See Also: Oracle Database PL/SQL User's Guide and Reference for
details on cursor variables

Declaring and Opening Cursor Variables
Memory is usually allocated for a cursor variable in the client application using the
appropriate ALLOCATE statement. In Pro*C, use the EXEC SQL ALLOCATE
 statement. In OCI, use the Cursor Data Area.
You can also use cursor variables in applications that run entirely in a single server
session. You can declare cursor variables in PL/SQL subprograms, open them, and use
them as parameters for other PL/SQL subprograms.

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Cursor Variables

Examples of Cursor Variables
This section includes several examples of cursor variable usage in PL/SQL. For
additional cursor variable examples that use the programmatic interfaces, refer to the
following manuals:
■

Pro*C/C++ Programmer's Guide

■

Pro*COBOL Programmer's Guide

■

Oracle Call Interface Programmer's Guide

■

Oracle SQL*Module for Ada Programmer's Guide

Fetching Data
The following package defines a PL/SQL cursor variable type Emp_val_cv_type,
and two procedures. The first procedure, Open_emp_cv, opens the cursor variable
using a bind variable in the WHERE clause. The second procedure, Fetch_emp_data,
fetches rows from the Emp_tab table using the cursor variable.
CREATE OR REPLACE PACKAGE Emp_data AS
TYPE Emp_val_cv_type IS REF CURSOR RETURN Emp_tab%ROWTYPE;
PROCEDURE Open_emp_cv (Emp_cv
IN OUT Emp_val_cv_type,
Dept_number
IN
INTEGER);
PROCEDURE Fetch_emp_data (emp_cv
IN
Emp_val_cv_type,
emp_row
OUT
Emp_tab%ROWTYPE);
END Emp_data;
CREATE OR REPLACE PACKAGE BODY Emp_data AS
PROCEDURE Open_emp_cv (Emp_cv
IN OUT Emp_val_cv_type,
Dept_number IN
INTEGER) IS
BEGIN
OPEN emp_cv FOR SELECT * FROM Emp_tab WHERE deptno = dept_number;
END open_emp_cv;
PROCEDURE Fetch_emp_data (Emp_cv
IN Emp_val_cv_type,
Emp_row
OUT Emp_tab%ROWTYPE) IS
BEGIN
FETCH Emp_cv INTO Emp_row;
END Fetch_emp_data;
END Emp_data;

The following example shows how to call the Emp_data package procedures from a
PL/SQL block:
DECLARE
-- declare a cursor variable
Emp_curs Emp_data.Emp_val_cv_type;
Dept_number Dept_tab.Deptno%TYPE;
Emp_row Emp_tab%ROWTYPE;
BEGIN
Dept_number := 20;
-- open the cursor using a variable
Emp_data.Open_emp_cv(Emp_curs, Dept_number);
-- fetch the data and display it
LOOP
Emp_data.Fetch_emp_data(Emp_curs, Emp_row);
EXIT WHEN Emp_curs%NOTFOUND;
DBMS_OUTPUT.PUT(Emp_row.Ename || ' ');
DBMS_OUTPUT.PUT_LINE(Emp_row.Sal);
END LOOP;

Coding PL/SQL Procedures and Packages 7-23

Handling PL/SQL Compile-Time Errors

END;

Implementing Variant Records
The power of cursor variables comes from their ability to point to different cursors. In
the following package example, a discriminant is used to open a cursor variable to
point to one of two different cursors:
CREATE OR REPLACE PACKAGE Emp_dept_data AS
TYPE Cv_type IS REF CURSOR;
PROCEDURE Open_cv (Cv
IN OUT cv_type,
Discrim
IN
POSITIVE);
END Emp_dept_data;
CREATE OR REPLACE PACKAGE BODY Emp_dept_data AS
PROCEDURE Open_cv (Cv
IN OUT cv_type,
Discrim IN
POSITIVE) IS
BEGIN
IF Discrim = 1 THEN
OPEN Cv FOR SELECT * FROM Emp_tab WHERE Sal > 2000;
ELSIF Discrim = 2 THEN
OPEN Cv FOR SELECT * FROM Dept_tab;
END IF;
END Open_cv;
END Emp_dept_data;

You can call the Open_cv procedure to open the cursor variable and point it to either a
query on the Emp_tab table or the Dept_tab table. The following PL/SQL block
shows how to fetch using the cursor variable, and then use the ROWTYPE_MISMATCH
predefined exception to handle either fetch:
DECLARE
Emp_rec Emp_tab%ROWTYPE;
Dept_rec Dept_tab%ROWTYPE;
Cv
Emp_dept_data.CV_TYPE;
BEGIN
Emp_dept_data.open_cv(Cv, 1); -- Open Cv For Emp_tab Fetch
Fetch cv INTO Dept_rec;
-- but fetch into Dept_tab record
-- which raises ROWTYPE_MISMATCH
DBMS_OUTPUT.PUT(Dept_rec.Deptno);
DBMS_OUTPUT.PUT_LINE(' ' || Dept_rec.Loc);
EXCEPTION
WHEN ROWTYPE_MISMATCH THEN
BEGIN
DBMS_OUTPUT.PUT_LINE
('Row type mismatch, fetching Emp_tab data...');
FETCH Cv INTO Emp_rec;
DBMS_OUTPUT.PUT(Emp_rec.Deptno);
DBMS_OUTPUT.PUT_LINE(' ' || Emp_rec.Ename);
END;

Handling PL/SQL Compile-Time Errors
When you use SQL*Plus to submit PL/SQL code, and when the code contains errors,
you receive notification that compilation errors have occurred, but there is no
immediate indication of what the errors are. For example, if you submit a standalone
(or stored) procedure PROC1 in the file proc1.sql as follows:
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Handling PL/SQL Compile-Time Errors

SQL> @proc1

And, if there are one or more errors in the code, then you receive a notice such as the
following:
MGR-00072: Warning: Procedure proc1 created with compilation errors

In this case, use the SHOW ERRORS statement in SQL*Plus to get a list of the errors that
were found. SHOW ERRORS with no argument lists the errors from the most recent
compilation. You can qualify SHOW ERRORS using the name of a procedure, function,
package, or package body:
SQL> SHOW ERRORS PROC1
SQL> SHOW ERRORS PROCEDURE PROC1

SQL*Plus User's Guide and Reference for complete
information about the SHOW ERRORS statement

See Also:

Note: Before issuing the SHOW ERRORS statement, use the SET
LINESIZE statement to get long lines on output. The value 132 is
usually a good choice. For example:
SET LINESIZE 132

Assume that you want to create a simple procedure that deletes records from the
employee table using SQL*Plus:
CREATE OR REPLACE PROCEDURE Fire_emp(Emp_id NUMBER) AS
BEGIN
DELETE FROM Emp_tab WHER Empno = Emp_id;
END
/

Notice that the CREATE PROCEDURE statement has two errors: the DELETE statement
has an error (the E is absent from WHERE), and the semicolon is missing after END.
After the CREATE PROCEDURE statement is entered and an error is returned, a SHOW
ERRORS statement returns the following lines:
SHOW ERRORS;
ERRORS FOR PROCEDURE Fire_emp:
LINE/COL
ERROR
-------------- -------------------------------------------3/27
PL/SQL-00103: Encountered the symbol "EMPNO" wh. . .
5/0
PL/SQL-00103: Encountered the symbol "END" when . . .
2 rows selected.

Notice that each line and column number where errors were found is listed by the
SHOW ERRORS statement.
Alternatively, you can query the following data dictionary views to list errors when
using any tool or application:
■

USER_ERRORS

■

ALL_ERRORS

■

DBA_ERRORS

The error text associated with the compilation of a procedure is updated when the
procedure is replaced, and it is deleted when the procedure is dropped.
Coding PL/SQL Procedures and Packages 7-25

Handling Run-Time PL/SQL Errors

Original source code can be retrieved from the data dictionary using the following
views: ALL_SOURCE, USER_SOURCE, and DBA_SOURCE.
Oracle Database Reference for more information about
these data dictionary views

See Also:

Handling Run-Time PL/SQL Errors
Oracle Database allows user-defined errors in PL/SQL code to be handled so that
user-specified error numbers and messages are returned to the client application. After
received, the client application can handle the error based on the user-specified error
number and message returned by Oracle Database.
User-specified error messages are returned using the RAISE_APPLICATION_ERROR
procedure. For example:
RAISE_APPLICATION_ERROR(Error_number, 'text', Keep_error_stack)

This procedure stops procedure execution, rolls back any effects of the procedure, and
returns a user-specified error number and message (unless the error is trapped by an
exception handler). ERROR_NUMBER must be in the range of -20000 to -20999.
Error number -20000 should be used as a generic number for messages where it is
important to relay information to the user, but having a unique error number is not
required. Text must be a character expression, 2 Kbytes or less (longer messages are
ignored). Keep_error_stack can be TRUE if you want to add the error to any
already on the stack, or FALSE if you want to replace the existing errors. By default,
this option is FALSE.
Some of the Oracle Database packages, such as
DBMS_OUTPUT, DBMS_DESCRIBE, and DBMS_ALERT, use
application error numbers in the range -20000 to -20005. Refer to the
descriptions of these packages for more information.

Note:

The RAISE_APPLICATION_ERROR procedure is often used in exception handlers or in
the logic of PL/SQL code. For example, the following exception handler selects the
string for the associated user-defined error message and calls the
RAISE_APPLICATION_ERROR procedure:
...
WHEN NO_DATA_FOUND THEN
SELECT Error_string INTO Message
FROM Error_table,
V$NLS_PARAMETERS V
WHERE Error_number = -20101 AND Lang = v.value AND
v.parameter = "NLS_LANGUAGE";
Raise_application_error(-20101, Message);
...

See Also: "Handling Errors in Remote Procedures" on page 7-28
for information on exception handling when calling remote
procedures

The following section includes an example of passing a user-specified error number
from a trigger to a procedure.

7-26 Oracle Database Application Developer’s Guide - Fundamentals

Handling Run-Time PL/SQL Errors

Declaring Exceptions and Exception Handling Routines
User-defined exceptions are explicitly defined and signaled within the PL/SQL block
to control processing of errors specific to the application. When an exception is raised
(signaled), the usual execution of the PL/SQL block stops, and a routine called an
exception handler is called. Specific exception handlers can be written to handle any
internal or user-defined exception.
Application code can check for a condition that requires special attention using an IF
statement. If there is an error condition, then two options are available:
■

■

Enter a RAISE statement that names the appropriate exception. A RAISE
statement stops the execution of the procedure, and control passes to an exception
handler (if any).
Call the RAISE_APPLICATION_ERROR procedure to return a user-specified error
number and message.

You can also define an exception handler to handle user-specified error messages. For
example, Figure 7–2 on page 7-27 illustrates the following:
■
■

■

An exception and associated exception handler in a procedure
A conditional statement that checks for an error (such as transferring funds not
available) and enters a user-specified error number and message within a trigger
How user-specified error numbers are returned to the calling environment (in this
case, a procedure), and how that application can define an exception that
corresponds to the user-specified error number

Declare a user-defined exception in a procedure or package body (private exceptions),
or in the specification of a package (public exceptions). Define an exception handler in
the body of a procedure (standalone or package).
Figure 7–2

Exceptions and User-Defined Errors

Procedure fire_emp(empid NUMBER) IS
invalid_empid EXCEPTION;
PRAGMA EXCEPTION_INIT(invalid_empid, –20101);
BEGIN
DELETE FROM emp WHERE empno = empid;
EXCEPTION
WHEN invlid_empid THEN
INSERT INTO emp_audit
VALUES (empid, ’Fired before probation ended’);
END;

Table EMP

Error number
returned to
calling
environment

TRIGGER emp_probation
BEFORE DELETE ON emp
FOR EACH ROW
BEGIN
IF (sysdate–:old.hiredate)<30 THEN
raise_application_error(20101,
’Employee’||old.ename||’ on probation’)
END IF;
END;

Coding PL/SQL Procedures and Packages 7-27

Handling Run-Time PL/SQL Errors

Unhandled Exceptions
In database PL/SQL program units, an unhandled user-error condition or internal
error condition that is not trapped by an appropriate exception handler causes the
implicit rollback of the program unit. If the program unit includes a COMMIT statement
before the point at which the unhandled exception is observed, then the implicit
rollback of the program unit can only be completed back to the previous COMMIT.
Additionally, unhandled exceptions in database-stored PL/SQL program units
propagate back to client-side applications that call the containing program unit. In
such an application, only the application program unit call is rolled back (not the
entire application program unit), because it is submitted to the database as a SQL
statement.
If unhandled exceptions in database PL/SQL program units are propagated back to
database applications, then the database PL/SQL code should be modified to handle
the exceptions. Your application can also trap for unhandled exceptions when calling
database program units and handle such errors appropriately.

Handling Errors in Distributed Queries
You can use a trigger or a stored procedure to create a distributed query. This
distributed query is decomposed by the local Oracle Database instance into a
corresponding number of remote queries, which are sent to the remote nodes for
execution. The remote nodes run the queries and send the results back to the local
node. The local node then performs any necessary post-processing and returns the
results to the user or application.
If a portion of a distributed statement fails, possibly due to an integrity constraint
violation, then Oracle Database returns error number ORA-02055. Subsequent
statements, or procedure calls, return error number ORA-02067 until a rollback or a
rollback to savepoint is entered.
You should design your application to check for any returned error messages that
indicates that a portion of the distributed update has failed. If you detect a failure, then
you should rollback the entire transaction (or rollback to a savepoint) before allowing
the application to proceed.

Handling Errors in Remote Procedures
When a procedure is run locally or at a remote location, four types of exceptions can
occur:
■

PL/SQL user-defined exceptions, which must be declared using the keyword
EXCEPTION.

■

PL/SQL predefined exceptions, such as NO_DATA_FOUND.

■

SQL errors, such as ORA-00900 and ORA-02015.

■

Application exceptions, which are generated using the
RAISE_APPLICATION_ERROR() procedure.

When using local procedures, all of these messages can be trapped by writing an
exception handler, such as shown in the following example:
EXCEPTION
WHEN ZERO_DIVIDE THEN
/* ...handle the exception */

7-28 Oracle Database Application Developer’s Guide - Fundamentals

Debugging Stored Procedures

Notice that the WHEN clause requires an exception name. If the exception that is raised
does not have a name, such as those generated with RAISE_APPLICATION_ERROR,
then one can be assigned using PRAGMA_EXCEPTION_INIT, as shown in the
following example:
DECLARE
...
Null_salary EXCEPTION;
PRAGMA EXCEPTION_INIT(Null_salary, -20101);
BEGIN
...
RAISE_APPLICATION_ERROR(-20101, 'salary is missing');
...
EXCEPTION
WHEN Null_salary THEN
...

When calling a remote procedure, exceptions are also handled by creating a local
exception handler. The remote procedure must return an error number to the local
calling procedure, which then handles the exception, as shown in the previous
example. Because PL/SQL user-defined exceptions always return ORA-06510 to the
local procedure, these exceptions cannot be handled. All other remote exceptions can
be handled in the same manner as local exceptions.

Debugging Stored Procedures
Compiling a stored procedure involves fixing any syntax errors in the code. You might
need to do additional debugging to make sure that the procedure works correctly,
performs well, and recovers from errors. Such debugging might involve:
■

■

Adding extra output statements to verify execution progress and check data
values at certain points within the procedure.
Running a separate debugger to analyze execution in greater detail.

Oracle JDeveloper
Recent releases of Oracle JDeveloper have extensive features for debugging PL/SQL,
Java, and multi-language programs. You can get Oracle JDeveloper as part of various
Oracle product suites. Often, a more recent release is available as a download at
http://www.oracle.com/technology/.

Oracle Procedure Builder and TEXT_IO Package
Oracle Procedure Builder is an advanced client/server debugger that transparently
debugs your database applications. It lets you run PL/SQL procedures and triggers in
a controlled debugging environment, and you can set breakpoints, list the values of
variables, and perform other debugging tasks. Oracle Procedure Builder is part of the
Oracle Developer tool set. It also provides the TEXT_IO package that is useful for
printing debug information.

DBMS_OUTPUT Package
You can also debug stored procedures and triggers using the DBMS_OUTPUT supplied
package. Put PUT and PUT_LINE statements in your code to output the value of
variables and expressions to your terminal.

Coding PL/SQL Procedures and Packages 7-29

Debugging Stored Procedures

Privileges for Debugging PL/SQL and Java Stored Procedures
Starting with Oracle Database 10g, a new privilege model applies to debugging
PL/SQL and Java code running within the database. This model applies whether you
are using Oracle JDeveloper, Oracle Developer, or any of the various third-party
PL/SQL or Java development environments, and it affects both the DBMS_DEBUG and
DBMS_DEBUG_JDWP APIs.
For a session to connect to a debugger, the effective user at the time of the connect
operation must have the DEBUG CONNECT SESSION system privilege. This effective
user may be the owner of a definer's rights routine involved in making the connect
call.
When a debugger becomes connected to a session, the session login user and the
currently enabled session-level roles are fixed as the privilege environment for that
debugging connection. Any DEBUG or EXECUTE privileges needed for debugging must
be granted to that combination of user and roles.
■

■

To be able to display and change Java public variables or variables declared in a
PL/SQL package specification, the debugging connection must be granted either
EXECUTE or DEBUG privilege on the relevant code.
To be able to either display and change private variables or breakpoint and
execute code lines step by step, the debugging connection must be granted DEBUG
privilege on the relevant code
Caution: The DEBUG privilege effectively allows a debugging
session to do anything that the procedure being debugged could
have done if that action had been included in its code.

In addition to these privilege requirements, the ability to stop on individual code lines
and debugger access to variables are allowed only in code compiled with debug
information generated. The PLSQL_DEBUG parameter and the DEBUG keyword on
commands such as ALTER PACKAGE can be used to control whether the PL/SQL
compiler includes debug information in its results. If it does not, variables will not be
accessible, and neither stepping nor breakpoints will stop on code lines. The PL/SQL
compiler will never generate debug information for code that has been obfuscated
using the PL/SQL wrap utility.
See Also: Oracle Database PL/SQL User's Guide and Reference,
"Obfuscating PL/SQL Source Code"

The DEBUG ANY PROCEDURE system privilege is equivalent to the DEBUG privilege
granted on all objects in the database. Objects owned by SYS are included if the value
of the O7_DICTIONARY_ACCESSIBILITY parameter is TRUE.
A debug role mechanism is available to carry privileges needed for debugging that are
not normally enabled in the session. Refer to the documentation on the DBMS_DEBUG
and DBMS_DEBUG_JDWP packages for details on how to specify a debug role and any
necessary related password.
The JAVADEBUGPRIV role carries the DEBUG CONNECT SESSION and DEBUG ANY
PROCEDURE privileges. Grant it only with the care those privileges warrant.
Caution: Granting DEBUG ANY PROCEDURE privilege, or
granting DEBUG privilege on any object owned by SYS, means
granting complete rights to the database.

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Calling Stored Procedures

Writing Low-Level Debugging Code
If you are actually writing code that will be part of a debugger, you might need to use
packages such as DBMS_DEBUG_JDWP or DBMS_DEBUG.
DBMS_DEBUG_JDWP Package
The DBMS_DEBUG_JDWP package, provided starting with Oracle9i Release 2, provides
a framework for multi-language debugging that is expected to supersede the
DBMS_DEBUG package over time. It is especially useful for programs that combine
PL/SQL and Java.
DBMS_DEBUG Package
The DBMS_DEBUG package, provided starting with Oracle8i, implements server-side
debuggers and provides a way to debug server-side PL/SQL program units. Several of
the debuggers available, such as Oracle Procedure Builder and various third-party
vendor solutions, use this API.
See Also:
■
■

■

■
■

Oracle Procedure Builder Developer's Guide
Oracle Database PL/SQL Packages and Types Reference for more
information about the DBMS_DEBUG and DBMS_OUTPUT
packages and associated privileges
The Oracle JDeveloper documentation for information on using
package DBMS_DEBUG_JDWP
Oracle Database SQL Reference for more details on privileges
The PL/SQL page at
http://www.oracle.com/technology/ for information
about writing low-level debug code

Calling Stored Procedures
You may need to set up data structures, similar to the
following, for certain examples to work:

Note:

CREATE TABLE Emp_tab (
Empno
NUMBER(4) NOT NULL,
Ename
VARCHAR2(10),
Job
VARCHAR2(9),
Mgr
NUMBER(4),
Hiredate DATE,
Sal
NUMBER(7,2),
Comm
NUMBER(7,2),
Deptno
NUMBER(2));
CREATE OR REPLACE PROCEDURE fire_emp1(Emp_id NUMBER) AS
BEGIN
DELETE FROM Emp_tab WHERE Empno = Emp_id;
END;
VARIABLE Empnum NUMBER;

Procedures can be called from many different environments. For example:
■

A procedure can be called within the body of another procedure or a trigger.

Coding PL/SQL Procedures and Packages 7-31

Calling Stored Procedures

■
■

■

A procedure can be interactively called by a user using an Oracle Database tool.
A procedure can be explicitly called within an application, such as a SQL*Forms or
a precompiler application.
A stored function can be called from a SQL statement in a manner similar to
calling a built-in SQL function, such as LENGTH or ROUND.

This section includes some common examples of calling procedures from within these
environments.
See Also:

"Calling Stored Functions from SQL Expressions" on

page 7-36

A Procedure or Trigger Calling Another Procedure
A procedure or trigger can call another stored procedure. For example, included in the
body of one procedure might be the following line:
. . .
Sal_raise(Emp_id, 200);
. . .

This line calls the Sal_raise procedure. Emp_id is a variable within the context of
the procedure. Recursive procedure calls are allowed within PL/SQL: A procedure can
call itself.

Interactively Calling Procedures From Oracle Database Tools
A procedure can be called interactively from an Oracle Database tool, such as
SQL*Plus. For example, to call a procedure named SAL_RAISE, owned by you, you
can use an anonymous PL/SQL block, as follows:
BEGIN
Sal_raise(7369, 200);
END;

Interactive tools, such as SQL*Plus, require you to follow
these lines with a slash (/) to run the PL/SQL block.

Note:

An easier way to run a block is to use the SQL*Plus statement EXECUTE, which wraps
BEGIN and END statements around the code you enter. For example:
EXECUTE Sal_raise(7369, 200);

Some interactive tools allow session variables to be created. For example, when using
SQL*Plus, the following statement creates a session variable:
VARIABLE Assigned_empno NUMBER

After defined, any session variable can be used for the duration of the session. For
example, you might run a function and capture the return value using a session
variable:
EXECUTE :Assigned_empno := Hire_emp('JSMITH', 'President',
1032, SYSDATE, 5000, NULL, 10);
PRINT Assigned_empno;
ASSIGNED_EMPNO
-------------2893

7-32 Oracle Database Application Developer’s Guide - Fundamentals

Calling Stored Procedures

See Also:
■
■

SQL*Plus User's Guide and Reference
Your tools documentation for information about performing
similar operations using your development tool

Calling Procedures within 3GL Applications
A 3GL database application, such as a precompiler or an OCI application, can include
a call to a procedure within the code of the application.
To run a procedure within a PL/SQL block in an application, simply call the
procedure. The following line within a PL/SQL block calls the Fire_emp procedure:
Fire_emp1(:Empnun);

In this case, :Empno is a host (bind) variable within the context of the application.
To run a procedure within the code of a precompiler application, you must use the
EXEC call interface. For example, the following statement calls the Fire_emp
procedure in the code of a precompiler application:
EXEC SQL EXECUTE
BEGIN
Fire_emp1(:Empnum);
END;
END-EXEC;

See Also: For information about calling PL/SQL procedures from
within 3GL applications:
■

Oracle Call Interface Programmer's Guide

■

Pro*C/C++ Programmer's Guide

■

Oracle SQL*Module for Ada Programmer's Guide

Name Resolution When Calling Procedures
References to procedures and packages are resolved according to the algorithm
described in the "Rules for Name Resolution in SQL Statements" section of Chapter 2,
"Designing Schema Objects".

Privileges Required to Execute a Procedure
If you are the owner of a standalone procedure or package, then you can run the
standalone procedure or packaged procedure, or any public procedure or packaged
procedure at any time, as described in the previous sections. If you want to run a
standalone or packaged procedure owned by another user, then the following
conditions apply:
■

■

You must have the EXECUTE privilege for the standalone procedure or package
containing the procedure, or you must have the EXECUTE ANY PROCEDURE system
privilege. If you are executing a remote procedure, then you must be granted the
EXECUTE privilege or EXECUTE ANY PROCEDURE system privilege directly, not
through a role.
You must include the name of the owner in the call. For example:1
EXECUTE Jward.Fire_emp (1043);
EXECUTE Jward.Hire_fire.Fire_emp (1043);

Coding PL/SQL Procedures and Packages 7-33

Calling Remote Procedures

■

■

If the procedure is a definer's-rights procedure, then it runs with the privileges of
the procedure owner. The owner must have all the necessary object privileges for
any referenced objects.
If the procedure is an invoker's-rights procedure, then it runs with your privileges
(as the invoker). In this case, you also need privileges on all referenced objects; that
is, all objects accessed by the procedure through external references that are
resolved in your schema. You may hold these privileges directly or through a role.
Roles are enabled unless an invoker's-rights procedure is called directly or
indirectly by a definer's-rights procedure.

Specifying Values for Procedure Arguments
When you call a procedure, specify a value or parameter for each of the procedure's
arguments. Identify the argument values using either of the following methods, or a
combination of both:
■

List the values in the order the arguments appear in the procedure declaration.

■

Specify the argument names and corresponding values, in any order.

For example, these statements each call the procedure Sal_raise to increase the
salary of employee number 7369 by 500:
Sal_raise(7369, 500);
Sal_raise(Sal_incr=>500, Emp_id=>7369);
Sal_raise(7369, Sal_incr=>500);

The first statement identifies the argument values by listing them in the order in which
they appear in the procedure specification.
The second statement identifies the argument values by name and in an order different
from that of the procedure specification. If you use argument names, then you can list
the arguments in any order.
The third statement identifies the argument values using a combination of these
methods. If you use a combination of order and argument names, then values
identified in order must precede values identified by name.
If you used the DEFAULT option to define default values for IN parameters to a
subprogram (see the Oracle Database PL/SQL User's Guide and Reference),then you can
pass different numbers of actual parameters to the first subprogram, accepting or
overriding the default values as you please. If an actual value is not passed, then the
corresponding default value is used. If you want to assign a value to an argument that
occurs after an omitted argument (for which the corresponding default is used), then
you must explicitly designate the name of the argument, as well as its value.

Calling Remote Procedures
Call remote procedures using an appropriate database link and the procedure name.
The following SQL*Plus statement runs the procedure Fire_emp located in the
database and pointed to by the local database link named BOSTON_SERVER:
1

You may need to set up the following data structures for certain examples to work: CONNECT
SYS/password AS SYSDBA;CREATE USER Jward IDENTIFIED BY Jward;GRANT
CREATE ANY PACKAGE TO Jward;GRANT CREATE SESSION TO Jward;GRANT
EXECUTE ANY PROCEDURE TO Jward;CONNECT Scott/Tiger

7-34 Oracle Database Application Developer’s Guide - Fundamentals

Calling Remote Procedures

EXECUTE fire_emp1@boston_server(1043);

See Also: "Handling Errors in Remote Procedures" on page 7-28
for information on exception handling when calling remote
procedures

Remote Procedure Calls and Parameter Values
You must explicitly pass values to all remote procedure parameters, even if there are
defaults. You cannot access remote package variables and constants.

Referencing Remote Objects
Remote objects can be referenced within the body of a locally defined procedure. The
following procedure deletes a row from the remote employee table:
CREATE OR REPLACE PROCEDURE fire_emp(emp_id NUMBER) IS
BEGIN
DELETE FROM emp@boston_server WHERE empno = emp_id;
END;

The following list explains how to properly call remote procedures, depending on the
calling environment.
■

Remote procedures (standalone and packaged) can be called from within a
procedure, an OCI application, or a precompiler application by specifying the
remote procedure name, a database link, and the arguments for the remote
procedure.
CREATE OR REPLACE PROCEDURE local_procedure(arg IN NUMBER) AS
BEGIN
fire_emp1@boston_server(arg);
END;

■

In the previous example, you could create a synonym for
FIRE_EMP1@BOSTON_SERVER. This would enable you to call the remote
procedure from an Oracle Database tool application, such as a SQL*Forms
application, as well from within a procedure, OCI application, or precompiler
application.
CREATE SYNONYM synonym1 for fire_emp1@boston_server;
CREATE OR REPLACE PROCEDURE local_procedure(arg IN NUMBER) AS
BEGIN
synonym1(arg);
END;

■

If you do not want to use a synonym, then you could write a local cover procedure
to call the remote procedure.
DECLARE
arg NUMBER;
BEGIN
local_procedure(arg);
END;

Here, local_procedure is defined as in the first item of this list.
See Also:

"Synonyms for Procedures and Packages" on page 7-36

Coding PL/SQL Procedures and Packages 7-35

Calling Stored Functions from SQL Expressions

Caution: Unlike stored procedures, which use compile-time
binding, runtime binding is used when referencing remote
procedures. The user account to which you connect depends on the
database link.

All calls to remotely stored procedures are assumed to perform updates; therefore, this
type of referencing always requires two-phase commit of that transaction (even if the
remote procedure is read-only). Furthermore, if a transaction that includes a remote
procedure call is rolled back, then the work done by the remote procedure is also
rolled back.
A procedure called remotely can usually execute a COMMIT, ROLLBACK, or
SAVEPOINT statement, the same as a local procedure. However, there are some
differences in behavior:
■

■

■

If the transaction was originated by a non-Oracle database, as may be the case in
XA applications, these operations are not allowed in the remote procedure.
After doing one of these operations, the remote procedure cannot start any
distributed transactions of its own.
If the remote procedure does not commit or roll back its work, the commit is done
implicitly when the database link is closed. In the meantime, further calls to the
remote procedure are not allowed because it is still considered to be performing a
transaction.

A distributed update modifies data on two or more databases. A distributed update is
possible using a procedure that includes two or more remote updates that access data
on different databases. Statements in the construct are sent to the remote databases,
and the execution of the construct succeeds or fails as a unit. If part of a distributed
update fails and part succeeds, then a rollback (of the entire transaction or to a
savepoint) is required to proceed. Consider this when creating procedures that
perform distributed updates.
Pay special attention when using a local procedure that calls a remote procedure. If a
timestamp mismatch is found during execution of the local procedure, then the remote
procedure is not run, and the local procedure is invalidated.

Synonyms for Procedures and Packages
Synonyms can be created for standalone procedures and packages to do the following:
■
■

Hide the identity of the name and owner of a procedure or package.
Provide location transparency for remotely stored procedures (standalone or
within a package).

When a privileged user needs to call a procedure, an associated synonym can be used.
Because the procedures defined within a package are not individual objects (the
package is the object), synonyms cannot be created for individual procedures within a
package.

Calling Stored Functions from SQL Expressions
You can include user-written PL/SQL functions in SQL expressions. (You must be
using PL/SQL release 2.1 or higher.) By using PL/SQL functions in SQL statements,
you can do the following:

7-36 Oracle Database Application Developer’s Guide - Fundamentals

Calling Stored Functions from SQL Expressions

■

■

■

■

Increase user productivity by extending SQL. Expressiveness of the SQL statement
increases where activities are too complex, too awkward, or unavailable with SQL.
Increase query efficiency. Functions used in the WHERE clause of a query can filter
data using criteria that would otherwise need to be evaluated by the application.
Manipulate character strings to represent special datatypes (for example, latitude,
longitude, or temperature).
Provide parallel query execution: If the query is parallelized, then SQL statements
in your PL/SQL function may also be run in parallel (using the parallel query
option).

Using PL/SQL Functions
PL/SQL functions must be created as top-level functions or declared within a package
specification before they can be named within a SQL statement. Stored PL/SQL
functions are used in the same manner as built-in Oracle functions (such as SUBSTR or
ABS).
PL/SQL functions can be placed wherever an Oracle function can be placed within a
SQL statement, or, wherever expressions can occur in SQL. For example, they can be
called from the following:
■

The select list of the SELECT statement.

■

The condition of the WHERE and HAVING clause.

■

The CONNECT BY, START WITH, ORDER BY, and GROUP BY clauses.

■

The VALUES clause of the INSERT statement.

■

The SET clause of the UPDATE statement.

You cannot call stored PL/SQL functions from a CHECK constraint clause of a CREATE
or ALTER TABLE statement or use them to specify a default value for a column. These
situations require an unchanging definition.
Unlike functions, which are called as part of an expression,
procedures are called as statements. Therefore, PL/SQL procedures
are not directly callable from SQL statements. However, functions
called from a PL/SQL statement or referenced in a SQL expression
can call a PL/SQL procedure.

Note:

Syntax for SQL Calling a PL/SQL Function
Use the following syntax to reference a PL/SQL function from SQL:
[[schema.]package.]function_name[@dblink][(param_1...param_n)]

For example, to reference a function you created that is called My_func, in the
My_funcs_pkg package, in the Scott schema, that takes two numeric parameters,
you could call the following:
SELECT Scott.My_funcs_pkg.My_func(10,20) FROM dual;

Naming Conventions
If only one of the optional schema or package names is given, then the first identifier
can be either a schema name or a package name. For example, to determine whether

Coding PL/SQL Procedures and Packages 7-37

Calling Stored Functions from SQL Expressions

Payroll in the reference Payroll.Tax_rate is a schema or package name, Oracle
Database proceeds as follows:
■
■

■

Oracle Database first checks for the Payroll package in the current schema.
If the PAYROLL package is found in the current schema, then Oracle Database
looks for a Tax_rate function in the Payroll package. If a Tax_rate function
is not found in the Payroll package, then an error message is returned.
If a Payroll package is not found, then Oracle Database looks for a schema
named Payroll that contains a top-level Tax_rate function. If the Tax_rate
function is not found in the Payroll schema, then an error message is returned.

You can also refer to a stored top-level function using any synonym that you have
defined for it.

Name Precedence
In SQL statements, the names of database columns take precedence over the names of
functions with no parameters. For example, if schema Scott creates the following two
objects:
CREATE TABLE Emp_tab(New_sal NUMBER ...);
CREATE FUNCTION New_sal RETURN NUMBER IS ...;

Then, in the following two statements, the reference to New_sal refers to the column
Emp_tab.New_sal:
SELECT New_sal FROM Emp_tab;
SELECT Emp_tab.New_sal FROM Emp_tab;

To access the function new_sal, enter the following:
SELECT Scott.New_sal FROM Emp_tab;

Example of Calling a PL/SQL Function from SQL For example, to call the Tax_rate
PL/SQL function from schema Scott, run it against the Ss_no and sal columns in
Tax_table, and place the results in the variable Income_tax, specify the following:
You may need to set up data structures similar to the
following for certain examples to work:

Note:

CREATE TABLE Tax_table (
Ss_no NUMBER,
Sal
NUMBER);
CREATE OR REPLACE FUNCTION tax_rate (ssn IN NUMBER, salary IN
NUMBER) RETURN NUMBER IS
sal_out NUMBER;
BEGIN
sal_out := salary * 1.1;
END;

DECLARE
Tax_id
NUMBER;
Income_tax NUMBER;
BEGIN
SELECT scott.tax_rate (Ss_no, Sal)
INTO Income_tax
FROM Tax_table

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Calling Stored Functions from SQL Expressions

WHERE Ss_no = Tax_id;
END;

These sample calls to PL/SQL functions are allowed in SQL expressions:
Circle_area(Radius)
Payroll.Tax_rate(Empno)
scott.Payroll.Tax_rate@boston_server(Dependents, Empno)

Arguments
To pass any number of arguments to a function, supply the arguments within the
parentheses. You must use positional notation; named notation is not supported. For
functions that do not accept arguments, use ().

Using Default Values
The stored function Gross_pay initializes two of its formal parameters to default
values using the DEFAULT clause. For example:
CREATE OR REPLACE FUNCTION Gross_pay
(Emp_id IN NUMBER,
St_hrs IN NUMBER DEFAULT 40,
Ot_hrs IN NUMBER DEFAULT 0) RETURN NUMBER AS
...

When calling Gross_pay from a procedural statement, you can always accept the
default value of St_hrs. This is because you can use named notation, which lets you
skip parameters. For example:
IF Gross_pay(Eenum, Ot_hrs => Otime) > Pay_limit
THEN ...

However, when calling Gross_pay from a SQL expression, you cannot accept the
default value of St_hrs, unless you accept the default value of Ot_hrs. This is
because you cannot use named notation.

Privileges
To call a PL/SQL function from SQL, you must either own or have EXECUTE
privileges on the function. To select from a view defined with a PL/SQL function, you
must have SELECT privileges on the view. No separate EXECUTE privileges are
necessary to select from the view.

Requirements for Calling PL/SQL Functions from SQL Expressions
To be callable from SQL expressions, a user-defined PL/SQL function must meet the
following basic requirements:
■

■

■

■

■

It must be a stored function, not a function defined within a PL/SQL block or
subprogram.
It must be a row function, not a column (group) function; in other words, it cannot
take an entire column of data as its argument.
All its formal parameters must be IN parameters; none can be an OUT or IN OUT
parameter.
The datatypes of its formal parameters must be Oracle built-in types, such as
CHAR, DATE, or NUMBER, not PL/SQL types, such as BOOLEAN, RECORD, or TABLE.
Its return type (the datatype of its result value) must be an Oracle built-in type.
Coding PL/SQL Procedures and Packages 7-39

Calling Stored Functions from SQL Expressions

For example, the following stored function meets the basic requirements:
You may need to set up the following data structures for
certain examples to work:

Note:

CREATE TABLE Payroll(
Srate
NUMBER,
Orate
NUMBER,
Acctno
NUMBER);

CREATE FUNCTION Gross_pay
(Emp_id IN NUMBER,
St_hrs IN NUMBER DEFAULT 40,
Ot_hrs IN NUMBER DEFAULT 0) RETURN NUMBER AS
St_rate NUMBER;
Ot_rate NUMBER;
BEGIN
SELECT Srate, Orate INTO St_rate, Ot_rate FROM Payroll
WHERE Acctno = Emp_id;
RETURN St_hrs * St_rate + Ot_hrs * Ot_rate;
END Gross_pay;

Controlling Side Effects
The purity of a stored subprogram (function or procedure) refers to the side effects of
that subprogram on database tables or package variables. Side effects can prevent the
parallelization of a query, yield order-dependent (and therefore, indeterminate)
results, or require that package state be maintained across user sessions. Various side
effects are not allowed when a subprogram is called from a SQL query or DML
statement.
In releases prior to Oracle8i, Oracle Database leveraged the PL/SQL compiler to
enforce restrictions during the compilation of a stored subprogram or a SQL statement.
Starting with Oracle8i, the compile-time restrictions were relaxed, and a smaller set of
restrictions are enforced during execution.
This change provides uniform support for stored subprograms written in PL/SQL,
Java, and C, and it allows programmers the most flexibility possible.

Restrictions
When a SQL statement is run, checks are made to see if it is logically embedded within
the execution of an already running SQL statement. This occurs if the statement is run
from a trigger or from a subprogram that was in turn called from the already running
SQL statement. In these cases, further checks occur to determine if the new SQL
statement is safe in the specific context.
The following restrictions are enforced on subprograms:
■

■

■

A subprogram called from a query or DML statement may not end the current
transaction, create or rollback to a savepoint, or ALTER the system or session.
A subprogram called from a query (SELECT) statement or from a parallelized
DML statement may not execute a DML statement or otherwise modify the
database.
A subprogram called from a DML statement may not read or modify the
particular table being modified by that DML statement.

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These restrictions apply regardless of what mechanism is used to run the SQL
statement inside the subprogram or trigger. For example:
■

■
■

They apply to a SQL statement called from PL/SQL, whether embedded directly
in a subprogram or trigger body, run using the native dynamic mechanism
(EXECUTE IMMEDIATE), or run using the DBMS_SQL package.
They apply to statements embedded in Java with SQLJ syntax or run using JDBC.
They apply to statements run with OCI using the callback context from within an
"external" C function.

You can avoid these restrictions if the execution of the new SQL statement is not
logically embedded in the context of the already running statement. PL/SQL's
autonomous transactions provide one escape (see "Autonomous Transactions" on
page 2-20). Another escape is available using Oracle Call Interface (OCI) from an
external C function, if you create a new connection rather than using the handle
available from the OCIExtProcContext argument.

Declaring a Function
You can use the keywords DETERMINISTIC and PARALLEL_ENABLE in the syntax for
declaring a function. These are optimization hints that inform the query optimizer and
other software components about the following:
■

Functions that need not be called redundantly

■

Functions permitted within a parallelized query or parallelized DML statement

Only functions that are DETERMINISTIC are allowed in function-based indexes and in
certain snapshots and materialized views.
A deterministic function depends solely on the values passed into it as arguments and
does not reference or modify the contents of package variables or the database or have
other side-effects. Such a function produces the same result value for any combination
of argument values passed into it.
You place the DETERMINISTIC keyword after the return value type in a declaration of
the function. For example:
CREATE FUNCTION F1 (P1 NUMBER) RETURN NUMBER DETERMINISTIC IS
BEGIN
RETURN P1 * 2;
END;

You may place this keyword in the following places:
■

On a function defined in a CREATE FUNCTION statement

■

In a function declaration in a CREATE PACKAGE statement

■

On a method declaration in a CREATE TYPE statement

You should not repeat the keyword on the function or method body in a CREATE
PACKAGE BODY or CREATE TYPE BODY statement.
Certain performance optimizations occur on calls to functions that are marked
DETERMINISTIC without any other action being required. The following features
require that any function used with them be declared DETERMINISTIC:
■
■

Any user-defined function used in a function-based index.
Any function used in a materialized view, if that view is to qualify for Fast Refresh
or is marked ENABLE QUERY REWRITE.

Coding PL/SQL Procedures and Packages 7-41

Calling Stored Functions from SQL Expressions

The preceding functions features attempt to use previously calculated results rather
than calling the function when it is possible to do so.
Functions that fall in the following categories should typically be DETERMINISTIC:
■

Functions used in a WHERE, ORDER BY, or GROUP BY clause

■

Functions that MAP or ORDER methods of a SQL type

■

Functions that in any other way help determine whether or where a row should
appear in a result set

Oracle Database cannot require that you should explicitly declare functions in the
preceding categories as DETERMINISTIC without breaking existing applications, but
the use of the keyword might be a wise choice of style within your application.
Keep the following points in mind when you create DETERMINISTIC functions:
■

■

The database cannot recognize if the behavior of the function is indeed
deterministic. If the DETERMINISTIC keyword is applied to a function whose
behavior is not truly deterministic, then the result of queries involving that
function is unpredictable.
If you change the semantics of a DETERMINISTIC function and recompile it, then
existing function-based indexes and materialized views report results for the prior
version of the function. Thus, if you change the semantics of a function, you must
manually rebuild any dependent function-based indexes and materialized views.
See Also: Oracle Database SQL Reference for an account of CREATE
FUNCTION restrictions

Parallel Query and Parallel DML
Oracle Database's parallel execution feature divides the work of executing a SQL
statement across multiple processes. Functions called from a SQL statement which is
run in parallel may have a separate copy run in each of these processes, with each
copy called for only the subset of rows that are handled by that process.
Each process has its own copy of package variables. When parallel execution begins,
these are initialized based on the information in the package specification and body as
if a new user is logging into the system; the values in package variables are not copied
from the original login session. And changes made to package variables are not
automatically propagated between the various sessions or back to the original session.
Java STATIC class attributes are similarly initialized and modified independently in
each process. Because a function can use package (or Java STATIC) variables to
accumulate some value across the various rows it encounters, Oracle Database cannot
assume that it is safe to parallelize the execution of all user-defined functions.
For query (SELECT) statements in Oracle Database versions prior to 8.1.5, the parallel
query optimization looked to see if a function was noted as RNPS and WNPS in a
PRAGMA RESTRICT_REFERENCES declaration; those functions that were marked as
both RNPS and WNPS could be run in parallel. Functions defined with a CREATE
FUNCTION statement had their code implicitly examined to determine if they were
pure enough; parallelized execution might occur even though a pragma cannot be
specified on these functions.
See Also: "PRAGMA RESTRICT_REFERENCES – for Backward
Compatibility" on page 7-43

For DML statements in Oracle Database versions prior to 8.1.5, the parallelization
optimization looked to see if a function was noted as having all four of RNDS, WNDS,
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RNPS and WNPS specified in a PRAGMA RESTRICT_REFERENCES declaration; those
functions that were marked as neither reading nor writing to either the database or
package variables could run in parallel. Again, those functions defined with a CREATE
FUNCTION statement had their code implicitly examined to determine if they were
actually pure enough; parallelized execution might occur even though a pragma
cannot be specified on these functions.
Oracle Database versions 8.1.5 and later continue to parallelize those functions that
earlier versions recognize as parallelizable. The PARALLEL_ENABLE keyword is the
preferred way to mark your code as safe for parallel execution. This keyword is
syntactically similar to DETERMINISTIC as described in "Declaring a Function" on
page 7-41; it is placed after the return value type in a declaration of the function, as in:
CREATE FUNCTION F1 (P1 NUMBER) RETURN NUMBER PARALLEL_ENABLE IS
BEGIN
RETURN P1 * 2;
END;

A PL/SQL function defined with CREATE FUNCTION may still be run in parallel
without any explicit declaration that it is safe to do so, if the system can determine that
it neither reads nor writes package variables nor calls any function that might do so. A
Java method or C function is never seen by the system as safe to run in parallel, unless
the programmer explicitly indicates PARALLEL_ENABLE on the "call specification", or
provides a PRAGMA RESTRICT_REFERENCES indicating that the function is
sufficiently pure.
An additional runtime restriction is imposed on functions run in parallel as part of a
parallelized DML statement. Such a function is not permitted to in turn execute a DML
statement; it is subject to the same restrictions that are enforced on functions that are
run inside a query (SELECT) statement.
See Also:

"Restrictions" on page 7-40

PRAGMA RESTRICT_REFERENCES – for Backward Compatibility
In Oracle Database versions prior to 8.1.5 (Oracle8i), programmers used the pragma
RESTRICT_REFERENCES to assert the purity level of a subprogram. In subsequent
versions, use the hints parallel-enable and deterministic, instead, to
communicate subprogram purity to Oracle Database.
You can remove RESTRICT_REFERENCES from your code. However, this pragma
remains available for backward compatibility in situations where one of the following is
true:
■

■

It is impossible or impractical to edit existing code to remove
RESTRICT_REFERENCES completely. If you do not remove it from a subprogram
S1 that depends on another subprogram S2, then RESTRICT_REFERENCES might
also be needed in S2, so that S1 will compile.
Replacing RESTRICT_REFERENCES in existing code with hints
parallel-enable and deterministic would negatively affect the behavior of
new, dependent code. Use RESTRICT_REFERENCES to preserve the behavior of
the existing code.

An existing PL/SQL application can thus continue using the pragma even on new
functionality, to ease integration with the existing code. Do not use the pragma in a
wholly new application.
If you use the pragma RESTRICT_REFERENCES, place it in a package specification,
not in a package body. It must follow the declaration of a subprogram (function or

Coding PL/SQL Procedures and Packages 7-43

Calling Stored Functions from SQL Expressions

procedure), but it need not follow immediately. Only one pragma can reference a given
subprogram declaration.
The pragma RESTRICT_REFERENCES applies to both
functions and procedures. Purity levels are important for functions,
but also for procedures that are called by functions.

Note:

To code the pragma RESTRICT_REFERENCES, use the following syntax:
PRAGMA RESTRICT_REFERENCES (
Function_name, WNDS [, WNPS] [, RNDS] [, RNPS] [, TRUST] );

Where:
Keyword Description
WNDS

The subprogram writes no database state (does not modify database tables).

RNDS

The subprogram reads no database state (does not query database tables).

WNPS

The subprogram writes no package state (does not change the values of packaged
variables).

RNPS

The subprogram reads no package state (does not reference the values of packaged
variables).

TRUST

The other restrictions listed in the pragma are not enforced; they are simply
assumed to be true. This allows easy calling from functions that have
RESTRICT_REFERENCES declarations to those that do not.

You can pass the arguments in any order. If any SQL statement inside the subprogram
body violates a rule, then you get an error when the statement is parsed.
In the following example, the function compound neither reads nor writes database or
package state; therefore, you can assert the maximum purity level. Always assert the
highest purity level that a subprogram allows. That way, the PL/SQL compiler never
rejects the subprogram unnecessarily.
You may need to set up the following data structures for
certain examples here to work:

Note:

CREATE TABLE Accts (
Yrs
NUMBER,
Amt
NUMBER,
Acctno
NUMBER,
Rte
NUMBER);

CREATE PACKAGE Finance AS -- package specification
FUNCTION Compound
(Years IN NUMBER,
Amount IN NUMBER,
Rate
IN NUMBER) RETURN NUMBER;
PRAGMA RESTRICT_REFERENCES (Compound, WNDS, WNPS, RNDS, RNPS);
END Finance;
CREATE PACKAGE BODY Finance AS
FUNCTION Compound
(Years IN NUMBER,
Amount IN NUMBER,

--package body

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Rate
IN NUMBER) RETURN NUMBER IS
BEGIN
RETURN Amount * POWER((Rate / 100) + 1, Years);
END Compound;
-- no pragma in package body
END Finance;

Later, you might call compound from a PL/SQL block, as follows:
DECLARE
Interest NUMBER;
Acct_id NUMBER;
BEGIN
SELECT Finance.Compound(Yrs, Amt, Rte)
INTO
Interest
FROM
Accounts
WHERE Acctno = Acct_id;

-- function call

Using the Keyword TRUST The keyword TRUST in the RESTRICT_REFERENCES syntax
allows easy calling from functions that have RESTRICT_REFERENCES declarations to
those that do not. When TRUST is present, the restrictions listed in the pragma are not
actually enforced, but rather are simply assumed to be true.
When calling from a section of code that is using pragmas to one that is not, there are
two likely usage styles. One is to place a pragma on the routine to be called, for
example on a "call specification" for a Java method. Then, calls from PL/SQL to this
method will complain if the method is less restricted than the calling subprogram. For
example:
CREATE OR REPLACE PACKAGE P1 IS
FUNCTION F1 (P1 NUMBER) RETURN NUMBER IS
LANGUAGE JAVA NAME 'CLASS1.METHODNAME(int) return int';
PRAGMA RESTRICT_REFERENCES(F1,WNDS,TRUST);
FUNCTION F2 (P1 NUMBER) RETURN NUMBER;
PRAGMA RESTRICT_REFERENCES(F2,WNDS);
END;
CREATE OR REPLACE PACKAGE BODY P1 IS
FUNCTION F2 (P1 NUMBER) RETURN NUMBER IS
BEGIN
RETURN F1(P1);
END;
END;

Here, F2 can call F1, as F1 has been declared to be WNDS.
The other approach is to mark only the caller, which may then make a call to any
subprogram without complaint. For example:
CREATE OR REPLACE PACKAGE P1a IS
FUNCTION F1 (P1 NUMBER) RETURN NUMBER IS
LANGUAGE JAVA NAME 'CLASS1.METHODNAME(int) return int';
FUNCTION F2 (P1 NUMBER) RETURN NUMBER;
PRAGMA RESTRICT_REFERENCES(F2,WNDS,TRUST);
END;
CREATE OR REPLACE PACKAGE BODY P1a IS
FUNCTION F2 (P1 NUMBER) RETURN NUMBER IS
BEGIN
RETURN F1(P1);

Coding PL/SQL Procedures and Packages 7-45

Calling Stored Functions from SQL Expressions

END;
END;

Here, F2 can call F1 because while F2 is promised to be WNDS (because TRUST is
specified), the body of F2 is not actually examined to determine if it truly satisfies the
WNDS restriction. Because F2 is not examined, its call to F1 is allowed, even though
there is no PRAGMA RESTRICT_REFERENCES for F1.
Differences between Static and Dynamic SQL Statements. Static INSERT, UPDATE, and
DELETE statements do not violate RNDS if these statements do not explicitly read any
database states, such as columns of a table. However, dynamic INSERT, UPDATE, and
DELETE statements always violate RNDS, regardless of whether or not the statements
explicitly read database states.
The following INSERT violates RNDS if it is executed dynamically, but it does not
violate RNDS if it is executed statically.
INSERT INTO my_table values(3, 'SCOTT');

The following UPDATE always violates RNDS statically and dynamically, because it
explicitly reads the column name of my_table.
UPDATE my_table SET id=777 WHERE name='SCOTT';

Overloading Packaged PL/SQL Functions PL/SQL lets you overload packaged (but not
standalone) functions: You can use the same name for different functions if their
formal parameters differ in number, order, or datatype family.
However, a RESTRICT_REFERENCES pragma can apply to only one function
declaration. Therefore, a pragma that references the name of overloaded functions
always applies to the nearest preceding function declaration.
In this example, the pragma applies to the second declaration of valid:
CREATE PACKAGE Tests AS
FUNCTION Valid (x NUMBER) RETURN CHAR;
FUNCTION Valid (x DATE) RETURN CHAR;
PRAGMA RESTRICT_REFERENCES (valid, WNDS);
END;

Serially Reusable PL/SQL Packages
PL/SQL packages usually consume user global area (UGA) memory corresponding to
the number of package variables and cursors in the package. This limits scalability,
because the memory increases linearly with the number of users. The solution is to
allow some packages to be marked as SERIALLY_REUSABLE (using pragma syntax).
For serially reusable packages, the package global memory is not kept in the UGA for
each user; rather, it is kept in a small pool and reused for different users. This means
that the global memory for such a package is only used within a unit of work. At the
end of that unit of work, the memory can therefore be released to the pool to be reused
by another user (after running the initialization code for all the global variables).
The unit of work for serially reusable packages is implicitly a call to the server; for
example, an OCI call to the server, or a PL/SQL RPC call from a client to a server, or an
RPC call from a server to another server.

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Package States
The state of a nonreusable package (one not marked SERIALLY_REUSABLE) persists
for the lifetime of a session. A package state includes global variables, cursors, and so
on.
The state of a serially reusable package persists only for the lifetime of a call to the
server. On a subsequent call to the server, if a reference is made to the serially reusable
package, then Oracle Database creates a new instantiation of the serially reusable
package and initializes all the global variables to NULL or to the default values
provided. Any changes made to the serially reusable package state in the previous
calls to the server are not visible.
Note: Creating a new instantiation of a serially reusable package
on a call to the server does not necessarily imply that Oracle
Database allocates memory or configures the instantiation object.
Oracle Database looks for an available instantiation work area
(which is allocated and configured) for this package in a
least-recently used (LRU) pool in the SGA.

At the end of the call to the server, this work area is returned back
to the LRU pool. The reason for keeping the pool in the SGA is that
the work area can be reused across users who have requests for the
same package.

Why Serially Reusable Packages?
Because the state of a non-reusable package persists for the lifetime of the session, this
locks up UGA memory for the whole session. In applications, such as Oracle Office, a
log-on session can typically exist for days together. Applications often need to use
certain packages only for certain localized periods in the session and would ideally
like to de-instantiate the package state in the middle of the session, after they are done
using the package.
With SERIALLY_REUSABLE packages, application developers have a way of
modelling their applications to manage their memory better for scalability. Package
state that they care about only for the duration of a call to the server should be
captured in SERIALLY_REUSABLE packages.

Syntax of Serially Reusable Packages
A package can be marked serially reusable by a pragma. The syntax of the pragma is:
PRAGMA SERIALLY_REUSABLE;

A package specification can be marked serially reusable, whether or not it has a
corresponding package body. If the package has a body, then the body must have the
serially reusable pragma, if its corresponding specification has the pragma; it cannot
have the serially reusable pragma unless the specification also has the pragma.

Semantics of Serially Reusable Packages
A package that is marked SERIALLY_REUSABLE has the following properties:
■

Its package variables are meant for use only within the work boundaries, which
correspond to calls to the server (either OCI call boundaries or PL/SQL RPC calls
to the server).

Coding PL/SQL Procedures and Packages 7-47

Calling Stored Functions from SQL Expressions

If the application programmer makes a mistake and
depends on a package variable that is set in a previous unit of
work, then the application program can fail. PL/SQL cannot check
for such cases.

Note:

■

■

■

A pool of package instantiations is kept, and whenever a "unit of work" needs this
package, one of the instantiations is "reused", as follows:
–

The package variables are reinitialized (for example, if the package variables
have default values, then those values are reinitialized).

–

The initialization code in the package body is run again.

At the "end work" boundary, cleanup is done.
–

If any cursors were left open, then they are silently closed.

–

Some non-reusable secondary memory is freed (such as memory for collection
variables or long VARCHAR2s).

–

This package instantiation is returned back to the pool of reusable
instantiations kept for this package.

Serially reusable packages cannot be accessed from database triggers or other
PL/SQL subprograms that are called from SQL statements. If you try, then Oracle
Database generates an error.

Examples of Serially Reusable Packages
This section presents a few examples of serially reusable packages.
Example 1: How Package Variables Act Across Call Boundaries This example has a serially
reusable package specification (there is no body).
CONNECT Scott/Tiger
CREATE OR REPLACE PACKAGE Sr_pkg IS
PRAGMA SERIALLY_REUSABLE;
N NUMBER := 5;
-- default initialization
END Sr_pkg;

Suppose your Enterprise Manager (or SQL*Plus) application issues the following:
CONNECT Scott/Tiger
# first CALL to server
BEGIN
Sr_pkg.N := 10;
END;
# second CALL to server
BEGIN
DBMS_OUTPUT.PUT_LINE(Sr_pkg.N);
END;

This program prints:
5

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If the package had not had the pragma
SERIALLY_REUSABLE, the program would have printed '10'.

Note:

Example 2: How Package Variables Act Across Call Boundaries This example has both a
package specification and package body, which are serially reusable.
CONNECT Scott/Tiger
DROP PACKAGE Sr_pkg;
CREATE OR REPLACE PACKAGE Sr_pkg IS
PRAGMA SERIALLY_REUSABLE;
TYPE Str_table_type IS TABLE OF VARCHAR2(200) INDEX BY BINARY_INTEGER;
Num
NUMBER
:= 10;
Str
VARCHAR2(200) := 'default-init-str';
Str_tab STR_TABLE_TYPE;
PROCEDURE Print_pkg;
PROCEDURE Init_and_print_pkg(N NUMBER, V VARCHAR2);
END Sr_pkg;
CREATE OR REPLACE PACKAGE BODY Sr_pkg IS
-- the body is required to have the pragma because the
-- specification of this package has the pragma
PRAGMA SERIALLY_REUSABLE;
PROCEDURE Print_pkg IS
BEGIN
DBMS_OUTPUT.PUT_LINE('num: ' || Sr_pkg.Num);
DBMS_OUTPUT.PUT_LINE('str: ' || Sr_pkg.Str);
DBMS_OUTPUT.PUT_LINE('number of table elems: ' || Sr_pkg.Str_tab.Count);
FOR i IN 1..Sr_pkg.Str_tab.Count LOOP
DBMS_OUTPUT.PUT_LINE(Sr_pkg.Str_tab(i));
END LOOP;
END;
PROCEDURE Init_and_print_pkg(N NUMBER, V VARCHAR2) IS
BEGIN
-- init the package globals
Sr_pkg.Num := N;
Sr_pkg.Str := V;
FOR i IN 1..n LOOP
Sr_pkg.Str_tab(i) := V || ' ' || i;
END LOOP;
-- print the package
Print_pkg;
END;
END Sr_pkg;
SET SERVEROUTPUT ON;
Rem SR package access in a CALL:
BEGIN
-- initialize and print the package
DBMS_OUTPUT.PUT_LINE('Initing and printing pkg state..');
Sr_pkg.Init_and_print_pkg(4, 'abracadabra');
-- print it in the same call to the server.
-- we should see the initialized values.
DBMS_OUTPUT.PUT_LINE('Printing package state in the same CALL...');
Sr_pkg.Print_pkg;
END;

Coding PL/SQL Procedures and Packages 7-49

Calling Stored Functions from SQL Expressions

Initing and printing pkg state..
num: 4
str: abracadabra
number of table elems: 4
abracadabra 1
abracadabra 2
abracadabra 3
abracadabra 4
Printing package state in the same CALL...
num: 4
str: abracadabra
number of table elems: 4
abracadabra 1
abracadabra 2
abracadabra 3
abracadabra 4
REM SR package access in subsequent CALL:
BEGIN
-- print the package in the next call to the server.
-- We should that the package state is reset to the initial (default) values.
DBMS_OUTPUT.PUT_LINE('Printing package state in the next CALL...');
Sr_pkg.Print_pkg;
END;
Statement processed.
Printing package state in the next CALL...
num: 10
str: default-init-str
number of table elems: 0

Example 3: Open Cursors in Serially Reusable Packages at Call Boundaries This example
demonstrates that any open cursors in serially reusable packages get closed
automatically at the end of a work boundary (which is a call). Also, in a new call, these
cursors need to be opened again.
REM
REM
REM
REM
REM
REM

For serially reusable pkg: At the end work boundaries
(which is currently the OCI call boundary) all open
cursors will be closed.
Because the cursor is closed - every time we fetch we
will start at the first row again.

CONNECT Scott/Tiger
DROP PACKAGE Sr_pkg;
DROP TABLE People;
CREATE TABLE People (Name VARCHAR2(20));
INSERT INTO People VALUES ('ET');
INSERT INTO People VALUES ('RAMBO');
CREATE OR REPLACE PACKAGE Sr_pkg IS
PRAGMA SERIALLY_REUSABLE;
CURSOR C IS SELECT Name FROM People;
END Sr_pkg;
SQL> SET SERVEROUTPUT ON;
SQL>
CREATE OR REPLACE PROCEDURE Fetch_from_cursor IS
Name VARCHAR2(200);
BEGIN
IF (Sr_pkg.C%ISOPEN) THEN
DBMS_OUTPUT.PUT_LINE('cursor is already open.');
ELSE

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DBMS_OUTPUT.PUT_LINE('cursor is closed; opening now.');
OPEN Sr_pkg.C;
END IF;
-- fetching from cursor.
FETCH sr_pkg.C INTO name;
DBMS_OUTPUT.PUT_LINE('fetched: ' || Name);
FETCH Sr_pkg.C INTO name;
DBMS_OUTPUT.PUT_LINE('fetched: ' || Name);
-- Oops forgot to close the cursor (Sr_pkg.C).
-- But, because it is a Serially Reusable pkg's cursor,
-- it will be closed at the end of this CALL to the server.
END;
EXECUTE fetch_from_cursor;
cursor is closed; opening now.
fetched: ET
fetched: RAMBO

Returning Large Amounts of Data from a Function
In a data warehousing environment, you might use a PL/SQL function to transform
large amounts of data. Perhaps the data is passed through a series of transformations,
each performed by a different function. PL/SQL table functions let you perform such
transformations without significant memory overhead or the need to store the data in
tables between each transformation stage. These functions can accept and return
multiple rows, can return rows as they are ready rather than all at once, and can be
parallelized.
In this technique:
■
■

■

■

■

The producer function uses the PIPELINED keyword in its declaration.
The producer function uses an OUT parameter that is a record, corresponding to a
row in the result set.
As each output record is completed, it is sent to the consumer function using the
PIPE ROW keyword.
The producer function ends with a RETURN statement that does not specify any
return value.
The consumer function or SQL statement uses the TABLE keyword to treat the
resulting rows like a regular table.

For example:
CREATE FUNCTION StockPivot(p refcur_pkg.refcur_t) RETURN TickerTypeSet PIPELINED
IS
out_rec TickerType := TickerType(NULL,NULL,NULL);
in_rec p%ROWTYPE;
BEGIN
LOOP
-- Function accepts multiple rows through a REF CURSOR argument.
FETCH p INTO in_rec;
EXIT WHEN p%NOTFOUND;
-- Return value is a record type that matches the table definition.
out_rec.ticker := in_rec.Ticker;
out_rec.PriceType := 'O';
out_rec.price := in_rec.OpenPrice;
-- Once a result row is ready, we send it back to the calling program,
-- and continue processing.
PIPE ROW(out_rec);
Coding PL/SQL Procedures and Packages 7-51

Coding Your Own Aggregate Functions

-- This function outputs twice as many rows as it receives as input.
out_rec.PriceType := 'C';
out_rec.Price := in_rec.ClosePrice;
PIPE ROW(out_rec);
END LOOP;
CLOSE p;
-- The function ends with a RETURN statement that does not specify any value.
RETURN;
END;
/
-- Here we use the result of this function in a SQL query.
SELECT * FROM TABLE(StockPivot(CURSOR(SELECT * FROM StockTable)));
-- Here we use the result of this function in a PL/SQL block.
DECLARE
total NUMBER := 0;
price_type VARCHAR2(1);
BEGIN
FOR item IN (SELECT * FROM TABLE(StockPivot(CURSOR(SELECT * FROM StockTable))))
LOOP
-- Access the values of each output row.
-- We know the column names based on the declaration of the output type.
-- This computation is just for illustration.
total := total + item.price;
price_type := item.price_type;
END LOOP;
END;
/

Coding Your Own Aggregate Functions
To analyze a set of rows and compute a result value, you can code your own aggregate
function that works the same as a built-in aggregate like SUM:
■

■

■
■

Define a SQL object type that defines these member functions:
■

ODCIAggregateInitialize

■

ODCIAggregateIterate

■

ODCIAggregateMerge

■

ODCIAggregateTerminate

Code the member functions. In particular, ODCIAggregateIterate
accumulates the result as it is called once for each row that is processed. Store any
intermediate results using the attributes of the object type.
Create the aggregate function, and associate it with the new object type.
Call the aggregate function from SQL queries, DML statements, or other places
that you might use the built-in aggregates. You can include typical options such as
DISTINCT and ALL in the calls to the aggregate function.
See Also: Oracle Database Data Cartridge Developer's Guide for
details of this process and the requirements for the member
functions

7-52 Oracle Database Application Developer’s Guide - Fundamentals

8
Coding Dynamic SQL
This chapter covers the following topics:
■

What Is Dynamic SQL?

■

Why Use Dynamic SQL?

■

Developing with Native Dynamic SQL: Scenario

■

Choosing Between Native Dynamic SQL and the DBMS_SQL Package

■

Programming with Dynamic SQL

■

Avoiding SQL Injection in PL/SQL

What Is Dynamic SQL?
Unlike static SQL, which remains the same in each execution, dynamic SQL enables
you to build SQL statements as character strings at runtime. The strings contain the
text of a SQL statement or PL/SQL block and can also contain placeholders for bind
arguments.
You can create general purpose, flexible applications with dynamic SQL because the
full text of a SQL statement may be unknown at compilation. You can use dynamic
SQL in several different development environments, including PL/SQL, Pro*C/C++,
and Java.
For an example of an application that uses dynamic SQL, suppose that a reporting
application in a data warehouse environment does not know a table name until
runtime. These tables are named according to the starting month and year of the
quarter, for example, inv_01_2003, inv_04_2003, inv_07_2003, inv_10_2003,
inv_01_2004, and so on. You can use dynamic SQL in your reporting application to
specify the table name at runtime.
In a different example, suppose that you want to run a complex query with a
user-selectable sort order. Instead of coding the query twice with a different ORDER BY
clause in each query, you can construct the query dynamically to include a specified
ORDER BY clause.

Programming with Dynamic SQL
For the sake of consistency, this chapter discusses dynamic SQL mainly from the
perspective of PL/SQL. To process most dynamic SQL statements, you use the
EXECUTE IMMEDIATE statement. To process a multi-row query in a PL/SQL
procedure, you use the OPEN-FOR, FETCH, and CLOSE statements.

Coding Dynamic SQL

8-1

Why Use Dynamic SQL?

Oracle Database enables you to implement dynamic SQL in a PL/SQL application in
the following ways:
■

■

Using native dynamic SQL, which involves placing dynamic SQL statements
directly into PL/SQL blocks
Calling procedures in the DBMS_SQL package

Although this chapter discusses PL/SQL support for dynamic SQL, you can call
dynamic SQL from other languages:
■

■

■

If you use C/C++, you can call dynamic SQL with the Oracle Call Interface (OCI),
or you can use the Pro*C/C++ precompiler to add dynamic SQL extensions to
your C code.
If you use COBOL, you can use the Pro*COBOL precompiler to add dynamic SQL
extensions to your COBOL code.
If you use Java, you can develop applications that use dynamic SQL with JDBC.

If you have a program that uses OCI, Pro*C/C++, or Pro*COBOL to execute dynamic
SQL, consider switching to native dynamic SQL inside PL/SQL stored procedures and
functions. The network round-trips required to perform dynamic SQL operations from
client-side applications might hurt performance. Stored procedures can reside on the
server, eliminating network overhead. You can call the PL/SQL stored procedures and
stored functions from the OCI, Pro*C/C++, or Pro*COBOL application.
See Also: Oracle Database PL/SQL Packages and Types Reference for
details about the DBMS_SQL package. To learn about calling Oracle
Database stored procedures and stored functions from languages
other than PL/SQL, consult the following resources:
■

Oracle Call Interface Programmer's Guide

■

Pro*C/C++ Programmer's Guide

■

Pro*COBOL Programmer's Guide

Why Use Dynamic SQL?
Dynamic SQL and static SQL both have advantages and disadvantages. The full text of
static SQL statements is known at compilation, which provides the following benefits:
■

■

Successful compilation verifies that the SQL statements reference valid database
objects and that the necessary privileges are in place to access the objects.
Performance of static SQL is generally better than dynamic SQL.

Despite these advantages, static SQL has limitations that can be overcome with
dynamic SQL, as in the following cases:
■

■

■

You do not know the full text of the SQL statements that must be executed in a
PL/SQL procedure. These SQL statements may depend on user input or on
processing work performed by the program.
You want to execute DDL statements and other SQL statements that are not
supported in purely static SQL programs.
You want to write a program that can handle changes in data definitions without
the need to recompile. Dynamic SQL is more flexible than static SQL because it
enables you to write reusable code that can be adapted for different environments.

8-2 Oracle Database Application Developer’s Guide - Fundamentals

Why Use Dynamic SQL?

As a general rule you should use dynamic SQL only if you cannot use static SQL to
accomplish your goals, or if using static SQL is too cumbersome. The following
sections describe typical situations in which you should use dynamic SQL:
■

Executing DDL and SCL Statements in PL/SQL

■

Executing Dynamic Queries

■

Referencing Database Objects that Do Not Exist at Compilation

■

Optimizing Execution Dynamically

■

Executing Dynamic PL/SQL Blocks

■

Performing Dynamic Operations Using Invoker's Rights

Executing DDL and SCL Statements in PL/SQL
Only dynamic SQL can execute the following types of statements within PL/SQL
program units:
■

Data definition language (DDL) statements such as CREATE, DROP, GRANT, and
REVOKE

■

Session control language (SCL) statements such as ALTER SESSION and SET ROLE

■

The TABLE clause in the SELECT statement

The following native dynamic SQL example uses a SELECT statement with the TABLE
clause.
Example 8–1 Using SELECT . . . TABLE in Dynamic SQL
-- Create an object t_emp and a datatype t_emplist as a table of type t_emp
CREATE TYPE t_emp AS OBJECT (id NUMBER, name VARCHAR2(20))
/
CREATE TYPE t_emplist AS TABLE OF t_emp
/
-- Create a table with a nested table of type t_emplist
CREATE TABLE dept_new (id NUMBER, emps t_emplist)
NESTED TABLE emps STORE AS emp_table;
-- Populate the dept_new table with data
INSERT INTO dept_new VALUES
(
10,
t_emplist
(
t_emp(1, 'SCOTT'),
t_emp(2, 'BRUCE')
)
);
-- Write a PL/SQL block that queries table dept_new and nested table emps
-- SELECT ... FROM ... TABLE is not allowed in static SQL in PL/SQL
DECLARE
v_deptid NUMBER;
v_ename
VARCHAR2(20);
BEGIN
EXECUTE IMMEDIATE 'SELECT d.id, e.name
FROM dept_new d, TABLE(d.emps) e
WHERE e.id = 1'
INTO v_deptid, v_ename;
END;
/

Coding Dynamic SQL

8-3

Why Use Dynamic SQL?

Oracle Database SQL Reference for information about DDL
and SCL statements

See Also:

Executing Dynamic Queries
You can use dynamic SQL to create applications that execute dynamic queries whose
full text is not available until runtime. Many types of applications need to use dynamic
queries, including applications that do the following:
■

Allow users to input or choose query search or sorting criteria at runtime

■

Allow users to input or choose optimizer hints at run time

■

Query a database where the data definitions of tables are constantly changing

■

Query a database where new tables are created often
"Querying with Dynamic SQL: Example" on page 8-14 and
"Developing with Native Dynamic SQL: Scenario" on page 8-7

See Also:

Referencing Database Objects that Do Not Exist at Compilation
Many applications must interact with data that is generated periodically. For example,
an application knows the table definitions at compile time, but not the table names.
Dynamic SQL enables you to specify table names at runtime.
In the sample data warehouse application discussed in "What Is Dynamic SQL?" on
page 8-1, the system generates new tables every quarter. You could allow a user to
specify the name of the table at runtime with a dynamic SQL query similar to the
following sample procedure.
Example 8–2 Dynamically Specifying a Table Name
CREATE OR REPLACE PROCEDURE query_invoice
(p_month VARCHAR2, p_year VARCHAR2)
IS
TYPE cur_typ IS REF CURSOR;
v_inv_cursor cur_typ; -- Declare a cursor variable
v_inv_query VARCHAR2(200);
v_inv_num
NUMBER;
v_inv_cust
VARCHAR2(20);
v_inv_amt
NUMBER;
BEGIN
-- Write dynamic query receiving month and year as parameters
-- and using these values to form the table name, for example, inv_APR_2004
v_inv_query := 'SELECT num, cust, amt FROM inv_'
|| p_month
|| '_'
|| p_year
|| ' WHERE v_inv_num = :g_id';
-- Open a cursor variable
OPEN v_inv_cursor FOR v_inv_query USING v_inv_num;
-- Fetch row into variables
LOOP
FETCH v_inv_cursor
INTO v_inv_num, v_inv_cust, v_inv_amt;
EXIT WHEN v_inv_cursor%NOTFOUND;
-- process row here
END LOOP;
CLOSE v_inv_cursor;
END;

8-4 Oracle Database Application Developer’s Guide - Fundamentals

Why Use Dynamic SQL?

/

Optimizing Execution Dynamically
You can use dynamic SQL to build a SQL statement that optimizes execution by
concatenating hints into a dynamic SQL statement. This technique enables you change
the hints based on your current database statistics without recompiling. The following
sample procedure uses a variable called p_hint to allow users to pass a hint option to
the SELECT statement.
Example 8–3 Concatenating Hints
CREATE OR REPLACE PROCEDURE query_emp
(p_hint VARCHAR2)
IS
TYPE cur_typ IS REF CURSOR;
v_emp_cursor cur_typ;
BEGIN
OPEN v_emp_cursor FOR 'SELECT '
|| p_hint
||' empno, ename, sal, job FROM emp WHERE empno = 7566';
-- process ...
CLOSE v_emp_cursor;
END;
/

In Example 8–3, the user can pass values such as the following for p_hint:
p_hint = '/*+ ALL_ROWS */'
p_hint = '/*+ FIRST_ROWS */'
p_hint = '/*+ CHOOSE */'

See Also: Oracle Database Performance Tuning Guide to learn more
about using hints

Executing Dynamic PL/SQL Blocks
You can use the EXECUTE IMMEDIATE statement to execute anonymous PL/SQL
blocks. In this way you can add flexibility by constructing the contents of the block at
runtime.
For example, suppose that you want to write an application that takes an event
number and dispatches it to a handler for the event. The name of the handler is in the
form EVENT_HANDLER_event_num, where event_num is the number of the event. One
approach is to implement the dispatcher as a switch statement, where the code
handles each event by making a static call to its appropriate handler. This code is not
very extensible because the dispatcher code must be updated whenever a handler for a
new event is added.
Example 8–4 Event Dispatching with Static SQL
CREATE OR REPLACE PROCEDURE event_handler_1
(p_handle NUMBER)
IS
BEGIN
-- process event 1
RETURN;
END;
/

Coding Dynamic SQL

8-5

Why Use Dynamic SQL?

CREATE OR REPLACE PROCEDURE event_handler_2
(p_handle NUMBER)
IS
BEGIN
-- process event 2
RETURN;
END;
/
CREATE OR REPLACE PROCEDURE event_handler_3
(p_handle NUMBER)
IS
BEGIN
-- process event 3
RETURN;
END;
/
CREATE OR REPLACE PROCEDURE event_dispatcher
(p_event_num NUMBER, p_handle NUMBER)
IS
BEGIN
IF (p_event_num = 1) THEN
EVENT_HANDLER_1(p_handle);
ELSIF (p_event_num = 2) THEN
EVENT_HANDLER_2(p_handle);
ELSIF (p_event_num = 3) THEN
EVENT_HANDLER_3(p_handle);
END IF;
END;
/

By using native dynamic SQL, you can write a smaller, more flexible event dispatcher
as shown in the following example.
Example 8–5 Event Dispatching with Native Dynamic SQL
CREATE OR REPLACE PROCEDURE event_dispatcher
(p_event_num NUMBER, p_handle NUMBER)
IS
BEGIN
EXECUTE IMMEDIATE
'BEGIN
EVENT_HANDLER_' || TO_CHAR(p_event_num) || '(:1);
END;'
USING p_handle;
END;
/

Performing Dynamic Operations Using Invoker's Rights
By using the invoker's rights feature with dynamic SQL, you can build applications
that issue dynamic SQL statements under the privileges and schema of the invoker.
These two features—invoker's rights and dynamic SQL—enable you to build reusable
application subcomponents that can operate on and access the invoker's data and
modules.
See Also: Oracle Database PL/SQL User's Guide and Reference to learn
about invoker's rights and native dynamic SQL

8-6 Oracle Database Application Developer’s Guide - Fundamentals

Developing with Native Dynamic SQL: Scenario

Developing with Native Dynamic SQL: Scenario
This scenario shows you how to perform the following operations with native
dynamic SQL:
■

Execute DDL and DML operations.

■

Execute single row and multiple row queries.

The database in this scenario is used for human resources. A master table named
offices contains the list of all company locations. The offices table has the
following definition:
Column Name
LOCATION

Null?
NOT_NULL

Type
VARCHAR2(200)

Multiple emp_location tables contain the employee information, where location is the
name of city where the office is located. For example, a table named emp_houston
contains employee information for the company's Houston office, whereas a table
named emp_boston contains employee information for the company's Boston office.
Each emp_location table has the following definition:
Column Name
EMPNO
ENAME
JOB
SAL
DEPTNO

Null?
NOT_NULL
NOT_NULL
NOT_NULL
NOT_NULL
NOT_NULL

Type
NUMBER(4)
VARCHAR2(10)
VARCHAR2(9)
NUMBER(7,2)
NUMBER(2)

The following sections describe various native dynamic SQL operations that can be
performed on the data in the hr database.

Sample DML Operation Using Native Dynamic SQL
The following native dynamic SQL procedure gives a raise to all employees with a
particular job title.
Example 8–6 Performing DML in Native Dynamic SQL
CREATE OR REPLACE PROCEDURE salary_raise
(p_raise_percent NUMBER, p_job VARCHAR2)
IS
TYPE loc_array_type IS TABLE OF VARCHAR2(40)
INDEX BY binary_integer;
v_dml_str VARCHAR2(200);
v_loc_array loc_array_type;
BEGIN
-- bulk fetch the list of office locations
SELECT location BULK COLLECT
INTO v_loc_array
FROM offices;
-- for each location, give a raise to employees with the given 'job'
FOR i IN v_loc_array.first..v_loc_array.last LOOP
v_dml_str := 'UPDATE emp_'
|| v_loc_array(i)
|| ' SET sal = sal * (1+(:p_raise_percent/100))'
|| ' WHERE p_job = :g_job_title';
EXECUTE IMMEDIATE v_dml_str USING p_raise_percent, p_job;
END LOOP;
END;
/
Coding Dynamic SQL

8-7

Developing with Native Dynamic SQL: Scenario

SHOW ERRORS;

Sample DDL Operation Using Native Dynamic SQL
The EXECUTE IMMEDIATE statement can perform DDL operations. For example, the
following procedure adds an office location.
Example 8–7 Creating a Table with Native Dynamic SQL
CREATE OR REPLACE PROCEDURE add_location
(p_loc VARCHAR2)
IS
BEGIN
-- Insert new location in master table, for example, detroit
INSERT INTO offices VALUES (p_loc);
-- Create an employee information table
EXECUTE IMMEDIATE
-- Use the parameter value to concatenate the table name, as in emp_detroit
'CREATE TABLE emp_'
|| p_loc
|| '(
empno
NUMBER(4) NOT NULL,
ename
VARCHAR2(10),
job
VARCHAR2(9),
sal
NUMBER(7,2),
deptno NUMBER(2)
)';
END;
/
SHOW ERRORS;

The following procedure uses the same concatenation technique to drop a table.
Example 8–8 Dropping a Table with Native Dynamic SQL
CREATE OR REPLACE PROCEDURE drop_location
(p_loc VARCHAR2)
IS
BEGIN
-- Drop the employee table for location 'p_loc', for example, emp_detroit
EXECUTE IMMEDIATE 'DROP TABLE ' || 'emp_' || p_loc;
-- Remove location from master table
DELETE FROM offices WHERE location = p_loc;
END;
/
SHOW ERRORS;

Sample Single-Row Query Using Native Dynamic SQL
The EXECUTE IMMEDIATE statement can perform dynamic single-row queries. You
can specify bind variables in the USING clause and fetch the resulting row into the
target specified in the INTO clause of the statement. The following function retrieves
the number of employees at a particular location performing a specified job.
Example 8–9 Performing Single-Row Queries in Native Dynamic SQL
CREATE OR REPLACE FUNCTION get_num_of_employees
(p_loc VARCHAR2, p_job VARCHAR2)
RETURN NUMBER
IS
v_query_str VARCHAR2(1000);

8-8 Oracle Database Application Developer’s Guide - Fundamentals

Choosing Between Native Dynamic SQL and the DBMS_SQL Package

v_num_of_employees NUMBER;
BEGIN
-- Use concatenation to form the table name in the SELECT statement
v_query_str := 'SELECT COUNT(*) FROM emp_'
|| p_loc
|| ' WHERE job = :1';
-- Execute the query and put the result row in a variable
EXECUTE IMMEDIATE v_query_str
INTO v_num_of_employees
USING p_job;
RETURN v_num_of_employees;
END;
/
SHOW ERRORS;

Sample Multiple-Row Query with Native Dynamic SQL
The OPEN-FOR, FETCH, and CLOSE statements can perform dynamic multiple-row
queries. For example, the following procedure lists all of the employees with a
particular job at a specified location.
Example 8–10

Performing Multiple-Row Queries with Dynamic SQL

CREATE OR REPLACE PROCEDURE list_employees
(p_loc VARCHAR2, p_job VARCHAR2)
IS
TYPE cur_typ IS REF CURSOR;
-- Define a cursor variable
v_emp_cursor cur_typ;
v_query_str
VARCHAR2(1000);
v_emp_name
VARCHAR2(20);
v_emp_num
NUMBER;
BEGIN
-- Use concatenation to form the SELECT statement
v_query_str := 'SELECT ename, empno FROM emp_'
|| p_loc
|| ' WHERE job = :g_job_title';
-- Open a cursor variable for the query
OPEN v_emp_cursor FOR v_query_str USING p_job;
-- Loop through each row to find employees who perform the specified job
LOOP
-- Fetch the employee name and ID into variables
FETCH v_emp_cursor INTO v_emp_name, v_emp_num;
EXIT WHEN v_emp_cursor%NOTFOUND;
-- Process row here
END LOOP;
CLOSE v_emp_cursor;
END;
/
SHOW ERRORS;

Choosing Between Native Dynamic SQL and the DBMS_SQL Package
Oracle Database provides two methods for using dynamic SQL within PL/SQL: native
dynamic SQL and the DBMS_SQL package. Each method has advantages and
disadvantages. The following sections provide detailed information about the
advantages of both methods.

Coding Dynamic SQL

8-9

Choosing Between Native Dynamic SQL and the DBMS_SQL Package

Native dynamic SQL enables you to place dynamic SQL statements directly into
PL/SQL code. These dynamic statements include the following:
■

Queries and DML statements

■

PL/SQL anonymous blocks

■

DDL statements

■

Transaction control statements

■

Session control statements

To process most native dynamic SQL statements, use the EXECUTE IMMEDIATE
statement. To process a multi-row SELECT statement, use OPEN-FOR, FETCH, and
CLOSE statements.
Note: To use native dynamic SQL, you must set the COMPATIBLE
initialization parameter to 8.1.0 or higher.

As an alternative to native dynamic SQL, the DBMS_SQL package offers a PL/SQL API
to execute dynamic SQL statements. For example, the DBMS_SQL package contains
procedures to do the following:
■

Open a cursor

■

Parse a cursor

■

Supply binds

Programs that use the DBMS_SQL package make calls to this package to perform
dynamic SQL operations.
See Also:
■

■

■

Oracle Database PL/SQL User's Guide and Reference to learn about
native dynamic SQL
Oracle Database PL/SQL Packages and Types Reference to learn about
the DBMS_SQL package
Oracle Database Upgrade Guide to learn about the COMPATIBLE
initialization parameter

Advantages of Native Dynamic SQL
Native dynamic SQL provides the following advantages over the DBMS_SQL package:
■

Native Dynamic SQL is Easy to Use

■

Native Dynamic SQL is Faster than DBMS_SQL

■

Native Dynamic SQL Supports User-Defined Types

■

Native Dynamic SQL Supports Fetching into Records

Native Dynamic SQL is Easy to Use
Because native dynamic SQL is integrated with SQL, you can use it in the same way
that you use static SQL within PL/SQL code. Native dynamic SQL code is typically
more compact and readable than equivalent code that uses the DBMS_SQL package.

8-10 Oracle Database Application Developer’s Guide - Fundamentals

Choosing Between Native Dynamic SQL and the DBMS_SQL Package

With the DBMS_SQL package you must call many procedures and functions in a strict
sequence, which means that even simple operations require extensive code. You can
avoid this complexity by using native dynamic SQL instead.
Table 8–1 illustrates the difference in the amount of code required to perform the same
operation with the DBMS_SQL package and native dynamic SQL.
Table 8–1

Code Comparison of DBMS_SQL Package and Native Dynamic SQL

DBMS_SQL Package

Native Dynamic SQL

CREATE OR REPLACE PROCEDURE insert_into_table
(p_table_name VARCHAR2, p_deptnumber NUMBER,
p_deptname VARCHAR2,
p_location
VARCHAR2)
IS
v_cur_hdl
INTEGER;
v_stmt_str
VARCHAR2(200);
v_rows_processed BINARY_INTEGER;

CREATE OR REPLACE PROCEDURE insert_into_table
(p_table_name VARCHAR2, p_deptnumber NUMBER,
p_deptname VARCHAR2,
p_location VARCHAR2)
IS
v_stmt_str
VARCHAR2(200);

BEGIN
v_stmt_str := 'INSERT INTO '
|| p_table_name
|| ' VALUES (:g_deptno, :g_dname, :g_loc)';
v_cur_hdl := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(v_cur_hdl, v_stmt_str,
DBMS_SQL.NATIVE);
DBMS_SQL.BIND_VARIABLE (v_cur_hdl,':g_deptno',
p_deptnumber);
DBMS_SQL.BIND_VARIABLE (v_cur_hdl, ':g_dname',
p_deptname);
DBMS_SQL.BIND_VARIABLE (v_cur_hdl, ':g_loc',
p_location);
v_rows_processed := DBMS_SQL.EXECUTE(v_cur_hdl);
DBMS_SQL.CLOSE_CURSOR(v_cur_hdl);
END;
/

BEGIN
v_stmt_str := 'INSERT INTO '
|| p_table_name
|| ' VALUES (:g_deptno, :g_dname, :g_loc)';
EXECUTE IMMEDIATE v_stmt_str
USING p_deptnumber, p_deptname,
p_location;

END;
/

Native Dynamic SQL is Faster than DBMS_SQL
Native dynamic SQL in PL/SQL performs comparably to static SQL because the
PL/SQL interpreter has built-in support. Programs that use native dynamic SQL are
much faster than programs that use the DBMS_SQL package. Typically, native dynamic
SQL statements perform 1.5 to 3 times better than equivalent DBMS_SQL calls. Of
course, performance gains may vary depending on your application.
Native dynamic SQL bundles the statement preparation, binding, and execution steps
into a single operation, which minimizes the data copying and procedure call
overhead and improves performance.
The DBMS_SQL package is based on a procedural API and incurs high procedure call
and data copy overhead. Each time you bind a variable, the DBMS_SQL package copies
the PL/SQL bind variable into its space for use during execution. Each time you
execute a fetch, the data is copied into the space managed by the DBMS_SQL package
and then the fetched data is copied, one column at a time, into the appropriate
PL/SQL variables, resulting in substantial overhead.
Improving Performance Through Bind Variables When using either native dynamic SQL or
the DBMS_SQL package, you can improve performance by using bind variables
because bind variables allow Oracle Database to share a single cursor for multiple SQL
statements.

Coding Dynamic SQL 8-11

Choosing Between Native Dynamic SQL and the DBMS_SQL Package

In Example 8–11 the native dynamic SQL code uses a parameter instead of a bind
variable to construct the SQL statement.
Example 8–11

Using Native Dynamic SQL Without Bind Variables

CREATE OR REPLACE PROCEDURE del_dept
(p_department_id departments.department_id%TYPE)
IS
BEGIN
EXECUTE IMMEDIATE 'DELETE FROM departments WHERE department_id = '
|| TO_CHAR(p_department_id);
END;
/
SHOW ERRORS;

For each distinct p_department_id parameter, the procedure creates a new cursor,
which causes resource contention and poor performance. Instead, you can construct
the SQL statement by using a bind variable, as shown in Example 8–12.
Example 8–12

Using Native Dynamic SQL with Bind Variables

CREATE OR REPLACE PROCEDURE del_dept
(p_department_id departments.department_id%TYPE)
IS
BEGIN
EXECUTE IMMEDIATE 'DELETE FROM departments WHERE department_id = :1'
USING p_department_id;
END;
/
SHOW ERRORS;

In Example 8–12 the same cursor is reused for different values of the bind my_deptno,
which improves performance and scalability.

Native Dynamic SQL Supports User-Defined Types
Native dynamic SQL supports all of the types supported by static SQL in PL/SQL,
including user-defined types such as user-defined objects, collections, and REFs. The
DBMS_SQL package does not support these user-defined types.
The DBMS_SQL package provides limited support for arrays.
Refer to the Oracle Database PL/SQL Packages and Types Reference for
information.
Note:

Native Dynamic SQL Supports Fetching into Records
Native dynamic SQL and static SQL both support fetching into records, but the
DBMS_SQL package does not. With native dynamic SQL, the rows resulting from a
query can be directly fetched into PL/SQL records. In Example 8–13 the rows from a
query are fetched into the v_emp_rec variable.
Example 8–13

Using Native Dynamic SQL to Fetch into Records

DECLARE
TYPE EmpCurTyp
v_emp_cursor
emp_record
v_stmt_str
v_e_job

IS REF CURSOR;
EmpCurTyp;
emp%ROWTYPE;
VARCHAR2(200);
emp.job%TYPE;

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BEGIN
v_stmt_str := 'SELECT * FROM emp WHERE job = :1';
-- in a multi-row query
OPEN v_emp_cursor FOR v_stmt_str USING 'MANAGER';
LOOP
FETCH v_emp_cursor INTO emp_record;
EXIT WHEN v_emp_cursor%NOTFOUND;
END LOOP;
CLOSE v_emp_cursor;
-- in a single-row query
EXECUTE IMMEDIATE v_stmt_str INTO emp_record USING 'PRESIDENT';
END;
/

Advantages of the DBMS_SQL Package
The DBMS_SQL package provides the following advantages over native dynamic SQL:
■

DBMS_SQL is Supported in Client-Side Programs

■

DBMS_SQL Supports Statements with Unknown Number of Inputs or Outputs

■

DBMS_SQL Supports SQL Statements Larger than 32 KB

■

DBMS_SQL Lets You Reuse SQL Statements

DBMS_SQL is Supported in Client-Side Programs
The DBMS_SQL package is supported in client-side programs, but native dynamic SQL
is not. Every call to the DBMS_SQL package from the client-side program translates to a
PL/SQL remote procedure call (RPC). These calls occur when you need to do any of
the following:
■

Bind a variable

■

Define a variable

■

Execute a statement

DBMS_SQL Supports Statements with Unknown Number of Inputs or Outputs
Native dynamic SQL does not support statements with an unknown number of inputs
or outputs. The DBMS_SQL package does not have this limitation. One consequence is
that you can use the DESCRIBE_COLUMNS procedure in the DBMS_SQL package to
describe columns for a cursor opened and parsed through DBMS_SQL. This feature is
similar to the DESCRIBE command in SQL*Plus. Native dynamic SQL does not have a
DESCRIBE facility.
See Also: Oracle Database PL/SQL Packages and Types Reference for an
example of using DESCRIBE_COLUMNS to create a query in a situation
where the SELECT list is not known until runtime

DBMS_SQL Supports SQL Statements Larger than 32 KB
The DBMS_SQL package supports SQL statements larger than 32 KB. Native dynamic
SQL does not.

DBMS_SQL Lets You Reuse SQL Statements
The PARSE procedure in the DBMS_SQL package parses a SQL statement once. After
the initial parsing, you can use the statement multiple times with different sets of bind
arguments.
Coding Dynamic SQL 8-13

Choosing Between Native Dynamic SQL and the DBMS_SQL Package

Native dynamic SQL prepares a SQL statement each time the statement is used, which
typically involves parsing, optimization, and plan generation. Although the extra
prepare operations incur a small performance penalty, the decrease in speed is
typically outweighed by the performance benefits of native dynamic SQL.

Examples of DBMS_SQL Package Code and Native Dynamic SQL Code
The following examples illustrate the code differences necessary to complete
operations with the DBMS_SQL package and native dynamic SQL. Specifically, the
following types of examples are presented:
■

Query

■

DML operation

■

DML returning operation

In general, the native dynamic SQL code is more readable and compact, which can
improve developer productivity.

Querying with Dynamic SQL: Example
The following example includes a dynamic query statement with one bind variable
(:g_jobname) and two select columns (ename and sal):
v_stmt_str := 'SELECT ename, sal
FROM emp
WHERE job = :g_jobname';

This example queries for employees with the job description SALESMAN in the job
column of the emp table. Table 8–2 shows sample code that accomplishes this query
using the DBMS_SQL package and native dynamic SQL.

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Table 8–2

Querying Using the DBMS_SQL Package and Native Dynamic SQL

DBMS_SQL Query Operation

Native Dynamic SQL Query Operation

DECLARE
v_stmt_str
VARCHAR2(200);
v_cur_hdl
INT;
v_rows_processed INT;
v_name
VARCHAR2(10);
v_salary
INT;
BEGIN
v_cur_hdl := DBMS_SQL.OPEN_CURSOR; -- open cursor
v_stmt_str := 'SELECT ename, sal
FROM emp
WHERE job = :g_jobname';
DBMS_SQL.PARSE(v_cur_hdl,v_stmt_str,DBMS_SQL.NATIVE);
-- Supply binds (bind by name)
DBMS_SQL.BIND_VARIABLE(v_cur_hdl, 'g_jobname',
'SALESMAN');
-- Describe defines
DBMS_SQL.DEFINE_COLUMN(v_cur_hdl, 1, v_name, 200);
DBMS_SQL.DEFINE_COLUMN(v_cur_hdl, 2, v_salary);
-- Execute
v_rows_processed := DBMS_SQL.EXECUTE(v_cur_hdl);
LOOP
-- Fetch a row
IF DBMS_SQL.FETCH_ROWS(v_cur_hdl) > 0 THEN
-- Fetch columns from the row
DBMS_SQL.COLUMN_VALUE(v_cur_hdl, 1, v_name);
DBMS_SQL.COLUMN_VALUE(v_cur_hdl, 2, v_salary);
-- Process
ELSE
EXIT;
END IF;
END LOOP;
DBMS_SQL.CLOSE_CURSOR(v_cur_hdl); -- close cursor
END;
/

DECLARE
TYPE EmpCurTyp IS REF CURSOR;
v_emp_cursor
EmpCurTyp;
v_stmt_str
VARCHAR2(200);
v_name
VARCHAR2(20);
v_salary
NUMBER;
BEGIN
v_stmt_str := 'SELECT ename, sal
FROM emp
WHERE job = :1';
OPEN v_emp_cursor FOR v_stmt_str
USING 'SALESMAN';
LOOP
FETCH v_emp_cursor
INTO v_name, v_salary;
EXIT WHEN v_emp_cursor%NOTFOUND;
-- Process data
END LOOP;
CLOSE v_emp_cursor;

END;
/

Performing DML with Dynamic SQL: Example
The following example includes a dynamic INSERT statement for a table with three
columns.
v_stmt_str := 'INSERT INTO dept_new VALUES (:g_deptno, :g_dname, :g_loc)';

This example inserts a new row for which the column values are in the PL/SQL
variables deptnumber, deptname, and location. Table 8–3 shows sample code that
accomplishes this task with the DBMS_SQL package and native dynamic SQL.

Coding Dynamic SQL 8-15

Choosing Between Native Dynamic SQL and the DBMS_SQL Package

Table 8–3

DML Operation Using the DBMS_SQL Package and Native Dynamic SQL

DBMS_SQL DML Operation

Native Dynamic SQL DML Operation

DECLARE
v_stmt_str
VARCHAR2(200);
v_cur_hdl
NUMBER;
v_deptnumber
NUMBER := 99;
v_deptname
VARCHAR2(20);
v_location
VARCHAR2(10);
v_rows_processed
NUMBER;
BEGIN
v_stmt_str := 'INSERT INTO dept
VALUES (:g_deptno, :g_dname, :g_loc)';
v_cur_hdl := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE(v_cur_hdl, v_stmt_str,
DBMS_SQL.NATIVE);
-- Supply binds
DBMS_SQL.BIND_VARIABLE
(v_cur_hdl, ':g_deptno', v_deptnumber);
DBMS_SQL.BIND_VARIABLE
(v_cur_hdl, ':g_dname', v_deptname);
DBMS_SQL.BIND_VARIABLE
(v_cur_hdl, ':g_loc', v_location);
v_rows_processed := DBMS_SQL.EXECUTE(v_cur_hdl);
DBMS_SQL.CLOSE_CURSOR(v_cur_hdl);
END;
/

DECLARE
v_stmt_str
v_deptnumber
v_deptname
v_location

VARCHAR2(200);
NUMBER := 99;
VARCHAR2(20);
VARCHAR2(10);

BEGIN
v_stmt_str := 'INSERT INTO dept
VALUES (:g_deptno, :g_dname, :g_loc)';
EXECUTE IMMEDIATE v_stmt_str
USING v_deptnumber, v_deptname,
v_location;

END;
/

Performing DML with RETURNING Clause Using Dynamic SQL: Example
The following example uses a dynamic UPDATE statement to update the location of a
department, then returns the name of the department:
v_stmt_str := 'UPDATE dept_new
SET loc = :g_newloc
WHERE deptno = :g_deptno
RETURNING dname INTO :g_dname';

Table 8–4 shows sample code that accomplishes this operation using both the
DBMS_SQL package and native dynamic SQL.

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Table 8–4

DML Returning Operation Using the DBMS_SQL Package and Native Dynamic SQL

DBMS_SQL DML Returning Operation

Native Dynamic SQL DML Returning Operation

DECLARE
deptname_array
DBMS_SQL.VARCHAR2_TABLE;
v_cur_hdl
INT;
v_stmt_str
VARCHAR2(200);
v_location
VARCHAR2(20);
v_deptnumber
NUMBER := 10;
v_rows_procsd
NUMBER;
BEGIN
v_stmt_str := 'UPDATE dept
SET loc = :g_newloc
WHERE deptno = :g_deptno
RETURNING dname INTO :g_dname';
v_cur_hdl := DBMS_SQL.OPEN_CURSOR;
DBMS_SQL.PARSE
(v_cur_hdl, v_stmt_str, DBMS_SQL.NATIVE);
-- Supply binds
DBMS_SQL.BIND_VARIABLE
(v_cur_hdl, ':g_newloc', v_location);
DBMS_SQL.BIND_VARIABLE
(v_cur_hdl, ':g_deptno', v_deptnumber);
DBMS_SQL.BIND_ARRAY
(v_cur_hdl, ':g_dname', deptname_array);
-- Execute cursor
v_rows_procsd := DBMS_SQL.EXECUTE(v_cur_hdl);
-- Get RETURNING column into OUT bind array
DBMS_SQL.VARIABLE_VALUE
(v_cur_hdl, ':g_dname', deptname_array);
DBMS_SQL.CLOSE_CURSOR(v_cur_hdl);
END;
/

DECLARE
deptname_array
v_stmt_str
v_location
v_deptnumber
v_deptname

DBMS_SQL.VARCHAR2_TABLE;
VARCHAR2(200);
VARCHAR2(20);
NUMBER := 10;
VARCHAR2(20);

BEGIN
v_stmt_str := 'UPDATE dept
SET loc = :g_newloc
WHERE deptno = :g_deptno
RETURNING dname INTO :g_dname';
EXECUTE IMMEDIATE v_stmt_str
USING v_location, v_deptnumber, OUT
v_deptname;

END;
/

Avoiding SQL Injection in PL/SQL
SQL injection is a technique for maliciously exploiting applications that use
client-supplied data in SQL statements. The purpose of this technique is to gain
unauthorized access to a database in order to query or manipulate restricted data. This
section describes SQL injection vulnerabilities in PL/SQL and explains how you can
guard against them.
This section contains the following topics:
■

Overview of SQL Injection Techniques

■

Guarding Against SQL Injection

Overview of SQL Injection Techniques
Although SQL injection techniques differ, they all exploit a single vulnerability: string
input is not correctly validated and is concatenated into a dynamic SQL statement. For
the purposes of this discussion, SQL injection attacks can be classified as follows:
■

Statement Modification

■

Statement Injection

The following sections describe these techniques. To try the examples in your sample
database, run the script in Example 8–14.

Coding Dynamic SQL 8-17

Avoiding SQL Injection in PL/SQL

Example 8–14

Setup for Injection Examples

CONNECT hr/hr
SET SERVEROUTPUT ON
SET LINESIZE 150
SET ECHO OFF
DROP TABLE user_pwd;
CREATE TABLE user_pwd( username VARCHAR2(100), password VARCHAR2(100) );
INSERT INTO user_pwd VALUES('whitehat', 'secret');
DROP TABLE delemp;
CREATE TABLE delemp AS SELECT * FROM employees;
COMMIT;

Statement Modification
SQL modification involves deliberately altering a dynamic SQL statement so that it
executes in a way unintended by the application developer. Typically, the user
retrieves unauthorized data by changing the WHERE clause of a query or by inserting a
UNION ALL clause. The classic example of this technique is bypassing password
authentication by making a WHERE clause always TRUE.
Suppose a Web form prompts a user to enter a username and password. When the
user clicks Submit, the form invokes a PL/SQL stored procedure that concatenates the
username and password entered in the form to build a dynamic SQL statement. The
procedure executes the query to authenticate the user.
Example 8–15 illustrates an authentication procedure that you can test in SQL*Plus.
The ckpwd procedure uses concatenation to build a SQL query of the user_pwd table.
If the user enters a username and password stored in the table, then the execution of
the query retrieves a single row and the user is authenticated. The ckpwd procedure
also displays the concatenated query so that you can see which query is executed.
Example 8–15

ckpwd Procedure

CREATE OR REPLACE PROCEDURE ckpwd (p_user IN VARCHAR2, p_pass IN VARCHAR2)
IS
v_query VARCHAR2(100);
v_output NUMBER;
BEGIN
v_query :=
q'{SELECT COUNT(*) FROM user_pwd }'
||
q'{WHERE username = '}'
||
p_user
||
q'{' AND password = '}'
||
p_pass
||
q'{'}';
DBMS_OUTPUT.PUT_LINE(CHR(10)||'Built the following query:'||CHR(10)||v_query);
EXECUTE IMMEDIATE v_query
INTO v_output;
IF v_output = 1 THEN
DBMS_OUTPUT.PUT_LINE(CHR(10)||p_user||' is authenticated');
ELSE
DBMS_OUTPUT.PUT_LINE(CHR(10)||'access denied');
END IF;
END;
/

Suppose that the user whitehat enters the password secret in a Web form. You can
simulate this scenario by invoking the code shown in Example 8–16 in SQL*Plus to
authenticate whitehat (sample output included).

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Avoiding SQL Injection in PL/SQL

Example 8–16

Authenticating a User with the ckpwd Procedure

BEGIN
ckpwd
(
p_user => q'{whitehat}',
p_pass => q'{secret}'
);
END;
/
Built the following query:
SELECT COUNT(*) FROM user_pwd WHERE username = 'whitehat' AND password = 'secret'
whitehat is authenticated

A malicious user could exploit the concatenation vulnerability and enter the username
x in the Web-based form and the text shown in Example 8–17 as a password.
Example 8–17

Password String Entered in Form

x' OR 'x' = 'x

You can simulate this scenario by executing the code in Example 8–18 in SQL*Plus
(sample output included).
Example 8–18

Performing Statement Modification with the ckpwd Procedure

BEGIN
ckpwd
(
p_user => q'{x}',
p_pass => q'{x' OR 'x' = 'x}'
);
END;
/
Built the following query:
SELECT COUNT(*) FROM user_pwd WHERE username = 'x' AND password = 'x' OR 'x' = 'x'
x is authenticated

By using the cleverly constructed password in Example 8–17, you alter the
concatenated SQL statement so that the OR condition always returns TRUE. Thus, the
query of the user_pwd table always succeeds no matter which username is entered.

Statement Injection
Statement injection occurs when a user appends one or more new SQL statements to a
dynamically generated SQL statement. Anonymous PL/SQL blocks are vulnerable to
this technique.
Suppose a Web form prompts a user to enter a username and password. When the
user clicks Submit, the form invokes a PL/SQL stored procedure that concatenates the
username and password entered in the form into an anonymous PL/SQL block. The
procedure then executes the anonymous block to authenticate the user.
Example 8–19 illustrates an authentication procedure that you can test in SQL*Plus.
The call_ckpwd procedure uses concatenation to build an anonymous block that
invokes the ckpwd procedure from Example 8–15. If the user enters a username and
password stored in the user_pwd table, then the execution of the block retrieves a
Coding Dynamic SQL 8-19

Avoiding SQL Injection in PL/SQL

single row and the user is authenticated. The call_ckpwd procedure also prints the
concatenated text so that you can see which block is executed.
Example 8–19

call_ckpwd Procedure

CREATE OR REPLACE PROCEDURE call_ckpwd (p_user IN VARCHAR2, p_pass IN VARCHAR2)
IS
v_block VARCHAR2(100);
BEGIN
v_block := q'{BEGIN ckpwd( '}'
||
p_user
|| q'{' , '}'
||
p_pass
|| q'{' ); END; }';
DBMS_OUTPUT.PUT_LINE(CHR(10)||
'Built the following anonymous block:'||CHR(10)||v_block);
EXECUTE IMMEDIATE v_block;
END;
/

Suppose that the user whitehat enters the password secret in a Web-based form.
You can simulate this scenario by invoking the call_ckpwd procedure shown in
Example 8–20 in SQL*Plus (sample output included).
Example 8–20

Authenticating a User with the call_ckpwd Procedure

BEGIN
call_ckpwd
(
p_user => q'{whitehat}',
p_pass => q'{secret}'
);
END;
/
Built the following anonymous block:
BEGIN ckpwd( 'whitehat' , 'secret' ); END;
Built the following query:
SELECT COUNT(*) FROM user_pwd WHERE username = 'whitehat' AND password = 'secret'
whitehat is authenticated

If whitehat turns bad, then he could enter the string shown in Example 8–21 as the
password in a Web form.
Example 8–21

Bogus Password Entered in Form

secret'); DELETE FROM hr.delemp WHERE UPPER('x') = UPPER('x

You can simulate this technique by invoking the call_ckpwd procedure shown in
Example 8–22 in SQL*Plus (sample output included).
Example 8–22 Performing Statement Injection with the call_ckpwd Procedure
BEGIN
call_ckpwd
(
p_user => q'{whitehat}',

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Avoiding SQL Injection in PL/SQL

p_pass => q'{secret'); DELETE FROM hr.delemp WHERE UPPER('x') = UPPER('x}'
);
END;
/
Built the following anonymous block:
BEGIN ckpwd( 'whitehat' , 'secret' ); DELETE FROM hr.delemp WHERE UPPER('x') = UPPER('x'); END;
Built the following query:
SELECT COUNT(*) FROM user_pwd WHERE username = 'whitehat' AND password = 'secret'
whitehat is authenticated

The bogus password in Example 8–21 causes the system to authenticate whitehat
and silently execute the injected DELETE statement. A query of the delemp table
shows that the injected statement silently removed all rows from the table:
SELECT * FROM delemp;
no rows selected

Guarding Against SQL Injection
If you use dynamic SQL in your PL/SQL applications, then you must check the input
text to ensure that it is exactly and only what is expected. You have the following
useful techniques at your disposal:
■

Using Bind Variables to Guard Against SQL Injection

■

Using Validation Checks to Guard Against SQL Injection

Using Bind Variables to Guard Against SQL Injection
"Improving Performance Through Bind Variables" on page 8-11 shows how you can
use bind variables to improve performance in dynamic SQL. Besides improving
performance, binding placeholders renders your PL/SQL code immune to SQL
injection attacks.
The ckpwd procedure shown in Example 8–15 used concatenation instead of bind
variables. Example 8–23 shows how you could rewrite the procedure to use bind
variables instead of concatenation.
Example 8–23

ckpwd_bind Procedure

CREATE OR REPLACE PROCEDURE ckpwd_bind (p_user IN VARCHAR2, p_pass IN VARCHAR2)
IS
v_query VARCHAR2(100);
v_output NUMBER;
BEGIN
v_query :=
q'{SELECT COUNT(*) FROM user_pwd WHERE username = :1 AND password = :2}';
DBMS_OUTPUT.PUT_LINE(CHR(10)||'Built the following query:'||CHR(10)||v_query);
EXECUTE IMMEDIATE v_query
INTO v_output
USING p_user, p_pass;
IF v_output = 1 THEN
DBMS_OUTPUT.PUT_LINE(CHR(10)||p_user||' is authenticated');
ELSE
DBMS_OUTPUT.PUT_LINE(CHR(10)||'access denied');
END IF;
END;

Coding Dynamic SQL 8-21

Avoiding SQL Injection in PL/SQL

/

If the user tries to pass the bogus password shown in Example 8–17 to the
ckpwd_bind procedure, then the technique fails to authenticate the user. You can
execute the block shown in Example 8–24 in SQL*Plus to test the revised version of the
code (sample output included).
Example 8–24

Preventing Statement Modification with the ckpwd_bind Procedure

BEGIN
ckpwd_bind
(
p_user => q'{x}',
p_pass => q'{x' OR 'x' = 'x}'
);
END;
/
Built the following query:
SELECT COUNT(*) FROM user_pwd WHERE username = :1 AND password = :2
access denied

The same binding technique fixes the vulnerable call_ckpwd procedure shown in
Example 8–19. By using bind variables exclusively in your code, you avoid
concatenating SQL statements and thereby prevent malicious users from altering or
injecting additional statements. Oracle database uses the value of the bind variable
exclusively and does not interpret its contents in any way. This technique is the most
effective way to prevent SQL injection in PL/SQL programs.

Using Validation Checks to Guard Against SQL Injection
A program should always validate user input to ensure that it is what is intended. For
example, if the user is passing in a department number for a DELETE statement, then
check the validity of this department number by querying the departments table.
Similarly, if a user enters the name of a table to be deleted, check that this table exists
by querying the ALL_TABLES view.

8-22 Oracle Database Application Developer’s Guide - Fundamentals

9
Coding Triggers
Triggers are procedures that are stored in the database and are implicitly run, or fired,
when something happens.
Traditionally, triggers supported the execution of a PL/SQL block when an INSERT,
UPDATE, or DELETE occurred on a table or view. Triggers support system and other
data events on DATABASE and SCHEMA. Oracle Database also supports the execution
of PL/SQL or Java procedures.
This chapter discusses DML triggers, INSTEAD OF triggers, and system triggers
(triggers on DATABASE and SCHEMA). Topics include:
■

Designing Triggers

■

Creating Triggers

■

Coding the Trigger Body

■

Compiling Triggers

■

Modifying Triggers

■

Enabling and Disabling Triggers

■

Viewing Information About Triggers

■

Examples of Trigger Applications

■

Responding to System Events through Triggers

Designing Triggers
Use the following guidelines when designing your triggers:
■

■

■

■

Use triggers to guarantee that when a specific operation is performed, related
actions are performed.
Do not define triggers that duplicate features already built into Oracle Database.
For example, do not define triggers to reject bad data if you can do the same
checking through declarative integrity constraints.
Limit the size of triggers. If the logic for your trigger requires much more than 60
lines of PL/SQL code, it is better to include most of the code in a stored procedure
and call the procedure from the trigger.
Use triggers only for centralized, global operations that should be fired for the
triggering statement, regardless of which user or database application issues the
statement.

Coding Triggers

9-1

Creating Triggers

■

■

Do not create recursive triggers. For example, creating an AFTER UPDATE statement
trigger on the Emp_tab table that itself issues an UPDATE statement on Emp_tab,
causes the trigger to fire recursively until it has run out of memory.
Use triggers on DATABASE judiciously. They are executed for every user every time
the event occurs on which the trigger is created.

Creating Triggers
Triggers are created using the CREATE TRIGGER statement. This statement can be used
with any interactive tool, such as SQL*Plus or Enterprise Manager. When using an
interactive tool, a single slash (/) on the last line is necessary to activate the CREATE
TRIGGER statement.
The following statement creates a trigger for the Emp_tab table.
CREATE OR REPLACE TRIGGER Print_salary_changes
BEFORE DELETE OR INSERT OR UPDATE ON Emp_tab
FOR EACH ROW
WHEN (new.Empno > 0)
DECLARE
sal_diff number;
BEGIN
sal_diff := :new.sal - :old.sal;
dbms_output.put('Old salary: ' || :old.sal);
dbms_output.put(' New salary: ' || :new.sal);
dbms_output.put_line(' Difference ' || sal_diff);
END;
/

The trigger is fired when DML operations (INSERT, UPDATE, and DELETE statements)
are performed on the table. You can choose what combination of operations should fire
the trigger.
Because the trigger uses the BEFORE keyword, it can access the new values before they
go into the table, and can change the values if there is an easily-corrected error by
assigning to :NEW.column_name. You might use the AFTER keyword if you want the
trigger to query or change the same table, because triggers can only do that after the
initial changes are applied and the table is back in a consistent state.
Because the trigger uses the FOR EACH ROW clause, it might be executed multiple
times, such as when updating or deleting multiple rows. You might omit this clause if
you just want to record the fact that the operation occurred, but not examine the data
for each row.
Once the trigger is created, entering the following SQL statement:
UPDATE Emp_tab SET sal = sal + 500.00 WHERE deptno = 10;

fires the trigger once for each row that is updated, in each case printing the new salary,
old salary, and the difference.
The CREATE (or CREATE OR REPLACE) statement fails if any errors exist in the
PL/SQL block.
Note:

The size of the trigger cannot be more than 32K.

The following sections use this example to illustrate the way that parts of a trigger are
specified.
9-2 Oracle Database Application Developer’s Guide - Fundamentals

Creating Triggers

See Also: "Examples of Trigger Applications" on page 9-23 for more
realistic examples of CREATE TRIGGER statements

Types of Triggers
A trigger is either a stored PL/SQL block or a PL/SQL, C, or Java procedure
associated with a table, view, schema, or the database itself. Oracle Database
automatically executes a trigger when a specified event takes place, which may be in
the form of a system event or a DML statement being issued against the table.
Triggers can be:
■

DML triggers on tables.

■

INSTEAD OF triggers on views.

■

System triggers on DATABASE or SCHEMA: With DATABASE, triggers fire for each
event for all users; with SCHEMA, triggers fire for each event for that specific user.
Oracle Database SQL Reference for information on trigger
creation syntax

See Also:

Overview of System Events
You can create triggers to be fired on any of the following:
■

DML statements (DELETE, INSERT, UPDATE)

■

DDL statements (CREATE, ALTER, DROP)

■

Database operations (SERVERERROR, LOGON, LOGOFF, STARTUP, SHUTDOWN)

Getting the Attributes of System Events
You can get certain event-specific attributes when the trigger is fired.
Creating a trigger on DATABASE implies that the triggering event is outside the scope
of a user (for example, database STARTUP and SHUTDOWN), and it applies to all users
(for example, a trigger created on LOGON event by the DBA).
Creating a trigger on SCHEMA implies that the trigger is created in the current user's
schema and is fired only for that user.
For each trigger, publication can be specified on DML and system events.
See Also: "Responding to System Events through Triggers" on
page 9-37

Naming Triggers
Trigger names must be unique with respect to other triggers in the same schema.
Trigger names do not need to be unique with respect to other schema objects, such as
tables, views, and procedures. For example, a table and a trigger can have the same
name (however, to avoid confusion, this is not recommended).

When Is the Trigger Fired?
A trigger is fired based on a triggering statement, which specifies:
■

The SQL statement or the system event, database event, or DDL event that fires
the trigger body. The options include DELETE, INSERT, and UPDATE. One, two, or
all three of these options can be included in the triggering statement specification.

Coding Triggers

9-3

Creating Triggers

■

The table, view, DATABASE, or SCHEMA associated with the trigger.
Exactly one table or view can be specified in the triggering
statement. If the INSTEAD OF option is used, then the triggering
statement may only specify a view; conversely, if a view is specified in
the triggering statement, then only the INSTEAD OF option may be
used.
Note:

For example, the PRINT_SALARY_CHANGES trigger fires after any DELETE, INSERT,
or UPDATE on the Emp_tab table. Any of the following statements trigger the
PRINT_SALARY_CHANGES trigger given in the previous example:
DELETE
INSERT
INSERT
UPDATE

FROM Emp_tab;
INTO Emp_tab VALUES ( ... );
INTO Emp_tab SELECT ... FROM ... ;
Emp_tab SET ... ;

Do Import and SQL*Loader Fire Triggers?
INSERT triggers fire during SQL*Loader conventional loads. (For direct loads, triggers
are disabled before the load.)
The IGNORE parameter of the IMP command determines whether triggers fire during
import operations:
■

■

■

If IGNORE=N (default) and the table already exists, then import does not change
the table and no existing triggers fire.
If the table does not exist, then import creates and loads it before any triggers are
defined, so again no triggers fire.
If IGNORE=Y, then import loads rows into existing tables. Any existing triggers
fire, and indexes are updated to account for the imported data.

How Column Lists Affect UPDATE Triggers
An UPDATE statement might include a list of columns. If a triggering statement
includes a column list, the trigger is fired only when one of the specified columns is
updated. If a triggering statement omits a column list, the trigger is fired when any
column of the associated table is updated. A column list cannot be specified for
INSERT or DELETE triggering statements.
The previous example of the PRINT_SALARY_CHANGES trigger could include a
column list in the triggering statement. For example:
... BEFORE DELETE OR INSERT OR UPDATE OF ename ON Emp_tab ...

Notes:
■
■

■

You cannot specify a column list for UPDATE with INSTEAD OF triggers.
If the column specified in the UPDATE OF clause is an object column, then the
trigger is also fired if any of the attributes of the object are modified.
You cannot specify UPDATE OF clauses on collection columns.

Controlling When a Trigger Is Fired (BEFORE and AFTER Options)
The BEFORE or AFTER option in the CREATE TRIGGER statement specifies exactly
when to fire the trigger body in relation to the triggering statement that is being run. In

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Creating Triggers

a CREATE TRIGGER statement, the BEFORE or AFTER option is specified just before the
triggering statement. For example, the PRINT_SALARY_CHANGES trigger in the
previous example is a BEFORE trigger.
In general, you use BEFORE or AFTER triggers to achieve the following results:
■

Use BEFORE row triggers to modify the row before the row data is written to disk.

■

Use AFTER row triggers to obtain, and perform operations, using the row ID.
BEFORE row triggers are slightly more efficient than AFTER
row triggers. With AFTER row triggers, affected data blocks must be
read (logical read, not physical read) once for the trigger and then
again for the triggering statement. Alternatively, with BEFORE row
triggers, the data blocks must be read only once for both the triggering
statement and the trigger.
Note:

BEFORE Triggers Fired Multiple Times
If an UPDATE or DELETE statement detects a conflict with a concurrent UPDATE, then
Oracle Database performs a transparent ROLLBACK to SAVEPOINT and restarts the
update. This can occur many times before the statement completes successfully. Each
time the statement is restarted, the BEFORE statement trigger is fired again. The
rollback to savepoint does not undo changes to any package variables referenced in
the trigger. Your package should include a counter variable to detect this situation.

Ordering of Triggers
A relational database does not guarantee the order of rows processed by a SQL
statement. Therefore, do not create triggers that depend on the order in which rows are
processed. For example, do not assign a value to a global package variable in a row
trigger if the current value of the global variable is dependent on the row being
processed by the row trigger. Also, if global package variables are updated within a
trigger, then it is best to initialize those variables in a BEFORE statement trigger.
When a statement in a trigger body causes another trigger to be fired, the triggers are
said to be cascading. Oracle Database allows up to 32 triggers to cascade at any one
time. However, you can effectively limit the number of trigger cascades using the
initialization parameter OPEN_CURSORS, because a cursor must be opened for every
execution of a trigger.

Trigger Evaluation Order
Although any trigger can run a sequence of operations either in-line or by calling
procedures, using multiple triggers of the same type enhances database administration
by permitting the modular installation of applications that have triggers on the same
tables.
Oracle Database executes all triggers of the same type before executing triggers of a
different type. If you have multiple triggers of the same type on a single table, then
Oracle Database chooses an arbitrary order to execute these triggers.
Oracle Database Concepts for more information on the firing
order of triggers

See Also:

Each subsequent trigger sees the changes made by the previously fired triggers. Each
trigger can see the old and new values. The old values are the original values, and the

Coding Triggers

9-5

Creating Triggers

new values are the current values, as set by the most recently fired UPDATE or INSERT
trigger.
To ensure that multiple triggered actions occur in a specific order, you must
consolidate these actions into a single trigger (for example, by having the trigger call a
series of procedures).

Modifying Complex Views (INSTEAD OF Triggers)
An updatable view is one that lets you perform DML on the underlying table. Some
views are inherently updatable, but others are not because they were created with one
or more of the constructs listed in"Views that Require INSTEAD OF Triggers".
Any view that contains one of those constructs can be made updatable by using an
INSTEAD OF trigger. INSTEAD OF triggers provide a transparent way of modifying
views that cannot be modified directly through UPDATE, INSERT, and DELETE
statements. These triggers are called INSTEAD OF triggers because, unlike other types
of triggers, Oracle Database fires the trigger instead of executing the triggering
statement. The trigger must determine what operation was intended and perform
UPDATE, INSERT, or DELETE operations directly on the underlying tables.
With an INSTEAD OF trigger, you can write normal UPDATE, INSERT, and DELETE
statements against the view, and the INSTEAD OF trigger works invisibly in the
background to make the right actions take place.
INSTEAD OF triggers can only be activated for each row.
See Also: "Firing Triggers One or Many Times (FOR EACH ROW
Option)" on page 9-9

Note:
■

■

■

The INSTEAD OF option can only be used for triggers created over
views.
The BEFORE and AFTER options cannot be used for triggers
created over views.
The CHECK option for views is not enforced when inserts or
updates to the view are done using INSTEAD OF triggers. The
INSTEAD OF trigger body must enforce the check.

Views that Require INSTEAD OF Triggers
A view cannot be modified by UPDATE, INSERT, or DELETE statements if the view
query contains any of the following constructs:
■

A set operator

■

A DISTINCT operator

■

An aggregate or analytic function

■

A GROUP BY, ORDER BY, MODEL, CONNECT BY, or START WITH clause

■

A collection expression in a SELECT list

■

A subquery in a SELECT list

■

A subquery designated WITH READ ONLY

9-6 Oracle Database Application Developer’s Guide - Fundamentals

Creating Triggers

■

Joins, with some exceptions, as documented in Oracle Database Administrator's
Guide

If a view contains pseudocolumns or expressions, then you can only update the view
with an UPDATE statement that does not refer to any of the pseudocolumns or
expressions.

INSTEAD OF Trigger Example
You may need to set up the following data structures for this
example to work:

Note:

CREATE TABLE
Prj_level
Projno
Resp_dept
CREATE TABLE
Empno
Ename
Job
Mgr
Hiredate
Sal
Comm
Deptno

Project_tab (
NUMBER,
NUMBER,
NUMBER);
Emp_tab (
NUMBER NOT NULL,
VARCHAR2(10),
VARCHAR2(9),
NUMBER(4),
DATE,
NUMBER(7,2),
NUMBER(7,2),
NUMBER(2) NOT NULL);

CREATE TABLE
Deptno
Dname
Loc
Mgr_no
Dept_type

Dept_tab (
NUMBER(2) NOT NULL,
VARCHAR2(14),
VARCHAR2(13),
NUMBER,
NUMBER);

The following example shows an INSTEAD OF trigger for inserting rows into the
MANAGER_INFO view.
CREATE OR REPLACE VIEW manager_info AS
SELECT e.ename, e.empno, d.dept_type, d.deptno, p.prj_level,
p.projno
FROM
Emp_tab e, Dept_tab d, Project_tab p
WHERE e.empno = d.mgr_no
AND
d.deptno = p.resp_dept;
CREATE OR REPLACE TRIGGER manager_info_insert
INSTEAD OF INSERT ON manager_info
REFERENCING NEW AS n
-- new manager information
FOR EACH ROW
DECLARE
rowcnt number;
BEGIN
SELECT COUNT(*) INTO rowcnt FROM Emp_tab WHERE empno = :n.empno;
IF rowcnt = 0 THEN
INSERT INTO Emp_tab (empno,ename) VALUES (:n.empno, :n.ename);
ELSE
UPDATE Emp_tab SET Emp_tab.ename = :n.ename
WHERE Emp_tab.empno = :n.empno;
END IF;
SELECT COUNT(*) INTO rowcnt FROM Dept_tab WHERE deptno = :n.deptno;
Coding Triggers

9-7

Creating Triggers

IF rowcnt = 0 THEN
INSERT INTO Dept_tab (deptno, dept_type)
VALUES(:n.deptno, :n.dept_type);
ELSE
UPDATE Dept_tab SET Dept_tab.dept_type = :n.dept_type
WHERE Dept_tab.deptno = :n.deptno;
END IF;
SELECT COUNT(*) INTO rowcnt FROM Project_tab
WHERE Project_tab.projno = :n.projno;
IF rowcnt = 0 THEN
INSERT INTO Project_tab (projno, prj_level)
VALUES(:n.projno, :n.prj_level);
ELSE
UPDATE Project_tab SET Project_tab.prj_level = :n.prj_level
WHERE Project_tab.projno = :n.projno;
END IF;
END;

The actions shown for rows being inserted into the MANAGER_INFO view first test to
see if appropriate rows already exist in the base tables from which MANAGER_INFO is
derived. The actions then insert new rows or update existing rows, as appropriate.
Similar triggers can specify appropriate actions for UPDATE and DELETE.

Object Views and INSTEAD OF Triggers
INSTEAD OF triggers provide the means to modify object view instances on the
client-side through OCI calls.
See Also:

Oracle Call Interface Programmer's Guide

To modify an object materialized by an object view in the client-side object cache and
flush it back to the persistent store, you must specify INSTEAD OF triggers, unless the
object view is modifiable. If the object is read only, then it is not necessary to define
triggers to pin it.

Triggers on Nested Table View Columns
INSTEAD OF triggers can also be created over nested table view columns. These
triggers provide a way of updating elements of the nested table. They fire for each
nested table element being modified. The row correlation variables inside the trigger
correspond to the nested table element. This type of trigger also provides an additional
correlation name for accessing the parent row that contains the nested table being
modified.
Note:
■
■

These triggers:

Can only be defined over nested table columns in views.
Fire only when the nested table elements are modified using the
THE() or TABLE() clauses. They do not fire when a DML statement
is performed on the view.

For example, consider a department view that contains a nested table of employees.
CREATE OR REPLACE VIEW Dept_view AS
SELECT d.Deptno, d.Dept_type, d.Dept_name,
CAST (MULTISET ( SELECT e.Empno, e.Empname, e.Salary)
FROM Emp_tab e

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Creating Triggers

WHERE e.Deptno = d.Deptno) AS Amp_list_ Emplist
FROM Dept_tab d;

The CAST (MULTISET..) operator creates a multi-set of employees for each department.
If you want to modify the emplist column, which is the nested table of employees,
then you can define an INSTEAD OF trigger over the column to handle the operation.
The following example shows how an insert trigger might be written:
CREATE OR REPLACE TRIGGER Dept_emplist_tr
INSTEAD OF INSERT ON NESTED TABLE Emplist OF Dept_view
REFERENCING NEW AS Employee
PARENT AS Department
FOR EACH ROW
BEGIN
-- The insert on the nested table is translated to an insert on the base table:
INSERT INTO Emp_tab VALUES (
:Employee.Empno, :Employee.Empname,:Employee.Salary, :Department.Deptno);
END;

Any INSERT into the nested table fires the trigger, and the Emp_tab table is filled with
the correct values. For example:
INSERT INTO TABLE (SELECT d.Emplist FROM Dept_view d WHERE Deptno = 10)
VALUES (1001, 'John Glenn', 10000);

The :department.deptno correlation variable in this example would have a value of
10.

Firing Triggers One or Many Times (FOR EACH ROW Option)
The FOR EACH ROW option determines whether the trigger is a row trigger or a
statement trigger. If you specify FOR EACH ROW, then the trigger fires once for each row
of the table that is affected by the triggering statement. The absence of the FOR EACH
ROW option indicates that the trigger fires only once for each applicable statement, but
not separately for each row affected by the statement.
For example, you define the following trigger:
You may need to set up the following data structures for
certain examples to work:

Note:

CREATE TABLE Emp_log (
Emp_id
NUMBER,
Log_date
DATE,
New_salary NUMBER,
Action
VARCHAR2(20));

CREATE OR REPLACE TRIGGER Log_salary_increase
AFTER UPDATE ON Emp_tab
FOR EACH ROW
WHEN (new.Sal > 1000)
BEGIN
INSERT INTO Emp_log (Emp_id, Log_date, New_salary, Action)
VALUES (:new.Empno, SYSDATE, :new.SAL, 'NEW SAL');
END;

Then, you enter the following SQL statement:

Coding Triggers

9-9

Coding the Trigger Body

UPDATE Emp_tab SET Sal = Sal + 1000.0
WHERE Deptno = 20;

If there are five employees in department 20, then the trigger fires five times when this
statement is entered, because five rows are affected.
The following trigger fires only once for each UPDATE of the Emp_tab table:
CREATE OR REPLACE TRIGGER Log_emp_update
AFTER UPDATE ON Emp_tab
BEGIN
INSERT INTO Emp_log (Log_date, Action)
VALUES (SYSDATE, 'Emp_tab COMMISSIONS CHANGED');
END;

See Also:

Oracle Database Concepts for the order of trigger firing

The statement level triggers are useful for performing validation checks for the entire
statement.

Firing Triggers Based on Conditions (WHEN Clause)
Optionally, a trigger restriction can be included in the definition of a row trigger by
specifying a Boolean SQL expression in a WHEN clause.
Note: A WHEN clause cannot be included in the definition of a
statement trigger.

If included, then the expression in the WHEN clause is evaluated for each row that the
trigger affects.
If the expression evaluates to TRUE for a row, then the trigger body is fired on behalf of
that row. However, if the expression evaluates to FALSE or NOT TRUE for a row
(unknown, as with nulls), then the trigger body is not fired for that row. The
evaluation of the WHEN clause does not have an effect on the execution of the triggering
SQL statement (in other words, the triggering statement is not rolled back if the
expression in a WHEN clause evaluates to FALSE).
For example, in the PRINT_SALARY_CHANGES trigger, the trigger body is not run if
the new value of Empno is zero, NULL, or negative. In more realistic examples, you
might test if one column value is less than another.
The expression in a WHEN clause of a row trigger can include correlation names, which
are explained later. The expression in a WHEN clause must be a SQL expression, and it
cannot include a subquery. You cannot use a PL/SQL expression (including
user-defined functions) in the WHEN clause.
Note:

You cannot specify the WHEN clause for INSTEAD OF triggers.

Coding the Trigger Body
The trigger body is a CALL procedure or a PL/SQL block that can include SQL and
PL/SQL statements. The CALL procedure can be either a PL/SQL or a Java procedure
that is encapsulated in a PL/SQL wrapper. These statements are run if the triggering
statement is entered and if the trigger restriction (if included) evaluates to TRUE.

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Coding the Trigger Body

The trigger body for row triggers has some special constructs that can be included in
the code of the PL/SQL block: correlation names and the REFERENCEING option, and
the conditional predicates INSERTING, DELETING, and UPDATING.
The INSERTING, DELETING, and UPDATING conditional
predicates cannot be used for the CALL procedures; they can only be
used in a PL/SQL block.

Note:

Example: Monitoring Logons with a Trigger

You may need to set up data structures similar to the following
for certain examples to work:

Note:

CONNECT system/manager
GRANT ADMINISTER DATABASE TRIGGER TO scott;
CONNECT scott/tiger
CREATE TABLE audit_table (
seq number,
user_at VARCHAR2(10),
time_now DATE,
term
VARCHAR2(10),
job
VARCHAR2(10),
proc
VARCHAR2(10),
enum
NUMBER);

CREATE OR REPLACE PROCEDURE foo (c VARCHAR2) AS
BEGIN
INSERT INTO Audit_table (user_at) VALUES(c);
END;
CREATE OR REPLACE TRIGGER logontrig AFTER LOGON ON DATABASE
-- Just call an existing procedure. The ORA_LOGIN_USER is a function
-- that returns information about the event that fired the trigger.
CALL foo (ora_login_user)
/

Example: Calling a Java Procedure from a Trigger

Although triggers are declared using PL/SQL, they can call procedures in other
languages, such as Java:
CREATE OR REPLACE PROCEDURE Before_delete (Id IN NUMBER, Ename VARCHAR2)
IS language Java
name 'thjvTriggers.beforeDelete (oracle.sql.NUMBER, oracle.sql.CHAR)';
CREATE OR REPLACE TRIGGER Pre_del_trigger BEFORE DELETE ON Tab
FOR EACH ROW
CALL Before_delete (:old.Id, :old.Ename)
/

The corresponding Java file is thjvTriggers.java:
import
import
import
import
public
{

java.sql.*
java.io.*
oracle.sql.*
oracle.oracore.*
class thjvTriggers

Coding Triggers 9-11

Coding the Trigger Body

public state void
beforeDelete (NUMBER old_id, CHAR old_name)
Throws SQLException, CoreException
{
Connection conn = JDBCConnection.defaultConnection();
Statement stmt = conn.CreateStatement();
String sql = "insert into logtab values
("+ old_id.intValue() +", '"+ old_ename.toString() + ", BEFORE DELETE');
stmt.executeUpdate (sql);
stmt.close();
return;
}
}

Accessing Column Values in Row Triggers
Within a trigger body of a row trigger, the PL/SQL code and SQL statements have
access to the old and new column values of the current row affected by the triggering
statement. Two correlation names exist for every column of the table being modified:
one for the old column value, and one for the new column value. Depending on the
type of triggering statement, certain correlation names might not have any meaning.
■

■

■

A trigger fired by an INSERT statement has meaningful access to new column
values only. Because the row is being created by the INSERT, the old values are
null.
A trigger fired by an UPDATE statement has access to both old and new column
values for both BEFORE and AFTER row triggers.
A trigger fired by a DELETE statement has meaningful access to :old column
values only. Because the row no longer exists after the row is deleted, the :new
values are NULL. However, you cannot modify :new values because ORA-4084 is
raised if you try to modify :new values.

The new column values are referenced using the new qualifier before the column
name, while the old column values are referenced using the old qualifier before the
column name. For example, if the triggering statement is associated with the Emp_tab
table (with the columns SAL, COMM, and so on), then you can include statements in the
trigger body. For example:
IF :new.Sal > 10000 ...
IF :new.Sal < :old.Sal ...

Old and new values are available in both BEFORE and AFTER row triggers. A new
column value can be assigned in a BEFORE row trigger, but not in an AFTER row
trigger (because the triggering statement takes effect before an AFTER row trigger is
fired). If a BEFORE row trigger changes the value of new.column, then an AFTER row
trigger fired by the same statement sees the change assigned by the BEFORE row
trigger.
Correlation names can also be used in the Boolean expression of a WHEN clause. A
colon (:) must precede the old and new qualifiers when they are used in a trigger
body, but a colon is not allowed when using the qualifiers in the WHEN clause or the
REFERENCING option.

Example: Modifying LOB Columns with a Trigger
You can treat LOB columns the same as other columns, using regular SQL and
PL/SQL functions with CLOB columns, and calls to the DBMS_LOB package with BLOB
columns:

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Coding the Trigger Body

drop table tab1;
create table tab1 (c1 clob);
insert into tab1 values ('
HTML Document Fragment
Some text.');
create or replace trigger trg1
before update on tab1
for each row
begin
dbms_output.put_line('Old value of CLOB column: '||:OLD.c1);
dbms_output.put_line('Proposed new value of CLOB column: '||:NEW.c1);
-- Previously, we couldn't change the new value for a LOB.
-- Now, we can replace it, or construct a new value using SUBSTR, INSTR...
-- operations for a CLOB, or DBMS_LOB calls for a BLOB.
:NEW.c1 := :NEW.c1 || to_clob('
Standard footer paragraph.');
dbms_output.put_line('Final value of CLOB column: '||:NEW.c1);
end;
/
set serveroutput on;
update tab1 set c1 = '
Different Document Fragment
Different text.';
select * from tab1;

INSTEAD OF Triggers on Nested Table View Columns
In the case of INSTEAD OF triggers on nested table view columns, the new and old
qualifiers correspond to the new and old nested table elements. The parent row
corresponding to this nested table element can be accessed using the parent qualifier.
The parent correlation name is meaningful and valid only inside a nested table trigger.

Avoiding Name Conflicts with Triggers (REFERENCING Option)
The REFERENCING option can be specified in a trigger body of a row trigger to avoid
name conflicts among the correlation names and tables that might be named old or
new. Because this is rare, this option is infrequently used.
For example, assume you have a table named new with columns field1 (number)
and field2 (character). The following CREATE TRIGGER example shows a trigger
associated with the new table that can use correlation names and avoid naming
conflicts between the correlation names and the table name:
You may need to set up the following data structures for
certain examples to work:

Note:

CREATE TABLE new (
field1
NUMBER,
field2
VARCHAR2(20));

CREATE OR REPLACE TRIGGER Print_salary_changes
BEFORE UPDATE ON new
REFERENCING new AS Newest
FOR EACH ROW
BEGIN
:Newest.Field2 := TO_CHAR (:newest.field1);
END;

Coding Triggers 9-13

Coding the Trigger Body

Notice that the new qualifier is renamed to newest using the REFERENCING option,
and it is then used in the trigger body.

Detecting the DML Operation That Fired a Trigger
If more than one type of DML operation can fire a trigger (for example, ON INSERT OR
DELETE OR UPDATE OF Emp_tab), the trigger body can use the conditional predicates
INSERTING, DELETING, and UPDATING to check which type of statement fire the
trigger.
Within the code of the trigger body, you can execute blocks of code depending on the
kind of DML operation fired the trigger:
IF INSERTING THEN ... END IF;
IF UPDATING THEN ... END IF;

The first condition evaluates to TRUE only if the statement that fired the trigger is an
INSERT statement; the second condition evaluates to TRUE only if the statement that
fired the trigger is an UPDATE statement.
In an UPDATE trigger, a column name can be specified with an UPDATING conditional
predicate to determine if the named column is being updated. For example, assume a
trigger is defined as the following:
CREATE OR REPLACE TRIGGER ...
... UPDATE OF Sal, Comm ON Emp_tab ...
BEGIN
... IF UPDATING ('SAL') THEN ... END IF;
END;

The code in the THEN clause runs only if the triggering UPDATE statement updates the
SAL column. This way, the trigger can minimize its overhead when the column of
interest is not being changed.

Error Conditions and Exceptions in the Trigger Body
If a predefined or user-defined error condition or exception is raised during the
execution of a trigger body, then all effects of the trigger body, as well as the triggering
statement, are rolled back (unless the error is trapped by an exception handler).
Therefore, a trigger body can prevent the execution of the triggering statement by
raising an exception. User-defined exceptions are commonly used in triggers that
enforce complex security authorizations or integrity constraints.
The only exception to this is when the event under consideration is database STARTUP,
SHUTDOWN, or LOGIN when the user logging in is SYSTEM. In these scenarios, only the
trigger action is rolled back.

Triggers on Object Tables
You can use the OBJECT_VALUE pseudocolumn in a trigger on an object table since
10g Release 1 (10.1). OBJECT_VALUE means the object as a whole. This is one example
of its use. You can also call a PL/SQL function with OBJECT_VALUE as the datatype of
an IN formal parameter.
Here is an example of the use of OBJECT_VALUE in a trigger. To keep track of updates
to values in an object table tbl, a history table, tbl_history, is also created in the
following example. For tbl, the values 1 through 5 are inserted into n, while m is kept
at 0. The trigger is a row-level trigger that executes once for each row affected by a

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Coding the Trigger Body

DML statement. The trigger causes the old and new values of the object t in tbl to be
written in tbl_history when tbl is updated. These old and new values are
:OLD.OBJECT_VALUE and :NEW.OBJECT_VALUE. An update of the table tbl is done
(each value of n is increased by 1). A select from the history table to check that the
trigger works is then shown at the end of the example:
CREATE OR REPLACE TYPE t AS OBJECT (n NUMBER, m NUMBER)
/
CREATE TABLE tbl OF t
/
BEGIN
FOR j IN 1..5 LOOP
INSERT INTO tbl VALUES (t(j, 0));
END LOOP;
END;
/
CREATE TABLE tbl_history ( d DATE, old_obj t, new_obj t)
/
CREATE OR REPLACE TRIGGER Tbl_Trg
AFTER UPDATE ON tbl
FOR EACH ROW
BEGIN
INSERT INTO tbl_history (d, old_obj, new_obj)
VALUES (SYSDATE, :OLD.OBJECT_VALUE, :NEW.OBJECT_VALUE);
END Tbl_Trg;
/
-------------------------------------------------------------------------------UPDATE tbl SET tbl.n = tbl.n+1
/
BEGIN
FOR j IN (SELECT d, old_obj, new_obj FROM tbl_history) LOOP
Dbms_Output.Put_Line (
j.d||
' -- old: '||j.old_obj.n||' '||j.old_obj.m||
' -- new: '||j.new_obj.n||' '||j.new_obj.m);
END LOOP;
END;
/

The result of the select shows that the values of the column n have been all increased
by 1. The value of m remains 0. The output of the select is:
23-MAY-05
23-MAY-05
23-MAY-05
23-MAY-05
23-MAY-05

------

old:
old:
old:
old:
old:

1
2
3
4
5

0
0
0
0
0

------

new:
new:
new:
new:
new:

2
3
4
5
6

0
0
0
0
0

Triggers and Handling Remote Exceptions
A trigger that accesses a remote site cannot do remote exception handling if the
network link is unavailable. For example:
CREATE OR REPLACE TRIGGER Example
AFTER INSERT ON Emp_tab
FOR EACH ROW
BEGIN
INSERT INTO Emp_tab@Remote
-- <- compilation fails here
VALUES ('x');
-when dblink is inaccessible
EXCEPTION
WHEN OTHERS THEN
Coding Triggers 9-15

Coding the Trigger Body

INSERT INTO Emp_log
VALUES ('x');
END;

A trigger is compiled when it is created. Thus, if a remote site is unavailable when the
trigger must compile, then Oracle Database cannot validate the statement accessing
the remote database, and the compilation fails. The previous example exception
statement cannot run, because the trigger does not complete compilation.
Because stored procedures are stored in a compiled form, the work-around for the
previous example is as follows:
CREATE OR REPLACE TRIGGER Example
AFTER INSERT ON Emp_tab
FOR EACH ROW
BEGIN
Insert_row_proc;
END;
CREATE OR REPLACE PROCEDURE Insert_row_proc AS
BEGIN
INSERT INTO Emp_tab@Remote
VALUES ('x');
EXCEPTION
WHEN OTHERS THEN
INSERT INTO Emp_log
VALUES ('x');
END;

The trigger in this example compiles successfully and calls the stored procedure,
which already has a validated statement for accessing the remote database; thus, when
the remote INSERT statement fails because the link is down, the exception is caught.

Restrictions on Creating Triggers
Coding triggers requires some restrictions that are not required for standard PL/SQL
blocks. The following sections discuss these restrictions.

Maximum Trigger Size
The size of a trigger cannot be more than 32K.

SQL Statements Allowed in Trigger Bodies
The body of a trigger can contain DML SQL statements. It can also contain SELECT
statements, but they must be SELECT... INTO... statements or the SELECT statement in
the definition of a cursor.
DDL statements are not allowed in the body of a trigger. Also, no transaction control
statements are allowed in a trigger. ROLLBACK, COMMIT, and SAVEPOINT cannot be
used.For system triggers, {CREATE/ALTER/DROP} TABLE statements and
ALTER...COMPILE are allowed.
A procedure called by a trigger cannot run the previous
transaction control statements, because the procedure runs within the
context of the trigger body.

Note:

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Coding the Trigger Body

Statements inside a trigger can reference remote schema objects. However, pay special
attention when calling remote procedures from within a local trigger. If a timestamp or
signature mismatch is found during execution of the trigger, then the remote
procedure is not run, and the trigger is invalidated.

Trigger Restrictions on LONG and LONG RAW Datatypes
LONG and LONG RAW datatypes in triggers are subject to the following restrictions:
■

■

A SQL statement within a trigger can insert data into a column of LONG or LONG
RAW datatype.
If data from a LONG or LONG RAW column can be converted to a constrained
datatype (such as CHAR and VARCHAR2), then a LONG or LONG RAW column can be
referenced in a SQL statement within a trigger. The maximum length for these
datatypes is 32000 bytes.

■

Variables cannot be declared using the LONG or LONG RAW datatypes.

■

:NEW and :PARENT cannot be used with LONG or LONG RAW columns.

Trigger Restrictions on Mutating Tables
A mutating table is a table that is being modified by an UPDATE, DELETE, or INSERT
statement, or a table that might be updated by the effects of a DELETE CASCADE
constraint.
The session that issued the triggering statement cannot query or modify a mutating
table. This restriction prevents a trigger from seeing an inconsistent set of data.
This restriction applies to all triggers that use the FOR EACH ROW clause. Views being
modified in INSTEAD OF triggers are not considered mutating.
When a trigger encounters a mutating table, a runtime error occurs, the effects of the
trigger body and triggering statement are rolled back, and control is returned to the
user or application.
Consider the following trigger:
CREATE OR REPLACE TRIGGER Emp_count
AFTER DELETE ON Emp_tab
FOR EACH ROW
DECLARE
n INTEGER;
BEGIN
SELECT COUNT(*) INTO n FROM Emp_tab;
DBMS_OUTPUT.PUT_LINE(' There are now ' || n ||
' employees.');
END;

If the following SQL statement is entered:
DELETE FROM Emp_tab WHERE Empno = 7499;

An error is returned because the table is mutating when the row is deleted:
ORA-04091: table SCOTT.Emp_tab is mutating, trigger/function may not see it

If you delete the line "FOR EACH ROW" from the trigger, it becomes a statement trigger
which is not subject to this restriction, and the trigger.
If you need to update a mutating table, you could bypass these restrictions by using a
temporary table, a PL/SQL table, or a package variable. For example, in place of a
single AFTER row trigger that updates the original table, resulting in a mutating table
Coding Triggers 9-17

Coding the Trigger Body

error, you might use two triggers—an AFTER row trigger that updates a temporary
table, and an AFTER statement trigger that updates the original table with the values
from the temporary table.
Declarative integrity constraints are checked at various times with respect to row
triggers.
Oracle Database Concepts for information about the
interaction of triggers and integrity constraints

See Also:

Because declarative referential integrity constraints are not supported between tables
on different nodes of a distributed database, the mutating table restrictions do not
apply to triggers that access remote nodes. These restrictions are also not enforced
among tables in the same database that are connected by loop-back database links. A
loop-back database link makes a local table appear remote by defining an Oracle Net
path back to the database that contains the link.
Restrictions on Mutating Tables Relaxed
The mutating error, discussed earlier in this section, still prevents the trigger from
reading or modifying the table that the parent statement is modifying. However,
starting in Oracle Database Release 8.1, a delete against the parent table causes
before/after statement triggers to be fired once. That way, you can create triggers (just
not row triggers) to read and modify the parent and child tables.
This allows most foreign key constraint actions to be implemented through their
obvious after-row trigger, providing the constraint is not self-referential. Update
cascade, update set null, update set default, delete set default, inserting a missing
parent, and maintaining a count of children can all be implemented easily. For
example, this is an implementation of update cascade:
create table p
create table f
create trigger
update f set
end;
/

(p1 number constraint ppk primary key);
(f1 number constraint ffk references p);
pt after update on p for each row begin
f1 = :new.p1 where f1 = :old.p1;

This implementation requires care for multirow updates. For example, if a table p has
three rows with the values (1), (2), (3), and table f also has three rows with the values
(1), (2), (3), then the following statement updates p correctly but causes problems
when the trigger updates f:
update p set p1 = p1+1;

The statement first updates (1) to (2) in p, and the trigger updates (1) to (2) in f, leaving
two rows of value (2) in f. Then the statement updates (2) to (3) in p, and the trigger
updates both rows of value (2) to (3) in f. Finally, the statement updates (3) to (4) in p,
and the trigger updates all three rows in f from (3) to (4). The relationship of the data
in p and f is lost.
To avoid this problem, you must forbid multirow updates to p that change the primary
key and reuse existing primary key values. It could also be solved by tracking which
foreign key values have already been updated, then modifying the trigger so that no
row is updated twice.
That is the only problem with this technique for foreign key updates. The trigger
cannot miss rows that have been changed but not committed by another transaction,
because the foreign key constraint guarantees that no matching foreign key rows are
locked before the after-row trigger is called.
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System Trigger Restrictions
Depending on the event, different event attribute functions are available. For example,
certain DDL operations may not be allowed on DDL events. Check "Event Attribute
Functions" on page 9-38 before using an event attribute function, because its effects
might be undefined rather than producing an error condition.
Only committed triggers are fired. For example, if you create a trigger that should be
fired after all CREATE events, then the trigger itself does not fire after the creation,
because the correct information about this trigger was not committed at the time when
the trigger on CREATE events was fired.
For example, if you execute the following SQL statement:
CREATE OR REPLACE TRIGGER my_trigger AFTER CREATE ON DATABASE
BEGIN null;
END;

Then, trigger my_trigger is not fired after the creation of my_trigger. Oracle
Database does not fire a trigger that is not committed.
Foreign Function Callouts
All restrictions on foreign function callouts also apply.

Who Is the Trigger User?
The following statement, inside a trigger, returns the owner of the trigger, not the
name of user who is updating the table:
SELECT Username FROM USER_USERS;

Privileges Needed to Work with Triggers
To create a trigger in your schema, you must have the CREATE TRIGGER system
privilege, and either:
■

Own the table specified in the triggering statement, or

■

Have the ALTER privilege for the table in the triggering statement, or

■

Have the ALTER ANY TABLE system privilege

To create a trigger in another user's schema, or to reference a table in another schema
from a trigger in your schema, you must have the CREATE ANY TRIGGER system
privilege. With this privilege, the trigger can be created in any schema and can be
associated with any user's table. In addition, the user creating the trigger must also
have EXECUTE privilege on the referenced procedures, functions, or packages.
To create a trigger on DATABASE, you must have the ADMINISTER DATABASE
TRIGGER privilege. If this privilege is later revoked, then you can drop the trigger, but
not alter it.
The object privileges to the schema objects referenced in the trigger body must be
granted to the trigger owner explicitly (not through a role). The statements in the
trigger body operate under the privilege domain of the trigger owner, not the privilege
domain of the user issuing the triggering statement. This is similar to the privilege
model for stored procedures.

Coding Triggers 9-19

Compiling Triggers

Compiling Triggers
Triggers are similar to PL/SQL anonymous blocks with the addition of the :new and
:old capabilities, but their compilation is different. A PL/SQL anonymous block is
compiled each time it is loaded into memory. Compilation involves three stages:
1.

Syntax checking: PL/SQL syntax is checked, and a parse tree is generated.

2.

Semantic checking: Type checking and further processing on the parse tree.

3.

Code generation: The pcode is generated.

Triggers, in contrast, are fully compiled when the CREATE TRIGGER statement is
entered, and the pcode is stored in the data dictionary. Hence, firing the trigger no
longer requires the opening of a shared cursor to run the trigger action. Instead, the
trigger is executed directly.
If errors occur during the compilation of a trigger, then the trigger is still created. If a
DML statement fires this trigger, then the DML statement fails. (Runtime that trigger
errors always cause the DML statement to fail.) You can use the SHOW ERRORS
statement in SQL*Plus or Enterprise Manager to see any compilation errors when you
create a trigger, or you can SELECT the errors from the USER_ERRORS view.

Dependencies for Triggers
Compiled triggers have dependencies. They become invalid if a depended-on object,
such as a stored procedure or function called from the trigger body, is modified.
Triggers that are invalidated for dependency reasons are recompiled when next
invoked.
You can examine the ALL_DEPENDENCIES view to see the dependencies for a trigger.
For example, the following statement shows the dependencies for the triggers in the
SCOTT schema:
SELECT NAME, REFERENCED_OWNER, REFERENCED_NAME, REFERENCED_TYPE
FROM ALL_DEPENDENCIES
WHERE OWNER = 'SCOTT' and TYPE = 'TRIGGER';

Triggers may depend on other functions or packages. If the function or package
specified in the trigger is dropped, then the trigger is marked invalid. An attempt is
made to validate the trigger on occurrence of the event. If the trigger cannot be
validated successfully, then it is marked VALID WITH ERRORS, and the event fails.
Note:
■

■

There is an exception for STARTUP events: STARTUP events
succeed even if the trigger fails. There are also exceptions for
SHUTDOWN events and for LOGON events if you login as SYSTEM.
Because the DBMS_AQ package is used to enqueue a message,
dependency between triggers and queues cannot be maintained.

Recompiling Triggers
Use the ALTER TRIGGER statement to recompile a trigger manually. For example, the
following statement recompiles the PRINT_SALARY_CHANGES trigger:
ALTER TRIGGER Print_salary_changes COMPILE;

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To recompile a trigger, you must own the trigger or have the ALTER ANY TRIGGER
system privilege.

Modifying Triggers
Like a stored procedure, a trigger cannot be explicitly altered: It must be replaced with
a new definition. (The ALTER TRIGGER statement is used only to recompile, enable, or
disable a trigger.)
When replacing a trigger, you must include the OR REPLACE option in the CREATE
TRIGGER statement. The OR REPLACE option is provided to allow a new version of an
existing trigger to replace the older version, without affecting any grants made for the
original version of the trigger.
Alternatively, the trigger can be dropped using the DROP TRIGGER statement, and you
can rerun the CREATE TRIGGER statement.
To drop a trigger, the trigger must be in your schema, or you must have the DROP ANY
TRIGGER system privilege.

Debugging Triggers
You can debug a trigger using the same facilities available for stored procedures.
See Also:

"Debugging Stored Procedures" on page 7-29

Enabling and Disabling Triggers
A trigger can be in one of two distinct modes:
Enabled. An enabled trigger executes its trigger body if a triggering statement is
entered and the trigger restriction (if any) evaluates to TRUE.
Disabled. A disabled trigger does not execute its trigger body, even if a triggering
statement is entered and the trigger restriction (if any) evaluates to TRUE.

Enabling Triggers
By default, a trigger is automatically enabled when it is created; however, it can later
be disabled. After you have completed the task that required the trigger to be disabled,
re-enable the trigger, so that it fires when appropriate.
Enable a disabled trigger using the ALTER TRIGGER statement with the ENABLE
option. To enable the disabled trigger named REORDER of the INVENTORY table, enter
the following statement:
ALTER TRIGGER Reorder ENABLE;

All triggers defined for a specific table can be enabled with one statement using the
ALTER TABLE statement with the ENABLE clause with the ALL TRIGGERS option. For
example, to enable all triggers defined for the INVENTORY table, enter the following
statement:
ALTER TABLE Inventory
ENABLE ALL TRIGGERS;

Disabling Triggers
You might temporarily disable a trigger if:

Coding Triggers 9-21

Viewing Information About Triggers

■
■

■

An object it references is not available.
You need to perform a large data load, and you want it to proceed quickly without
firing triggers.
You are reloading data.

By default, triggers are enabled when first created. Disable a trigger using the ALTER
TRIGGER statement with the DISABLE option.
For example, to disable the trigger named REORDER of the INVENTORY table, enter the
following statement:
ALTER TRIGGER Reorder DISABLE;

All triggers associated with a table can be disabled with one statement using the
ALTER TABLE statement with the DISABLE clause and the ALL TRIGGERS option. For
example, to disable all triggers defined for the INVENTORY table, enter the following
statement:
ALTER TABLE Inventory
DISABLE ALL TRIGGERS;

Viewing Information About Triggers
The following data dictionary views reveal information about triggers:
■

USER_TRIGGERS

■

ALL_TRIGGERS

■

DBA_TRIGGERS

The new column, BASE_OBJECT_TYPE, specifies whether the trigger is based on
DATABASE, SCHEMA, table, or view. The old column, TABLE_NAME, is null if the base
object is not table or view.
The column ACTION_TYPE specifies whether the trigger is a call type trigger or a
PL/SQL trigger.
The column TRIGGER_TYPE includes two additional values: BEFORE EVENT and
AFTER EVENT, applicable only to system events.
The column TRIGGERING_EVENT includes all system and DML events.
Oracle Database Reference for a complete description of
these data dictionary views

See Also:

For example, assume the following statement was used to create the REORDER trigger:
Caution: You may need to set up data structures for certain examples
to work:
CREATE OR REPLACE TRIGGER Reorder
AFTER UPDATE OF Parts_on_hand ON Inventory
FOR EACH ROW
WHEN(new.Parts_on_hand < new.Reorder_point)
DECLARE
x NUMBER;
BEGIN
SELECT COUNT(*) INTO x

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Examples of Trigger Applications

FROM Pending_orders
WHERE Part_no = :new.Part_no;
IF x = 0 THEN
INSERT INTO Pending_orders
VALUES (:new.Part_no, :new.Reorder_quantity,
sysdate);
END IF;
END;

The following two queries return information about the REORDER trigger:
SELECT Trigger_type, Triggering_event, Table_name
FROM USER_TRIGGERS
WHERE Trigger_name = 'REORDER';
TYPE
TRIGGERING_STATEMENT
TABLE_NAME
---------------- -------------------------- -----------AFTER EACH ROW
UPDATE
INVENTORY
SELECT Trigger_body
FROM USER_TRIGGERS
WHERE Trigger_name = 'REORDER';
TRIGGER_BODY
-------------------------------------------DECLARE
x NUMBER;
BEGIN
SELECT COUNT(*) INTO x
FROM Pending_orders
WHERE Part_no = :new.Part_no;
IF x = 0
THEN INSERT INTO Pending_orders
VALUES (:new.Part_no, :new.Reorder_quantity,
sysdate);
END IF;
END;

Examples of Trigger Applications
You can use triggers in a number of ways to customize information management in
Oracle Database. For example, triggers are commonly used to:
■

Provide sophisticated auditing

■

Prevent invalid transactions

■

Enforce referential integrity (either those actions not supported by declarative
integrity constraints or across nodes in a distributed database)

■

Enforce complex business rules

■

Enforce complex security authorizations

■

Provide transparent event logging

■

Automatically generate derived column values

■

Enable building complex views that are updatable

■

Track system events

Coding Triggers 9-23

Examples of Trigger Applications

This section provides an example of each of these trigger applications. These examples
are not meant to be used exactly as written: They are provided to assist you in
designing your own triggers.

Auditing with Triggers: Example
Triggers are commonly used to supplement the built-in auditing features of Oracle
Database. Although triggers can be written to record information similar to that
recorded by the AUDIT statement, triggers should be used only when more detailed
audit information is required. For example, use triggers to provide value-based
auditing for each row.
Sometimes, the AUDIT statement is considered a security audit facility, while triggers
can provide financial audit facility.
When deciding whether to create a trigger to audit database activity, consider what
Oracle Database's auditing features provide, compared to auditing defined by triggers,
as shown in Table 9–1.
Table 9–1

Comparison of Built-in Auditing and Trigger-Based Auditing

Audit Feature

Description

DML and DDL
Auditing

Standard auditing options permit auditing of DML and DDL statements
regarding all types of schema objects and structures. Comparatively, triggers
permit auditing of DML statements entered against tables, and DDL auditing
at SCHEMA or DATABASE level.

Centralized
Audit Trail

All database audit information is recorded centrally and automatically using
the auditing features of Oracle Database.

Declarative
Method

Auditing features enabled using the standard Oracle Database features are
easier to declare and maintain, and less prone to errors, when compared to
auditing functions defined by triggers.

Auditing
Options can be
Audited

Any changes to existing auditing options can also be audited to guard against
malicious database activity.

Session and
Execution time
Auditing

Using the database auditing features, records can be generated once every
time an audited statement is entered (BY ACCESS) or once for every session
that enters an audited statement (BY SESSION). Triggers cannot audit by
session; an audit record is generated each time a trigger-audited table is
referenced.

Auditing of
Unsuccessful
Data Access

Database auditing can be set to audit when unsuccessful data access occurs.
However, unless autonomous transactions are used, any audit information
generated by a trigger is rolled back if the triggering statement is rolled back.
For more information on autonomous transactions, refer to Oracle Database
Concepts.

Sessions can be
Audited

Connections and disconnections, as well as session activity (physical I/Os,
logical I/Os, deadlocks, and so on), can be recorded using standard database
auditing.

When using triggers to provide sophisticated auditing, AFTER triggers are normally
used. By using AFTER triggers, auditing information is recorded after the triggering
statement is subjected to any applicable integrity constraints, preventing cases where
the audit processing is carried out unnecessarily for statements that generate
exceptions to integrity constraints.
Choosing between AFTER row and AFTER statement triggers depends on the
information being audited. For example, row triggers provide value-based auditing
for each table row. Triggers can also require the user to supply a "reason code" for

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Examples of Trigger Applications

issuing the audited SQL statement, which can be useful in both row and
statement-level auditing situations.
The following example demonstrates a trigger that audits modifications to the
Emp_tab table for each row. It requires that a "reason code" be stored in a global
package variable before the update. This shows how triggers can be used to provide
value-based auditing and how to use public package variables.
You may need to set up the following data structures for the
examples to work:

Note:

CREATE OR REPLACE PACKAGE Auditpackage AS
Reason VARCHAR2(10);
PROCEDURE Set_reason(Reason VARCHAR2);
END;
CREATE TABLE Emp99 (
Empno
NOT NULL
NUMBER(4),
Ename
VARCHAR2(10),
Job
VARCHAR2(9),
Mgr
NUMBER(4),
Hiredate
DATE,
Sal
NUMBER(7,2),
Comm
NUMBER(7,2),
Deptno
NUMBER(2),
Bonus
NUMBER,
Ssn
NUMBER,
Job_classification NUMBER);
CREATE TABLE Audit_employee (
Oldssn
NUMBER,
Oldname
VARCHAR2(10),
Oldjob
VARCHAR2(2),
Oldsal
NUMBER,
Newssn
NUMBER,
Newname
VARCHAR2(10),
Newjob
VARCHAR2(2),
Newsal
NUMBER,
Reason
VARCHAR2(10),
User1
VARCHAR2(10),
Systemdate
DATE);

CREATE OR REPLACE TRIGGER Audit_employee
AFTER INSERT OR DELETE OR UPDATE ON Emp99
FOR EACH ROW
BEGIN
/* AUDITPACKAGE is a package with a public package
variable REASON. REASON could be set by the
application by a command such as EXECUTE
AUDITPACKAGE.SET_REASON(reason_string). Note that a
package variable has state for the duration of a
session and that each session has a separate copy of
all package variables. */
IF Auditpackage.Reason IS NULL THEN
Raise_application_error(-20201, 'Must specify reason'
|| ' with AUDITPACKAGE.SET_REASON(Reason_string)');
END IF;
/* If the preceding conditional evaluates to TRUE, the

Coding Triggers 9-25

Examples of Trigger Applications

user-specified error number and message is raised,
the trigger stops execution, and the effects of the
triggering statement are rolled back. Otherwise, a
new row is inserted into the predefined auditing
table named AUDIT_EMPLOYEE containing the existing
and new values of the Emp_tab table and the reason code
defined by the REASON variable of AUDITPACKAGE. Note
that the "old" values are NULL if triggering
statement is an INSERT and the "new" values are NULL
if the triggering statement is a DELETE. */
INSERT INTO Audit_employee VALUES
(:old.Ssn, :old.Ename, :old.Job_classification, :old.Sal,
:new.Ssn, :new.Ename, :new.Job_classification, :new.Sal,
auditpackage.Reason, User, Sysdate );
END;

Optionally, you can also set the reason code back to NULL if you wanted to force the
reason code to be set for every update. The following simple AFTER statement trigger
sets the reason code back to NULL after the triggering statement is run:
CREATE OR REPLACE TRIGGER Audit_employee_reset
AFTER INSERT OR DELETE OR UPDATE ON Emp_tab
BEGIN
auditpackage.set_reason(NULL);
END;

Notice that the previous two triggers are both fired by the same type of SQL statement.
However, the AFTER row trigger is fired once for each row of the table affected by the
triggering statement, while the AFTER statement trigger is fired only once after the
triggering statement execution is completed.
This next trigger also uses triggers to do auditing. It tracks changes made to the
Emp_tab table and stores this information in AUDIT_TABLE and
AUDIT_TABLE_VALUES.
You may need to set up the following data structures for the
example to work:

Note:

CREATE TABLE Audit_table (
Seq
NUMBER,
User_at VARCHAR2(10),
Time_now DATE,
Term
VARCHAR2(10),
Job
VARCHAR2(10),
Proc
VARCHAR2(10),
enum
NUMBER);
CREATE SEQUENCE Audit_seq;
CREATE TABLE Audit_table_values (
Seq
NUMBER,
Dept
NUMBER,
Dept1
NUMBER,
Dept2
NUMBER);

CREATE OR REPLACE TRIGGER Audit_emp
AFTER INSERT OR UPDATE OR DELETE ON Emp_tab
FOR EACH ROW
DECLARE
Time_now DATE;

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Examples of Trigger Applications

Terminal CHAR(10);
BEGIN
-- get current time, and the terminal of the user:
Time_now := SYSDATE;
Terminal := USERENV('TERMINAL');
-- record new employee primary key
IF INSERTING THEN
INSERT INTO Audit_table
VALUES (Audit_seq.NEXTVAL, User, Time_now,
Terminal, 'Emp_tab', 'INSERT', :new.Empno);
-- record primary key of the deleted row:
ELSIF DELETING THEN
INSERT INTO Audit_table
VALUES (Audit_seq.NEXTVAL, User, Time_now,
Terminal, 'Emp_tab', 'DELETE', :old.Empno);
-- for updates, record the primary key
-- of the row being updated:
ELSE
INSERT INTO Audit_table
VALUES (audit_seq.NEXTVAL, User, Time_now,
Terminal, 'Emp_tab', 'UPDATE', :old.Empno);
-- and for SAL and DEPTNO, record old and new values:
IF UPDATING ('SAL') THEN
INSERT INTO Audit_table_values
VALUES (Audit_seq.CURRVAL, 'SAL',
:old.Sal, :new.Sal);
ELSIF UPDATING ('DEPTNO') THEN
INSERT INTO Audit_table_values
VALUES (Audit_seq.CURRVAL, 'DEPTNO',
:old.Deptno, :new.DEPTNO);
END IF;
END IF;
END;

Integrity Constraints and Triggers: Examples
Triggers and declarative integrity constraints can both be used to constrain data input.
However, triggers and integrity constraints have significant differences.
Declarative integrity constraints are statements about the database that are always
true. A constraint applies to existing data in the table and any statement that
manipulates the table.
See Also: Chapter 6, "Maintaining Data Integrity in Application
Development"

Triggers constrain what a transaction can do. A trigger does not apply to data loaded
before the definition of the trigger; therefore, it is not known if all data in a table
conforms to the rules established by an associated trigger.
Although triggers can be written to enforce many of the same rules supported by
Oracle Database's declarative integrity constraint features, triggers should only be
used to enforce complex business rules that cannot be defined using standard integrity
constraints. The declarative integrity constraint features provided with Oracle
Database offer the following advantages when compared to constraints defined by
triggers:
Centralized integrity checks. All points of data access must adhere to the global set of
rules defined by the integrity constraints corresponding to each schema object.

Coding Triggers 9-27

Examples of Trigger Applications

Declarative method. Constraints defined using the standard integrity constraint features
are much easier to write and are less prone to errors, when compared with comparable
constraints defined by triggers.
While most aspects of data integrity can be defined and enforced using declarative
integrity constraints, triggers can be used to enforce complex business constraints not
definable using declarative integrity constraints. For example, triggers can be used to
enforce:
■
■

■

UPDATE SET NULL, and UPDATE and DELETE SET DEFAULT referential actions.
Referential integrity when the parent and child tables are on different nodes of a
distributed database.
Complex check constraints not definable using the expressions allowed in a CHECK
constraint.

Referential Integrity Using Triggers
There are many cases where referential integrity can be enforced using triggers. Note,
however, you should only use triggers when there is no declarative support for the
action you are performing.
When using triggers to maintain referential integrity, declare the PRIMARY (or
UNIQUE) KEY constraint in the parent table. If referential integrity is being maintained
between a parent and child table in the same database, then you can also declare the
foreign key in the child table, but disable it; this prevents the corresponding PRIMARY
KEY constraint from being dropped (unless the PRIMARY KEY constraint is explicitly
dropped with the CASCADE option).
To maintain referential integrity using triggers:
■

■

A trigger must be defined for the child table that guarantees values inserted or
updated in the foreign key correspond to values in the parent key.
One or more triggers must be defined for the parent table. These triggers
guarantee the desired referential action (RESTRICT, CASCADE, or SET NULL) for
values in the foreign key when values are updated or deleted in the parent key. No
action is required for inserts into the parent table (no dependent foreign keys
exist).

The following sections provide examples of the triggers necessary to enforce
referential integrity. The Emp_tab and Dept_tab table relationship is used in these
examples.
Several of the triggers include statements that lock rows (SELECT... FOR UPDATE). This
operation is necessary to maintain concurrency as the rows are being processed.
Foreign Key Trigger for Child Table The following trigger guarantees that before an
INSERT or UPDATE statement affects a foreign key value, the corresponding value
exists in the parent key. The mutating table exception included in the following
example allows this trigger to be used with the UPDATE_SET_DEFAULT and
UPDATE_CASCADE triggers. This exception can be removed if this trigger is used
alone.
CREATE OR REPLACE TRIGGER Emp_dept_check
BEFORE INSERT OR UPDATE OF Deptno ON Emp_tab
FOR EACH ROW WHEN (new.Deptno IS NOT NULL)
-- Before a row is inserted, or DEPTNO is updated in the Emp_tab
-- table, fire this trigger to verify that the new foreign
-- key value (DEPTNO) is present in the Dept_tab table.

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DECLARE
Dummy
INTEGER; -- to be used for cursor fetch
Invalid_department EXCEPTION;
Valid_department
EXCEPTION;
Mutating_table
EXCEPTION;
PRAGMA EXCEPTION_INIT (Mutating_table, -4091);
-- Cursor used to verify parent key value exists.
-- present, lock parent key's row so it can't be
-- deleted by another transaction until this
-- transaction is committed or rolled back.
CURSOR Dummy_cursor (Dn NUMBER) IS
SELECT Deptno FROM Dept_tab
WHERE Deptno = Dn
FOR UPDATE OF Deptno;
BEGIN
OPEN Dummy_cursor (:new.Deptno);
FETCH Dummy_cursor INTO Dummy;

If

---IF

Verify parent key. If not found, raise user-specified
error number and message. If found, close cursor
before allowing triggering statement to complete:
Dummy_cursor%NOTFOUND THEN
RAISE Invalid_department;
ELSE
RAISE valid_department;
END IF;
CLOSE Dummy_cursor;
EXCEPTION
WHEN Invalid_department THEN
CLOSE Dummy_cursor;
Raise_application_error(-20000, 'Invalid Department'
|| ' Number' || TO_CHAR(:new.deptno));
WHEN Valid_department THEN
CLOSE Dummy_cursor;
WHEN Mutating_table THEN
NULL;
END;

The following trigger is
defined on the DEPT_TAB table to enforce the UPDATE and DELETE RESTRICT
referential action on the primary key of the DEPT_TAB table:
UPDATE and DELETE RESTRICT Trigger for Parent Table

CREATE OR REPLACE TRIGGER Dept_restrict
BEFORE DELETE OR UPDATE OF Deptno ON Dept_tab
FOR EACH ROW
-- Before a row is deleted from Dept_tab or the primary key
-- (DEPTNO) of Dept_tab is updated, check for dependent
-- foreign key values in Emp_tab; rollback if any are found.
DECLARE
Dummy
INTEGER;
-- to be used for cursor fetch
Employees_present
EXCEPTION;
employees_not_present EXCEPTION;
-- Cursor used to check for dependent foreign key values.
CURSOR Dummy_cursor (Dn NUMBER) IS
SELECT Deptno FROM Emp_tab WHERE Deptno = Dn;
BEGIN

Coding Triggers 9-29

Examples of Trigger Applications

OPEN Dummy_cursor (:old.Deptno);
FETCH Dummy_cursor INTO Dummy;
-- If dependent foreign key is found, raise user-specified
-- error number and message. If not found, close cursor
-- before allowing triggering statement to complete.
IF Dummy_cursor%FOUND THEN
RAISE Employees_present;
-- dependent rows exist
ELSE
RAISE Employees_not_present; -- no dependent rows
END IF;
CLOSE Dummy_cursor;
EXCEPTION
WHEN Employees_present THEN
CLOSE Dummy_cursor;
Raise_application_error(-20001, 'Employees Present in'
|| ' Department ' || TO_CHAR(:old.DEPTNO));
WHEN Employees_not_present THEN
CLOSE Dummy_cursor;
END;

This trigger does not work with self-referential tables
(tables with both the primary/unique key and the foreign key). Also,
this trigger does not allow triggers to cycle (such as, A fires B fires A).

Caution:

UPDATE and DELETE SET NULL Triggers for Parent Table: Example The following
trigger is defined on the DEPT_TAB table to enforce the UPDATE and DELETE SET
NULL referential action on the primary key of the DEPT_TAB table:
CREATE OR REPLACE TRIGGER Dept_set_null
AFTER DELETE OR UPDATE OF Deptno ON Dept_tab
FOR EACH ROW
-- Before a row is deleted from Dept_tab or the primary key
-- (DEPTNO) of Dept_tab is updated, set all corresponding
-- dependent foreign key values in Emp_tab to NULL:
BEGIN
IF UPDATING AND :OLD.Deptno != :NEW.Deptno OR DELETING THEN
UPDATE Emp_tab SET Emp_tab.Deptno = NULL
WHERE Emp_tab.Deptno = :old.Deptno;
END IF;
END;

DELETE Cascade Trigger for Parent Table: Example The following trigger on the
DEPT_TAB table enforces the DELETE CASCADE referential action on the primary key
of the DEPT_TAB table:
CREATE OR REPLACE TRIGGER Dept_del_cascade
AFTER DELETE ON Dept_tab
FOR EACH ROW
-- Before a row is deleted from Dept_tab, delete all
-- rows from the Emp_tab table whose DEPTNO is the same as
-- the DEPTNO being deleted from the Dept_tab table:
BEGIN
DELETE FROM Emp_tab
WHERE Emp_tab.Deptno = :old.Deptno;
END;

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Examples of Trigger Applications

Typically, the code for DELETE CASCADE is combined with the
code for UPDATE SET NULL or UPDATE SET DEFAULT to account for
both updates and deletes.

Note:

The following trigger ensures
that if a department number is updated in the Dept_tab table, then this change is
propagated to dependent foreign keys in the Emp_tab table:

UPDATE Cascade Trigger for Parent Table: Example

-- Generate a sequence number to be used as a flag for
-- determining if an update has occurred on a column:
CREATE SEQUENCE Update_sequence
INCREMENT BY 1 MAXVALUE 5000
CYCLE;
CREATE OR REPLACE PACKAGE Integritypackage AS
Updateseq NUMBER;
END Integritypackage;
CREATE OR REPLACE PACKAGE BODY Integritypackage AS
END Integritypackage;
-- create flag col:
ALTER TABLE Emp_tab ADD Update_id NUMBER;
CREATE OR REPLACE TRIGGER Dept_cascade1 BEFORE UPDATE OF Deptno ON Dept_tab
DECLARE
Dummy NUMBER;
-- Before updating the Dept_tab table (this is a statement
-- trigger), generate a new sequence number and assign
-- it to the public variable UPDATESEQ of a user-defined
-- package named INTEGRITYPACKAGE:
BEGIN
SELECT Update_sequence.NEXTVAL
INTO Dummy
FROM dual;
Integritypackage.Updateseq := Dummy;
END;
CREATE OR REPLACE TRIGGER Dept_cascade2 AFTER DELETE OR UPDATE
OF Deptno ON Dept_tab FOR EACH ROW
-- For each department number in Dept_tab that is updated,
-- cascade the update to dependent foreign keys in the
-- Emp_tab table. Only cascade the update if the child row
-- has not already been updated by this trigger:
BEGIN
IF UPDATING THEN
UPDATE Emp_tab
SET Deptno = :new.Deptno,
Update_id = Integritypackage.Updateseq
--from 1st
WHERE Emp_tab.Deptno = :old.Deptno
AND Update_id IS NULL;
/* only NULL if not updated by the 3rd trigger
fired by this same triggering statement */
END IF;
IF DELETING THEN
-- Before a row is deleted from Dept_tab, delete all

Coding Triggers 9-31

Examples of Trigger Applications

-- rows from the Emp_tab table whose DEPTNO is the same as
-- the DEPTNO being deleted from the Dept_tab table:
DELETE FROM Emp_tab
WHERE Emp_tab.Deptno = :old.Deptno;
END IF;
END;
CREATE OR REPLACE TRIGGER Dept_cascade3 AFTER UPDATE OF Deptno ON Dept_tab
BEGIN UPDATE Emp_tab
SET Update_id = NULL
WHERE Update_id = Integritypackage.Updateseq;
END;

Because this trigger updates the Emp_tab table, the
Emp_dept_check trigger, if enabled, is also fired. The resulting
mutating table error is trapped by the Emp_dept_check trigger. You
should carefully test any triggers that require error trapping to
succeed to ensure that they always work properly in your
environment.

Note:

Trigger for Complex Check Constraints: Example
Triggers can enforce integrity rules other than referential integrity. For example, this
trigger performs a complex check before allowing the triggering statement to run.
You may need to set up the following data structures for the
example to work:

Note:

CREATE TABLE Salgrade (
Grade
Losal
Hisal
Job_classification

NUMBER,
NUMBER,
NUMBER,
NUMBER)

CREATE OR REPLACE TRIGGER Salary_check
BEFORE INSERT OR UPDATE OF Sal, Job ON Emp99
FOR EACH ROW
DECLARE
Minsal
NUMBER;
Maxsal
NUMBER;
Salary_out_of_range
EXCEPTION;
BEGIN
/* Retrieve the minimum and maximum salary for the
employee's new job classification from the SALGRADE
table into MINSAL and MAXSAL: */
SELECT Minsal, Maxsal INTO Minsal, Maxsal FROM Salgrade
WHERE Job_classification = :new.Job;

/* If the employee's new salary is less than or greater
than the job classification's limits, the exception is
raised. The exception message is returned and the
pending INSERT or UPDATE statement that fired the
trigger is rolled back:*/
IF (:new.Sal < Minsal OR :new.Sal > Maxsal) THEN

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Examples of Trigger Applications

RAISE Salary_out_of_range;
END IF;
EXCEPTION
WHEN Salary_out_of_range THEN
Raise_application_error (-20300,
'Salary '||TO_CHAR(:new.Sal)||' out of range for '
||'job classification '||:new.Job
||' for employee '||:new.Ename);
WHEN NO_DATA_FOUND THEN
Raise_application_error(-20322,
'Invalid Job Classification '
||:new.Job_classification);
END;

Complex Security Authorizations and Triggers: Example
Triggers are commonly used to enforce complex security authorizations for table data.
Only use triggers to enforce complex security authorizations that cannot be defined
using the database security features provided with Oracle Database. For example, a
trigger can prohibit updates to salary data of the Emp_tab table during weekends,
holidays, and non-working hours.
When using a trigger to enforce a complex security authorization, it is best to use a
BEFORE statement trigger. Using a BEFORE statement trigger has these benefits:
■

■

The security check is done before the triggering statement is allowed to run, so
that no wasted work is done by an unauthorized statement.
The security check is performed only once for the triggering statement, not for
each row affected by the triggering statement.

This example shows a trigger used to enforce security.
You may need to set up the following data structures for the
example to work:

Note:

CREATE TABLE Company_holidays (Day DATE);

CREATE OR REPLACE TRIGGER Emp_permit_changes
BEFORE INSERT OR DELETE OR UPDATE ON Emp99
DECLARE
Dummy
INTEGER;
Not_on_weekends
EXCEPTION;
Not_on_holidays
EXCEPTION;
Non_working_hours EXCEPTION;
BEGIN
/* check for weekends: */
IF (TO_CHAR(Sysdate, 'DY') = 'SAT' OR
TO_CHAR(Sysdate, 'DY') = 'SUN') THEN
RAISE Not_on_weekends;
END IF;
/* check for company holidays:*/
SELECT COUNT(*) INTO Dummy FROM Company_holidays
WHERE TRUNC(Day) = TRUNC(Sysdate);
/* TRUNC gets rid of time parts of dates: */
IF dummy > 0 THEN
RAISE Not_on_holidays;
END IF;
/* Check for work hours (8am to 6pm): */
IF (TO_CHAR(Sysdate, 'HH24') < 8 OR

Coding Triggers 9-33

Examples of Trigger Applications

TO_CHAR(Sysdate, 'HH24') > 18) THEN
RAISE Non_working_hours;
END IF;
EXCEPTION
WHEN Not_on_weekends THEN
Raise_application_error(-20324,'May not change '
||'employee table during the weekend');
WHEN Not_on_holidays THEN
Raise_application_error(-20325,'May not change '
||'employee table during a holiday');
WHEN Non_working_hours THEN
Raise_application_error(-20326,'May not change '
||'Emp_tab table during non-working hours');
END;

See Also: Oracle Database Security Guide for details on database
security features

Transparent Event Logging and Triggers
Triggers are very useful when you want to transparently perform a related change in
the database following certain events.
The REORDER trigger example shows a trigger that reorders parts as necessary when
certain conditions are met. (In other words, a triggering statement is entered, and the
PARTS_ON_HAND value is less than the REORDER_POINT value.)
Derived Column Values and Triggers: Example
Triggers can derive column values automatically, based upon a value provided by an
INSERT or UPDATE statement. This type of trigger is useful to force values in specific
columns that depend on the values of other columns in the same row. BEFORE row
triggers are necessary to complete this type of operation for the following reasons:
■

■

The dependent values must be derived before the INSERT or UPDATE occurs, so
that the triggering statement can use the derived values.
The trigger must fire for each row affected by the triggering INSERT or UPDATE
statement.

The following example illustrates how a trigger can be used to derive new column
values for a table whenever a row is inserted or updated.
You may need to set up the following data structures for the
example to work:

Note:

ALTER TABLE Emp99 ADD(
Uppername
VARCHAR2(20),
Soundexname VARCHAR2(20));

CREATE OR REPLACE TRIGGER Derived
BEFORE INSERT OR UPDATE OF Ename ON Emp99
/* Before updating the ENAME field, derive the values for
the UPPERNAME and SOUNDEXNAME fields. Users should be
restricted from updating these fields directly: */
FOR EACH ROW
BEGIN
:new.Uppername := UPPER(:new.Ename);

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Examples of Trigger Applications

:new.Soundexname := SOUNDEX(:new.Ename);
END;

Building Complex Updatable Views Using Triggers: Example
Views are an excellent mechanism to provide logical windows over table data.
However, when the view query gets complex, the system implicitly cannot translate
the DML on the view into those on the underlying tables. INSTEAD OF triggers help
solve this problem. These triggers can be defined over views, and they fire instead of
the actual DML.
Consider a library system where books are arranged under their respective titles. The
library consists of a collection of book type objects. The following example explains the
schema.
CREATE OR REPLACE TYPE Book_t AS OBJECT
(
Booknum
NUMBER,
Title
VARCHAR2(20),
Author
VARCHAR2(20),
Available CHAR(1)
);
CREATE OR REPLACE TYPE Book_list_t AS TABLE OF Book_t;

Assume that the following tables exist in the relational schema:
Table Book_table (Booknum, Section, Title, Author, Available)
Booknum

Section

Title

Author

Available

121001

Classic

Iliad

Homer

Y

121002

Novel

Gone With the Wind

Mitchell M

N

Library consists of library_table(section).
Section
Geography
Classic

You can define a complex view over these tables to create a logical view of the library
with sections and a collection of books in each section.
CREATE OR REPLACE VIEW Library_view AS
SELECT i.Section, CAST (MULTISET (
SELECT b.Booknum, b.Title, b.Author, b.Available
FROM Book_table b
WHERE b.Section = i.Section) AS Book_list_t) BOOKLIST
FROM Library_table i;

Make this view updatable by defining an INSTEAD OF trigger over the view.
CREATE OR REPLACE TRIGGER Library_trigger INSTEAD OF INSERT ON Library_view FOR
EACH ROW
Bookvar BOOK_T;
i
INTEGER;
BEGIN
INSERT INTO Library_table VALUES (:NEW.Section);
FOR i IN 1..:NEW.Booklist.COUNT LOOP

Coding Triggers 9-35

Examples of Trigger Applications

Bookvar := Booklist(i);
INSERT INTO book_table
VALUES ( Bookvar.booknum, :NEW.Section, Bookvar.Title, Bookvar.Author,
bookvar.Available);
END LOOP;
END;
/

The library_view is an updatable view, and any INSERTs on the view are handled
by the trigger that gets fired automatically. For example:
INSERT INTO Library_view VALUES ('History', book_list_t(book_t(121330,
'Alexander', 'Mirth', 'Y');

Similarly, you can also define triggers on the nested table booklist to handle
modification of the nested table element.
Tracking System Events Using Triggers
Fine-Grained Access Control Using Triggers: Example System triggers can be used
to set application context. Application context is a relatively new feature that enhances
your ability to implement fine-grained access control. Application context is a secure
session cache, and it can be used to store session-specific attributes.

In the example that follows, procedure set_ctx sets the application context based on
the user profile. The trigger setexpensectx ensures that the context is set for every
user.
CONNECT secdemo/secdemo
CREATE OR REPLACE CONTEXT Expenses_reporting USING Secdemo.Exprep_ctx;
REM =================================================================
REM Creation of the package which implements the context:
REM =================================================================
CREATE OR REPLACE PACKAGE Exprep_ctx AS
PROCEDURE Set_ctx;
END;
SHOW ERRORS
CREATE OR REPLACE PACKAGE BODY Exprep_ctx IS
PROCEDURE Set_ctx IS
Empnum
NUMBER;
Countrec NUMBER;
Cc
NUMBER;
Role
VARCHAR2(20);
BEGIN
-- SET emp_number:
SELECT Employee_id INTO Empnum FROM Employee
WHERE Last_name = SYS_CONTEXT('userenv', 'session_user');
DBMS_SESSION.SET_CONTEXT('expenses_reporting','emp_number', Empnum);
-- SET ROLE:
SELECT COUNT (*) INTO Countrec FROM Cost_center WHERE Manager_id=Empnum;
IF (countrec > 0) THEN
DBMS_SESSION.SET_CONTEXT('expenses_reporting','exp_role','MANAGER');

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Responding to System Events through Triggers

ELSE
DBMS_SESSION.SET_CONTEXT('expenses_reporting','exp_role','EMPLOYEE');
END IF;
-- SET cc_number:
SELECT Cost_center_id INTO Cc FROM Employee
WHERE Last_name = SYS_CONTEXT('userenv','session_user');
DBMS_SESSION.SET_CONTEXT(expenses_reporting','cc_number',Cc);
END;
END;

CALL Syntax
CREATE OR REPLACE TRIGGER Secdemo.Setexpseetx
AFTER LOGON ON DATABASE
CALL Secdemo.Exprep_etx.Set_otx

Responding to System Events through Triggers
System event publication lets applications subscribe to database events, just like they
subscribe to messages from other applications. The system events publication
framework includes the following features:
■

■

■

Infrastructure for publish/subscribe, by making the database an active publisher
of events.
Integration of data cartridges in the server. The system events publication can be
used to notify cartridges of state changes in the server.
Integration of fine-grained access control in the server.

By creating a trigger, you can specify a procedure that runs when an event occurs.
DML events are supported on tables, and system events are supported on DATABASE
and SCHEMA. You can turn notification on and off by enabling and disabling the trigger
using the ALTER TRIGGER statement.
This feature is integrated with the Advanced Queueing engine. Publish/subscribe
applications use the DBMS_AQ.ENQUEUE() procedure, and other applications such as
cartridges use callouts.
See Also:
■
■

Oracle Database SQL Reference
Oracle Streams Advanced Queuing User's Guide and Reference for
details on how to subscribe to published events

How Events Are Published Through Triggers
When events are detected by the database, the trigger mechanism executes the action
specified in the trigger. As part of this action, you can use the DBMS_AQ package to
publish the event to a queue so that subscribers receive notifications.
Note: Only system-defined database events can be detected this way.
You cannot define your own event conditions.

When an event occurs, the database fires all triggers that are enabled on that event,
with some exceptions:

Coding Triggers 9-37

Responding to System Events through Triggers

■

■

If the trigger is actually the target of the triggering event, it is not fired. For
example, a trigger for all DROP events is not fired when it is dropped itself.
If a trigger has been modified but not committed within the same transaction as
the firing event. For example, recursive DDL within a system trigger might modify
a trigger, which prevents the modified trigger from being fired by events within
the same transaction.

You can create more than one trigger on an object. When an event fires more than one
trigger, the firing order is not defined and so you should not rely on the triggers being
fired in a particular order.

Publication Context
When an event is published, certain runtime context and attributes, as specified in the
parameter list, are passed to the callout procedure. A set of functions called event
attribute functions are provided.
See Also: "Event Attribute Functions" on page 9-38 for information
on event-specific attributes

For each supported system event, you can identify and predefine event-specific
attributes for the event. You can choose the parameter list to be any of these attributes,
along with other simple expressions. For callouts, these are passed as IN arguments.

Error Handling
Return status from publication callout functions for all events are ignored. For
example, with SHUTDOWN events, the database cannot do anything with the return
status.
See Also: "List of Database Events" on page 9-41 for details on
return status

Execution Model
Traditionally, triggers execute as the definer of the trigger. The trigger action of an
event is executed as the definer of the action (as the definer of the package or function
in callouts, or as owner of the trigger in queues). Because the owner of the trigger must
have EXECUTE privileges on the underlying queues, packages, or procedure, this
behavior is consistent.

Event Attribute Functions
When the database fires a trigger, you can retrieve certain attributes about the event
that fired the trigger. You can retrieve each attribute with a function call. Table 9–2
describes the system-defined event attributes.

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Note:
■

■

■

Table 9–2

To make these attributes available, you must first run the
CATPROC.SQL script.
The trigger dictionary object maintains metadata about events
that will be published and their corresponding attributes.
In earlier releases, these functions were accessed through the SYS
package. We recommend you use these public synonyms whose
names begin with ora_.

System-Defined Event Attributes

Attribute

Type

Description

Example

ora_client_ip_address

VARCHAR2

Returns IP address of
the client in a LOGON
event when the
underlying protocol is
TCP/IP

DECLARE
v_addr VARCHAR2(11);
IF (ora_sysevent = 'LOGON') THEN
v_addr := ora_client_ip_address;
END IF;
END;

ora_database_name

VARCHAR2(50)

Database name.

DECLARE
v_db_name VARCHAR2(50);
BEGIN
v_db_name := ora_database_name;
END;

ora_des_encrypted_password

VARCHAR2

The DES-encrypted
password of the user
being created or
altered.

IF (ora_dict_obj_type = 'USER') THEN
INSERT INTO event_table
VALUES (ora_des_encrypted_password);
END IF;

ora_dict_obj_name

VARCHAR(30)

Name of the dictionary
object on which the
DDL operation
occurred.

INSERT INTO event_table
VALUES ('Changed object is ' ||
ora_dict_obj_name);

ora_dict_obj_name_list
(name_list OUT
ora_name_list_t)

BINARY_INTEGER

Return the list of object
names of objects being
modified in the event.

IF (ora_sysevent='ASSOCIATE STATISTICS')
THEN number_modified :=
ora_dict_obj_name_list(name_list);
END IF;

ora_dict_obj_owner

VARCHAR(30)

Owner of the
dictionary object on
which the DDL
operation occurred.

INSERT INTO event_table
VALUES ('object owner is' ||
ora_dict_obj_owner);

ora_dict_obj_owner_list
(owner_list OUT
ora_name_list_t)

BINARY_INTEGER

Returns the list of
IF (ora_sysevent='ASSOCIATE STATISTICS')
object owners of objects
THEN number_of_modified_objects :=
being modified in the
ora_dict_obj_owner_list(owner_list);
event.
END IF;

ora_dict_obj_type

VARCHAR(20)

Type of the dictionary
object on which the
DDL operation
occurred.

INSERT INTO event_table
VALUES ('This object is a ' ||
ora_dict_obj_type);

ora_grantee
(user_list OUT
ora_name_list_t)

BINARY_INTEGER

Returns the grantees of
a grant event in the
OUT parameter;
returns the number of
grantees in the return
value.

IF (ora_sysevent = 'GRANT') THEN
number_of_users=ora_grantee(user_list);
END IF;

ora_instance_num

NUMBER

Instance number.

IF (ora_instance_num = 1) THEN
INSERT INTO event_table VALUES ('1');
END IF;

Coding Triggers 9-39

Responding to System Events through Triggers

Table 9–2 (Cont.) System-Defined Event Attributes
Attribute

Type

Description

Example

ora_is_alter_column
(column_name IN VARCHAR2)

BOOLEAN

Returns true if the
specified column is
altered.

IF (ora_sysevent = 'ALTER' AND
ora_dict_obj_type = 'TABLE') THEN
alter_column := ora_is_alter_column('C');
END IF;

ora_is_creating_nested_table

BOOLEAN

Returns true if the
current event is
creating a nested table

IF (ora_sysevent = 'CREATE' and
ora_dict_obj_type = 'TABLE' and
ora_is_creating_nested_table) THEN
INSERT INTO event_table
VALUES ('A nested table is created');
END IF;

ora_is_drop_column
(column_name IN VARCHAR2)

BOOLEAN

Returns true if the
specified column is
dropped.

IF (ora_sysevent = 'ALTER' AND
ora_dict_obj_type = 'TABLE') THEN
drop_column := ora_is_drop_column('C');
END IF;

ora_is_servererror

BOOLEAN

Returns TRUE if given
error is on error stack,
FALSE otherwise.

IF (ora_is_servererror(error_number)) THEN
INSERT INTO event_table
VALUES ('Server error!!');
END IF;

ora_login_user

VARCHAR2(30)

Login user name.

SELECT ora_login_user
FROM dual;

ora_partition_pos

BINARY_INTEGER

In an INSTEAD OF
trigger for CREATE
TABLE, the position
within the SQL text
where you could insert
a PARTITION clause.

-- Retrieve ora_sql_txt into
-- sql_text variable first.
v_n := ora_partition_pos;
v_new_stmt := SUBSTR(sql_text,1,v_n - 1)
|| ' ' || my_partition_clause
|| ' ' || SUBSTR(sql_text, v_n));

ora_privilege_list
(privilege_list
OUT ora_name_list_t)

BINARY_INTEGER

Returns the list of
IF (ora_sysevent = 'GRANT' OR
privileges being
ora_sysevent = 'REVOKE') THEN
granted by the grantee
number_of_privileges :=
or the list of privileges
ora_privilege_list(priv_list);
revoked from the
END IF;
revokees in the OUT
parameter; returns the
number of privileges in
the return value.

ora_revokee
(user_list OUT
ora_name_list_t)

BINARY_INTEGER

Returns the revokees of IF (ora_sysevent = 'REVOKE') THEN
a revoke event in the
number_of_users := ora_revokee(user_list);
OUT parameter; returns
the number of revokees
in the return value.

ora_server_error

NUMBER

Given a position (1 for
top of stack), it returns
the error number at
that position on error
stack

INSERT INTO event_table
VALUES ('top stack error ' ||
ora_server_error(1));

ora_server_error_depth

BINARY_INTEGER

Returns the total
number of error
messages on the error
stack.

n := ora_server_error_depth;
-- This value is used with other functions
-- such as ora_server_error

ora_server_error_msg
(position in binary_integer)

VARCHAR2

Given a position (1 for
top of stack), it returns
the error message at
that position on error
stack

INSERT INTO event_table
VALUES ('top stack error message' ||
ora_server_error_msg(1));

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Table 9–2 (Cont.) System-Defined Event Attributes
Attribute

Type

Description

Example

ora_server_error_num_params
(position in binary_integer)

BINARY_INTEGER

Given a position (1 for
top of stack), it returns
the number of strings
that have been
substituted into the
error message using a
format like %s.

n := ora_server_error_num_params(1);

ora_server_error_param
(position in binary_integer,
param in binary_integer)

VARCHAR2

Given a position (1 for
top of stack) and a
parameter number,
returns the matching
substitution value (%s,
%d, and so on) in the
error message.

-- For example, the second %s in a
-- message: "Expected %s, found %s"
param := ora_server_error_param(1,2);

ora_sql_txt
(sql_text out
ora_name_list_t)

BINARY_INTEGER

Returns the SQL text of
the triggering
statement in the OUT
parameter. If the
statement is long, it is
broken into multiple
PL/SQL table
elements. The function
return value shows the
number of elements are
in the PL/SQL table.

sql_text ora_name_list_t;
v_stmt VARCHAR2(2000);
...
n := ora_sql_txt(sql_text);
FOR i IN 1..n LOOP
v_stmt := v_stmt || sql_text(i);
END LOOP;
INSERT INTO event_table
VALUES ('text of triggering statement: '
|| v_stmt);

ora_sysevent

VARCHAR2(20)

System event firing the
trigger: Event name is
same as that in the
syntax.

INSERT INTO event_table
VALUES (ora_sysevent);

ora_with_grant_option

BOOLEAN

Returns true if the
privileges are granted
with grant option.

IF (ora_sysevent = 'GRANT' and
ora_with_grant_option = TRUE) THEN
INSERT INTO event_table
VALUES ('with grant option');
END IF;

space_error_info
(error_number OUT NUMBER,
error_type OUT VARCHAR2,
object_owner OUT VARCHAR2,
table_space_name OUT
VARCHAR2,
object_name OUT VARCHAR2,
sub_object_name OUT
VARCHAR2)

BOOLEAN

Returns true if the error IF (space_error_info(eno,typ,owner,ts,obj,
is related to an
subobj) = TRUE) THEN
out-of-space condition,
DBMS_OUTPUT.PUT_LINE('The object '|| obj
and fills in the OUT
|| ' owned by ' || owner ||
parameters with
' has run out of space.');
information about the
END
IF;
object that caused the
error.

List of Database Events
This section describes important system events and client events.

System Events
System events are related to entire instances or schemas, not individual tables or rows.
Triggers created on startup and shutdown events must be associated with the database
instance. Triggers created on error and suspend events can be associated with either
the database instance or a particular schema.
Table 9–3 contains a list of system manager events.

Coding Triggers 9-41

Responding to System Events through Triggers

Table 9–3

System Manager Events

Event

When Fired?

Conditions

STARTUP

When the database is opened. None
allowed

Restrictions

Transaction

Attribute Functions

No database
operations allowed
in the trigger.

Starts a separate
transaction and
commits it after
firing the triggers.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name

Starts a separate
transaction and
commits it after
firing the triggers.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name

Starts a separate
transaction and
commits it after
firing the triggers.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name

Starts a separate
transaction and
commits it after
firing the triggers.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_server_error
ora_is_servererror
space_error_info

Return status
ignored.
SHUTDOWN

Just before the server starts
the shutdown of an instance.

None
allowed

This lets the cartridge
shutdown completely. For
abnormal instance shutdown,
this event may not be fired.
DB_ROLE_CHANGE When the database is opened
for the first time after a role
change.
SERVERERROR

When the error eno occurs. If
no condition is given, then
this event fires whenever an
error occurs.

No database
operations allowed
in the trigger.
Return status
ignored.

None
allowed

Return status
ignored.

ERRNO = eno Depends on the
error.
Return status
ignored.

The trigger does not fire on
ORA-1034, ORA-1403,
ORA-1422, ORA-1423, and
ORA-4030 because they are
not true errors or are too
serious to continue
processing. It also fails to fire
on ORA-18 and ORA-20
because a process is not
available to connect to the
database to record the error.

Client Events
Client events are the events related to user logon/logoff, DML, and DDL operations.
For example:
CREATE OR REPLACE TRIGGER On_Logon
AFTER LOGON
ON The_user.Schema
BEGIN
Do_Something;
END;

The LOGON and LOGOFF events allow simple conditions on UID and USER. All other
events allow simple conditions on the type and name of the object, as well as functions
like UID and USER.
The LOGON event starts a separate transaction and commits it after firing the triggers.
All other events fire the triggers in the existing user transaction.
The LOGON and LOGOFF events can operate on any objects. For all other events, the
corresponding trigger cannot perform any DDL operations, such as DROP and ALTER,
on the object that caused the event to be generated.
The DDL allowed inside these triggers is altering, creating, or dropping a table,
creating a trigger, and compile operations.
If an event trigger becomes the target of a DDL operation (such as CREATE TRIGGER),
it cannot be fired later during the same transaction
Table 9–4 contains a list of client events.
9-42 Oracle Database Application Developer’s Guide - Fundamentals

Responding to System Events through Triggers

Table 9–4

Client Events

Event

When Fired?

Attribute Functions

BEFORE ALTER

When a catalog object is altered.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_type
ora_dict_obj_name
ora_dict_obj_owner
ora_des_encrypted_password
(for ALTER USER events)
ora_is_alter_column
(for ALTER TABLE events)
ora_is_drop_column
(for ALTER TABLE events)

When a catalog object is dropped.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_type
ora_dict_obj_name
ora_dict_obj_owner

When an analyze statement is issued

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner

When an associate statistics statement is issued

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner
ora_dict_obj_name_list
ora_dict_obj_owner_list

When an audit or noaudit statement is issued

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name

When an object is commented

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner

When a catalog object is created.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_type
ora_dict_obj_name
ora_dict_obj_owner
ora_is_creating_nested_table
(for CREATE TABLE events)

AFTER ALTER

BEFORE DROP
AFTER DROP

BEFORE ANALYZE
AFTER ANALYZE

BEFORE ASSOCIATE STATISTICS
AFTER ASSOCIATE STATISTICS

BEFORE AUDIT
AFTER AUDIT
BEFORE NOAUDIT
AFTER NOAUDIT
BEFORE COMMENT
AFTER COMMENT

BEFORE CREATE
AFTER CREATE

Coding Triggers 9-43

Responding to System Events through Triggers

Table 9–4 (Cont.) Client Events
Event

When Fired?

Attribute Functions

BEFORE DDL
AFTER DDL

When most SQL DDL statements are issued. Not
fired for ALTER DATABASE, CREATE
CONTROLFILE, CREATE DATABASE, and DDL issued
through the PL/SQL procedure interface, such as
creating an advanced queue.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner

BEFORE DISASSOCIATE STATISTICS

When a disassociate statistics statement is issued

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner
ora_dict_obj_name_list
ora_dict_obj_owner_list

When a grant statement is issued

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner
ora_grantee
ora_with_grant_option
ora_privileges

BEFORE LOGOFF

At the start of a user logoff

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name

AFTER LOGON

After a successful logon of a user.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_client_ip_address

BEFORE RENAME

When a rename statement is issued.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_owner
ora_dict_obj_type

AFTER DISASSOCIATE STATISTICS

BEFORE GRANT
AFTER GRANT

AFTER RENAME

9-44 Oracle Database Application Developer’s Guide - Fundamentals

Responding to System Events through Triggers

Table 9–4 (Cont.) Client Events
Event

When Fired?

Attribute Functions

BEFORE REVOKE

When a revoke statement is issued

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner
ora_revokee
ora_privileges

AFTER SUSPEND

After a SQL statement is suspended because of an
out-of-space condition. The trigger should correct the
condition so the statement can be resumed.

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_server_error
ora_is_servererror
space_error_info

BEFORE TRUNCATE

When an object is truncated

ora_sysevent
ora_login_user
ora_instance_num
ora_database_name
ora_dict_obj_name
ora_dict_obj_type
ora_dict_obj_owner

AFTER REVOKE

AFTER TRUNCATE

Coding Triggers 9-45

Responding to System Events through Triggers

9-46 Oracle Database Application Developer’s Guide - Fundamentals

10
Developing Flashback Applications
This chapter discusses the following flashback topics:
■

Overview of Flashback Features

■

Database Administration Tasks Before Using Flashback Features

■

Using Flashback Query (SELECT ... AS OF)

■

Using the DBMS_FLASHBACK Package

■

Using ORA_ROWSCN

■

Using Flashback Version Query

■

Using Flashback Transaction Query

■

Flashback Tips
See Also:
■

■

Oracle Database Backup and Recovery Advanced User's Guide and
Oracle Database Administrator's Guide for information on flashback
features designed for database administration tasks such as Oracle
Flashback Database and Oracle Flashback Table
Oracle Database SQL Reference for the syntax of SQL extensions for
flashback features

Overview of Flashback Features
Oracle Database has a group of features, known collectively as flashback, that provide
ways to view past states of database objects or to return database objects to a previous
state without using point-in-time media recovery.
You can use flashback features of the database to do the following:
■
■

■

Perform queries that return past data.
Perform queries that return metadata that shows a detailed history of changes to
the database.
Recover tables or rows to a previous point in time.

Flashback features use the Automatic Undo Management system to obtain metadata
and historical data for transactions. They rely on undo data, which are records of the
effects of individual transactions. For example, if a user executes an UPDATE statement
to change a salary from 1000 to 1100, then Oracle would store the value 1000 in the
undo data.

Developing Flashback Applications 10-1

Overview of Flashback Features

Undo data is persistent and survives a database shutdown. By using flashback
features, you can employ undo data to query past data or recover from logical
corruptions. Besides using it in flashback operations, Oracle Database uses undo data
to perform the following actions:
■

Roll back active transactions

■

Recover terminated transactions by using database or process recovery

■

Provide read consistency for SQL queries
Oracle Database Concepts for more information about
flashback features and automatic undo management

See Also:

Application Development Features
In application development, you can use flashback features to report on historical data
or undo erroneous changes. Flashback features include the following:
■

Oracle Flashback Query
You can use this feature to retrieve data for a time in the past that you specify
using the AS OF clause of the SELECT statement.

■

Oracle Flashback Version Query
You can use this feature to retrieve metadata and historical data for a specific time
interval. For example, you can view all the rows of a table that ever existed during
a given time interval. Metadata about the different versions of rows includes start
and end time, type of change operation, and identity of the transaction that
created the row version. You use the VERSIONS BETWEEN clause of the SELECT
statement to create a Flashback Version Query.

■

Oracle Flashback Transaction Query
You can use this feature to retrieve metadata and historical data for a given
transaction or for all transactions within a given time interval. You can also obtain
the SQL code to undo the changes to particular rows affected by a transaction.
Typically, you use Flashback Transaction Query in conjunction with a Flashback
Version Query that provides the transaction IDs for the rows of interest. To
perform a Flashback Transaction Query, select from the
FLASHBACK_TRANSACTION_QUERY view.

■

DBMS_FLASHBACK package
You can use this feature to set the internal Oracle clock to a time in the past so that
you can examine data current at that time.

Database Administration Features
You can use the following features for application development or interactively as a
database user or administrator:
■

DBMS_FLASHBACK package

■

Flashback Query

■

Flashback Version Query

■

Flashback Transaction Query

Typically, you use the following flashback features only in database administration:
■

Oracle Flashback Table

10-2 Oracle Database Application Developer’s Guide - Fundamentals

Database Administration Tasks Before Using Flashback Features

You can use this feature to recover a table to its state at a previous point in time.
You can restore table data while the database is on line, undoing changes to only
the specified table.
■

Oracle Flashback Drop
You can use this feature to recover a dropped table. This reverses the effects of a
DROP TABLE statement.

■

Oracle Flashback Database
You can use this feature to quickly return the database to an earlier point in time,
by undoing all of the changes that have taken place since then. This is fast, because
you do not have to restore database backups.

Flashback Database, Flashback Table, and Flashback Drop are primarily data recovery
mechanisms and are therefore documented elsewhere. The other flashback features,
while valuable in data recovery scenarios, are useful for application development.
They are the focus of this chapter.
See Also:
■
■

■

■

Oracle Database Backup and Recovery Advanced User's Guide
Oracle Database Administrator's Guide to learn about the Flashback
Drop feature
Oracle Database Administrator's Guide to learn about the Flashback
Table feature
Oracle Database Administrator's Guide to learn about Automatic
Undo Management

Database Administration Tasks Before Using Flashback Features
Before you can use flashback features in your application, you must perform the
following administrative tasks to configure your database. Consult with your database
administrator to perform these tasks:
■

■

Create an undo tablespace with enough space to keep the required data for
flashback operations. The more often users update the data, the more space is
required. Calculating the space requirements is usually performed by a database
administrator. You can find the calculation formula in the Oracle Database
Administrator's Guide.
Enable Automatic Undo Management, as explained in Oracle Database
Administrator's Guide. In particular, you must set the following database
initialization parameters:
–

UNDO_MANAGEMENT

–

UNDO_TABLESPACE

Note that for an undo tablespace with a fixed size, Oracle Database automatically
performs the following actions:
–

Tunes the system to give the best possible undo retention for the undo
tablespace.

For an automatically extensible undo tablespace, Oracle Database retains undo
data longer than the longest query duration as well as the low threshold of undo
retention specified by the UNDO_RETENTION parameter.

Developing Flashback Applications 10-3

Using Flashback Query (SELECT ... AS OF)

Note: You can query V$UNDOSTAT.TUNED_UNDORETENTION to
determine the amount of time for which undo is retained for the
current undo tablespace.

■

■

■

Specify the RETENTION GUARANTEE clause for the undo tablespace to ensure that
unexpired undo is not discarded. Setting UNDO_RETENTION is not, by itself, a
strict guarantee. If the system is under space pressure, then Oracle can overwrite
unexpired undo with freshly generated undo. Specifying RETENTION
GUARANTEE prevents this behavior.
Grant flashback privileges to users, roles, or applications that need to use
flashback features as follows:
–

For the DBMS_FLASHBACK package, grant the EXECUTE privilege on
DBMS_FLASHBACK to provide access to the features in this package.

–

For Flashback Query and Flashback Version Query, grant FLASHBACK and
SELECT privileges on specific objects to be accessed during queries or grant
the FLASHBACK ANY TABLE privilege to allow queries on all tables.

–

For Flashback Transaction Query, grant the SELECT ANY TRANSACTION
privilege.

–

For Execution of undo SQL code, grant SELECT, UPDATE, DELETE, and
INSERT privileges for specific tables, as appropriate, to permit execution of
undo SQL code retrieved by a Flashback Transaction Query.

To use the Flashback Transaction Query feature in Oracle Database 10g, the
database must be running with version 10.0 compatibility, and must have
supplemental logging turned on with the following SQL statement:
ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;

■

To enable flashback operations on specific LOB columns of a table, use the ALTER
TABLE command with the RETENTION option. Because undo data for LOB
columns can be voluminous, you must define which LOB columns to use with
flashback operations.
See Also:
■

■

Oracle Database Backup and Recovery Advanced User's Guide and
Oracle Database Administrator's Guide to learn about DBA tasks
such as setting up automatic undo management and granting
privileges
Oracle Database Application Developer's Guide - Large Objects to learn
about LOB storage and the RETENTION parameter

Using Flashback Query (SELECT ... AS OF)
You perform a Flashback Query by using a SELECT statement with an AS OF clause.
You use a Flashback Query to retrieve data as it existed at some time in the past. The
query explicitly references a past time by means of a timestamp or SCN. It returns
committed data that was current at that point in time.
Potential uses of Flashback Query include:
■

Recovering lost data or undoing incorrect, committed changes. For example, if you
mistakenly delete or update rows, and then commit them, you can immediately
undo the mistake.

10-4 Oracle Database Application Developer’s Guide - Fundamentals

Using Flashback Query (SELECT ... AS OF)

■

■

■

■
■

Comparing current data with the corresponding data at some time in the past. For
example, you might run a daily report that shows the change in data from
yesterday. You can compare individual rows of table data or find intersections or
unions of sets of rows.
Checking the state of transactional data at a particular time. For example, you
could verify the account balance of a certain day.
Simplifying application design, by removing the need to store some kinds of
temporal data. By using a Flashback Query, you can retrieve past data directly
from the database.
Applying packaged applications such as report generation tools to past data.
Providing self-service error correction for an application, thereby enabling users to
undo and correct their errors.
Oracle Database SQL Reference for details on the syntax of
the SELECT... AS OF statement

See Also:

Examining Past Data: Example
This example uses a Flashback Query to examine the state of a table at a previous time.
Suppose that a DBA discovers at 12:30 PM that the row for employee Chung had been
deleted from the employees table. The DBA also knows that at 9:30AM the data for
Chung was correctly stored in the database. The DBA can use a Flashback Query to
examine the contents of the table at 9:30 to find out what data had been lost. If
appropriate, the DBA can then re-insert the lost data.
Example 10–1 retrieves the state of the record for Chung at 9:30AM, April 4, 2004:
Example 10–1

Retrieving a Row with Flashback Query

SELECT * FROM employees AS OF TIMESTAMP
TO_TIMESTAMP('2004-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')
WHERE last_name = 'Chung';

The update in Example 10–2 restores Chung's information to the employees table:
Example 10–2

Reinserting a Row After a Flashback Query

INSERT INTO employees
(SELECT * FROM employees AS OF TIMESTAMP
TO_TIMESTAMP('2004-04-04 09:30:00', 'YYYY-MM-DD HH:MI:SS')
WHERE last_name = 'Chung');

Tips for Using Flashback Query
Keep the following in mind when using a Flashback Query (SELECT ... AS OF):
■

■

■

You can specify or omit the AS OF clause for each table and specify different times
for different tables. Use an AS OF clause in a query to perform DDL operations
(such as creating and truncating tables) or DML operations (such as inserting and
deleting) in the same session as the query.
To use the results of a Flashback Query in a DDL or DML statement that affects the
current state of the database, use an AS OF clause inside an INSERT or CREATE
TABLE AS SELECT statement.
When choosing whether to use a timestamp or an SCN in Flashback Query,
remember that Oracle Database uses SCNs internally and maps these to

Developing Flashback Applications 10-5

Using the DBMS_FLASHBACK Package

timestamps at a granularity of 3 seconds. If a possible 3-second error (maximum)
is important to a Flashback Query in your application, then use an SCN instead of
a timestamp. Refer to "Flashback Tips – General".
■

You can create a view that refers to past data by using the AS OF clause in the
SELECT statement that defines the view. If you specify a relative time by
subtracting from the current time on the database host, then the past time is
recalculated for each query. For example:
CREATE VIEW hour_ago AS
SELECT * FROM employees AS OF
TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE);
-- SYSTIMESTAMP refers to the time zone of the database host environment

■

You can use the AS OF clause in self-joins, or in set operations such as INTERSECT
and MINUS, to extract or compare data from two different times. You can store the
results by preceding a Flashback Query with a CREATE TABLE AS SELECT or
INSERT INTO TABLE SELECT statement. For example, the following query
reinserts into table employees the rows that existed an hour ago:
INSERT INTO employees
(SELECT * FROM employees AS OF
TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE))
-- SYSTIMESTAMP refers to the time zone of the database host environment
MINUS SELECT * FROM employees);

Using the DBMS_FLASHBACK Package
In general, the DBMS_FLASHBACK package provides the same functionality as
Flashback Query, but Flashback Query is sometimes more convenient.
The DBMS_FLASHBACK package acts as a time machine: you can turn back the clock,
carry out normal queries as if you were at that time in the past, and then return to the
present. Because you can use the DBMS_FLASHBACK package to perform queries on
past data without special clauses such as AS OF or VERSIONS BETWEEN, you can
reuse existing PL/SQL code to query the database at times in the past.
You must have the EXECUTE privilege on the DBMS_FLASHBACK package.
To use the DBMS_FLASHBACK package in your PL/SQL code:
1.

Call DBMS_FLASHBACK.ENABLE_AT_TIME or
DBMS_FLASHBACK.ENABLE_AT_SYSTEM_CHANGE_NUMBER to turn back the
clock to a specified time in the past. Afterwards all queries retrieve data that was
current at the specified time.

2.

Perform normal queries, that is, without any special flashback-feature syntax such
as AS OF. The database is automatically queried at the specified past time.
Perform only queries; do not try to perform DDL or DML operations.

3.

Call DBMS_FLASHBACK.DISABLE to return to the present. You must call DISABLE
before calling ENABLE again for a different time. You cannot nest ENABLE
/DISABLE pairs.

You can use a cursor to store the results of queries. To do this, open the cursor before
calling DBMS_FLASHBACK.DISABLE. After storing the results and then calling
DISABLE, you can do the following:
■

Perform INSERT or UPDATE operations to modify the current database state by
using the stored results from the past.

10-6 Oracle Database Application Developer’s Guide - Fundamentals

Using ORA_ROWSCN

■

Compare current data with the past data. After calling DISABLE, open a second
cursor. Fetch from the first cursor to retrieve past data; fetch from the second
cursor to retrieve current data. You can store the past data in a temporary table,
and then use set operators such as MINUS or UNION to contrast or combine the
past and current data.

You can call DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER at any time to obtain
the current System Change Number (SCN). Note that the current SCN is always
returned; this takes no account of previous calls to DBMS_FLASHBACK.ENABLE*.
See Also:
■

■

Oracle Database PL/SQL Packages and Types Reference for details
about the DBMS_FLASHBACK package
Oracle Database Reference and Oracle Database Backup and Recovery
Reference for information about SCNs

Using ORA_ROWSCN
ORA_ROWSCN is a pseudocolumn of any table that is not fixed or external. It represents
the SCN of the most recent change to a given row, that is, the latest COMMIT operation
for the row. For example:
SELECT ora_rowscn, last_name, salary
FROM employees
WHERE employee_id = 7788;
ORA_ROWSCN
---------202553

NAME
---Fudd

SALARY
-----3000

The latest COMMIT operation for the row took place at approximately SCN 202553.
You can use function SCN_TO_TIMESTAMP to convert an SCN, like ORA_ROWSCN, to
the corresponding TIMESTAMP value.
ORA_SCN is in fact a conservative upper bound of the latest commit time: the actual
commit SCN can be somewhat earlier. ORA_SCN is more precise (closer to the actual
commit SCN) for a row-dependent table (created using CREATE TABLE with the
ROWDEPENDENCIES clause).
Noteworthy uses of ORA_ROWSCN in application development include concurrency
control and client cache invalidation. To see how you might use it in concurrency
control, consider the following scenario.
Your application examines a row of data, and records the corresponding ORA_ROWSCN
as 202553. Later, the application needs to update the row, but only if its record of the
data is still accurate. That is, this particular update operation depends, logically, on the
row not having been changed. The operation is therefore made conditional on the
ORA_ROWSCN being still 202553. Here is an equivalent interactive command:
UPDATE employees
SET salary = salary + 100
WHERE employee_id = 7788
AND ora_rowscn = 202553;
0 rows updated.

Developing Flashback Applications 10-7

Using Flashback Version Query

The conditional update fails in this case, because the ORA_ROWSCN is no longer
202553. This means that some user or another application changed the row and
performed a COMMIT more recently than the recorded ORA_ROWSCN.
Your application queries again to obtain the new row data and ORA_ROWSCN. Suppose
that the ORA_ROWSCN is now 415639. The application tries the conditional update
again, using the new ORA_ROWSCN. This time, the update succeeds, and it is
committed. Here is an interactive equivalent:
SQL> UPDATE employees SET salary = salary + 100
WHERE empno = 7788 AND ora_rowscn = 415639;
1 row updated.
SQL> COMMIT;
Commit complete.
SQL> SELECT ora_rowscn, name, salary FROM employees WHERE empno = 7788;
ORA_ROWSCN
---------465461

NAME
---Fudd

SALARY
-----3100

The SCN corresponding to the new COMMIT is 465461.
Besides using ORA_ROWSCN in an UPDATE statement WHERE clause, you can use it in a
DELETE statement WHERE clause or the AS OF clause of a Flashback Query.
See Also:
■

Oracle Database SQL Reference

Using Flashback Version Query
You use a Flashback Version Query to retrieve the different versions of specific rows
that existed during a given time interval. A new row version is created whenever a
COMMIT statement is executed.
You specify a Flashback Version Query using the VERSIONS BETWEEN clause of the
SELECT statement. Here is the syntax:
VERSIONS {BETWEEN {SCN | TIMESTAMP} start AND end}

where start and end are expressions representing the start and end of the time
interval to be queried, respectively. The interval is closed at both ends: the upper and
lower limits specified (start and end) are both included in the time interval.
The Flashback Version Query returns a table with a row for each version of the row that
existed at any time during the time interval you specify. Each row in the table includes
pseudocolumns of metadata about the row version, described in Table 10–1. This
information can reveal when and how a particular change (perhaps erroneous)
occurred to your database.

10-8 Oracle Database Application Developer’s Guide - Fundamentals

Using Flashback Version Query

Table 10–1

Flashback Version Query Row Data Pseudocolumns

Pseudocolumn Name

Description

Starting System Change Number (SCN) or TIMESTAMP when the
row version was created. This identifies the time when the data first
VERSIONS_STARTTIME took on the values reflected in the row version. You can use this to
identify the past target time for a Flashback Table or Flashback
Query operation.
VERSIONS_STARTSCN

If this is NULL, then the row version was created before the lower
time bound of the query BETWEEN clause.
VERSIONS_ENDSCN
VERSIONS_ENDTIME

VERSIONS_XID

SCN or TIMESTAMP when the row version expired. This identifies
the row expiration time.
If this is NULL, then either the row version was still current at the
time of the query or the row corresponds to a DELETE operation.
Identifier of the transaction that created the row version.

VERSIONS_OPERATION Operation performed by the transaction: I for insertion, D for
deletion, or U for update. The version is that of the row that was
inserted, deleted, or updated; that is, the row after an INSERT
operation, the row before a DELETE operation, or the row affected by
an UPDATE operation.
Note: For user updates of an index key, a Flashback Version Query
may treat an UPDATE operation as two operations, DELETE plus
INSERT, represented as two version rows with a D followed by an I
VERSIONS_OPERATION.

A given row version is valid starting at its time VERSIONS_START* up to, but not
including, its time VERSIONS_END*. That is, it is valid for any time t such that
VERSIONS_START* <= t < VERSIONS_END*. For example, the following output
indicates that the salary was 10243 from September 9, 2002, included, to November 25,
2003, not included.
VERSIONS_START_TIME
------------------09-SEP-2003

VERSIONS_END_TIME
----------------25-NOV-2003

SALARY
-----10243

Here is a typical Flashback Version Query:
SELECT versions_startscn, versions_starttime,
versions_endscn, versions_endtime,
versions_xid, versions_operation,
name, salary
FROM employees
VERSIONS BETWEEN TIMESTAMP
TO_TIMESTAMP('2003-07-18 14:00:00', 'YYYY-MM-DD HH24:MI:SS')
AND TO_TIMESTAMP('2003-07-18 17:00:00', 'YYYY-MM-DD HH24:MI:SS')
WHERE name = 'JOE';

Pseudocolumn VERSIONS_XID provides a unique identifier for the transaction that
put the data in that state. You can use this value in connection with a Flashback
Transaction Query to locate metadata about this transaction in the
FLASHBACK_TRANSACTION_QUERY view, including the SQL required to undo the
row change and the user responsible for the change – see "Using Flashback Transaction
Query" on page 10-10.
Oracle Database SQL Reference for information on the
Flashback Version Query pseudocolumns and the syntax of the
VERSIONS clause

See Also:

Developing Flashback Applications 10-9

Using Flashback Transaction Query

Using Flashback Transaction Query
A Flashback Transaction Query is a query on the view
FLASHBACK_TRANSACTION_QUERY. You use a Flashback Transaction Query to obtain
transaction information, including SQL code that you can use to undo each of the
changes made by the transaction.
See Also: Oracle Database Backup and Recovery Advanced User's Guide.
and Oracle Database Administrator's Guide for information on how a
DBA can use the Flashback Table feature to restore an entire table,
rather than individual rows

As an example, the following statement queries the
FLASHBACK_TRANSACTION_QUERY view for transaction information, including the
transaction ID, the operation, the operation start and end SCNs, the user responsible
for the operation, and the SQL code to undo the operation:
SELECT xid, operation, start_scn,commit_scn, logon_user, undo_sql
FROM flashback_transaction_query
WHERE xid = HEXTORAW('000200030000002D');

As another example, the following query uses a Flashback Version Query as a
subquery to associate each row version with the LOGON_USER responsible for the row
data change.
SELECT xid, logon_user FROM flashback_transaction_query
WHERE xid IN (SELECT versions_xid FROM employees VERSIONS BETWEEN TIMESTAMP
TO_TIMESTAMP('2003-07-18 14:00:00', 'YYYY-MM-DD HH24:MI:SS') AND
TO_TIMESTAMP('2003-07-18 17:00:00', 'YYYY-MM-DD HH24:MI:SS'));

Flashback Transaction Query and Flashback Version Query: Example
This example demonstrates the use of a Flashback Transaction Query in conjunction
with a Flashback Version Query. The example assumes simple variations of the
employees and departments tables in the sample hr schema.
In this example, a DBA carries out the following series of actions in SQL*Plus:
connect hr/hr
CREATE TABLE emp
(empno
NUMBER PRIMARY KEY,
empname VARCHAR2(16),
salary NUMBER);
INSERT INTO emp VALUES (111, 'Mike', 555);
COMMIT;
CREATE TABLE dept
(deptno
NUMBER,
deptname VARCHAR2(32));
INSERT INTO dept VALUES (10, 'Accounting');
COMMIT;

At this point, emp and dept have one row each. In terms of row versions, each table
has one version of one row. Next, suppose that an erroneous transaction deletes
employee id 111 from table emp:
UPDATE emp SET salary = salary + 100 WHERE empno = 111;
INSERT INTO dept VALUES (20, 'Finance');
DELETE FROM emp WHERE empno = 111;
COMMIT;

10-10 Oracle Database Application Developer’s Guide - Fundamentals

Using Flashback Transaction Query

Subsequently, a new transaction reinserts employee id 111 with a new employee name
into the emp table.
INSERT INTO emp VALUES (111, 'Tom', 777);
UPDATE emp SET salary = salary + 100 WHERE empno = 111;
UPDATE emp SET salary = salary + 50 WHERE empno = 111;
COMMIT;

At this point, the DBA detects the application error and needs to diagnose the
problem. The DBA issues the following query to retrieve versions of the rows in the
emp table that correspond to empno 111. The query uses Flashback Version Query
pseudocolumns.
connect dba_name/password
SELECT versions_xid XID, versions_startscn START_SCN,
versions_endscn END_SCN, versions_operation OPERATION,
empname, salary FROM hr.emp
VERSIONS BETWEEN SCN MINVALUE AND MAXVALUE
where empno = 111;
XID
---------------0004000700000058
000200030000002D
000200030000002E
3 rows selected

START_SCN END_SCN
---------- --------113855
113564
112670
113564

OPERATION
---------I
D
I

EMPNAME
---------Tom
Mike
Mike

SALARY
---------927
555
555

The results table reads chronologically, from bottom to top. The third row corresponds
to the version of the row in emp that was originally inserted in the table when the table
was created. The second row corresponds to the row in emp that was deleted by the
erroneous transaction. The first row corresponds to the version of the row in emp that
was reinserted with a new employee name.
The DBA identifies transaction 000200030000002D as the erroneous transaction and
issues the following Flashback Transaction Query to audit all changes made by this
transaction:
SELECT

xid, start_scn START, commit_scn COMMIT,
operation OP, logon_user USER,
undo_sql FROM flashback_transaction_query
WHERE xid = HEXTORAW('000200030000002D');

XID
START
COMMIT OP
USER
UNDO_SQL
---------------- ---------- ------------------------------000200030000002D 195243 195244 DELETE
HR
insert into "HR"."EMP"
("EMPNO","EMPNAME","SALARY") values ('111','Mike','655');
000200030000002D 195243 195244 INSERT
where ROWID = 'AAAKD4AABAAAJ3BAAB';

HR

delete from "HR"."DEPT"

000200030000002D 195243 195244 UPDATE
HR
update "HR"."EMP"
set "SALARY" = '555' where ROWID = 'AAAKD2AABAAAJ29AAA';
000200030000002D

195243

113565

BEGIN

HR

4 rows selected

The rightmost column (undo_sql) contains the SQL code that will undo the
corresponding change operation. The DBA can execute this code to undo the changes

Developing Flashback Applications

10-11

Flashback Tips

made by that transaction. The USER column (logon_user) shows the user
responsible for the transaction.
A DBA might also be interested in knowing all changes made in a certain time
window. In our scenario, the DBA performs the following query to view the details of
all transactions that executed since the erroneous transaction identified earlier
(including the erroneous transaction itself):
SELECT xid, start_scn, commit_scn, operation, table_name, table_owner
FROM flashback_transaction_query
WHERE table_owner = 'HR' AND
start_timestamp >=
TO_TIMESTAMP ('2002-04-16 11:00:00','YYYY-MM-DD HH:MI:SS');
XID
---------------0004000700000058
0004000700000058
0004000700000058
000200030000002D
000200030000002D
000200030000002D

START_SCN
--------195245
195245
195245
195243
195243
195243

COMMIT_SCN
---------195246
195246
195246
195244
195244
195244

OPERATION
--------UPDATE
UPDATE
INSERT
DELETE
INSERT
UPDATE

TABLE_NAME
---------EMP
EMP
EMP
EMP
DEPT
EMP

TABLE_OWNER
----------HR
HR
HR
HR
HR
HR

6 rows selected

Flashback Tips
The following tips and restrictions apply to using flashback features.

Flashback Tips – Performance
■

■

■

■

■

■

For better performance, generate statistics on all tables involved in a Flashback
Query by using the DBMS_STATS package, and keep the statistics current.
Flashback Query always uses the cost-based optimizer, which relies on these
statistics.
The performance of a query into the past depends on how much undo data must
be accessed. For better performance, use queries to select small sets of past data
using indexes, not to scan entire tables. If you must do a full table scan, consider
adding a parallel hint to the query.
The performance cost in I/O is the cost of paging in data and undo blocks that are
not already in the buffer cache. The performance cost in CPU use is the cost of
applying undo information to affected data blocks. When operating on changes in
the recent past, flashback features essentially CPU bound.
Use index structures for Flashback Version Query: the database keeps undo data
for index changes as well as data changes. Performance of index lookup-based
Flashback Version Query is an order of magnitude faster than the full table scans
that are otherwise needed.
In a Flashback Transaction Query, the type of the xid column is RAW(8). To take
advantage of the index built on the xid column, use the HEXTORAW conversion
function: HEXTORAW(xid).
Flashback Query against a materialized view does not take advantage of query
rewrite optimizations.
See Also:

Oracle Database Performance Tuning Guide

10-12 Oracle Database Application Developer’s Guide - Fundamentals

Flashback Tips

Flashback Tips – General
■

■

■

■

Use the DBMS_FLASHBACK package or other flashback features? Use
ENABLE/DISABLE calls to the DBMS_FLASHBACK package around SQL code that
you do not control, or when you want to use the same past time for several
consecutive queries. Use Flashback Query, Flashback Version Query, or Flashback
Transaction Query for SQL that you write, for convenience. A Flashback Query, for
example, is flexible enough to do comparisons and store results in a single query.
To obtain an SCN to use later with a flashback feature, use
DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER.
You can compute or retrieve a past time to use in a query by using a function
return value as a timestamp or SCN argument. For example, you can perform date
and time calculations by adding or subtracting an INTERVAL value to the value of
the SYSTIMESTAMP function.
You can query locally or remotely (Flashback Query, Flashback Version Query, or
Flashback Transaction Query). for example here is a remote Flashback Query:
(SELECT * FROM employees@some_remote_host AS OF
TIMESTAMP (SYSTIMESTAMP - INTERVAL '60' MINUTE);

■

■

■

■

To ensure database consistency, always perform a COMMIT or ROLLBACK operation
before querying past data.
Remember that all flashback processing is done using the current session settings,
such as national language and character set, not the settings that were in effect at
the time being queried.
Some DDLs that alter the structure of a table, such as drop/modify column, move
table, drop partition, and truncate table/partition, invalidate any existing undo
data for the table. It is not possible to retrieve data from a point earlier than the
time such DDLs were executed. Trying such a query results in error ORA-1466.
This restriction does not apply to DDL operations that alter the storage attributes
of a table, such as PCTFREE, INITRANS, and MAXTRANS.
Use an SCN to query past data at a precise time. If you use a timestamp, the actual
time queried might be up to 3 seconds earlier than the time you specify. Internally,
Oracle Database uses SCNs; these are mapped to timestamps at a granularity of
every 3 seconds.
For example, assume that the SCN values 1000 and 1005 are mapped to the times
8:41 and 8:46 AM respectively. A query for a time between 8:41:00 and 8:45:59 AM
is mapped to SCN 1000; a Flashback Query for 8:46 AM is mapped to SCN 1005.
Due to this time-to-SCN mapping, if you specify a time that is slightly after a DDL
operation (such as a table creation) the database might actually use an SCN that is
just before the DDL operation. This can result in error ORA-1466.

■

You cannot retrieve past data from a V$ view in the data dictionary. Performing a
query on such a view always returns the current data. You can, however, perform
queries on past data in other views of the data dictionary, such as USER_TABLES.

Developing Flashback Applications

10-13

Flashback Tips

10-14 Oracle Database Application Developer’s Guide - Fundamentals

11
Developing Applications with the PL/SQL
Web Toolkit
Java is not the only language that can do network operations and produce dynamic
Web content. PL/SQL has a number of features that you can use to make your
database available on the Web and make back-office data accessible on the intranet.
This chapter discusses the following topics:
■

Developing PL/SQL Web Applications: Overview

■

Using the mod_plsql Gateway

■

Generating HTML Output with PL/SQL

■

Passing Parameters to a PL/SQL Web Application

■

Performing Network Operations within PL/SQL Stored Procedures

Developing PL/SQL Web Applications: Overview
This section contains the following topics:
■

Invoking a PL/SQL Web Application

■

Implementing a PL/SQL Web Application

Invoking a PL/SQL Web Application
Typically, a Web application written in PL/SQL is a set of stored procedures that
interact with Web browsers through HTTP. A set of interlinked, dynamically generated
HTML pages forms the user interface of a web application.
The program flow of a PL/SQL Web application is similar to that in a CGI Perl script.
Developers often use CGI scripts to produce Web pages dynamically, but such scripts
are often not optimal for accessing Oracle Database. Delivering Web content with
PL/SQL stored procedures provides the power and flexibility of database processing.
For example, you can use DML, dynamic SQL, and cursors. You also eliminate the
process overhead of forking a new CGI process to handle each HTTP request.
Figure 11–1 illustrates the generic process for a PL/SQL Web application.

Developing Applications with the PL/SQL Web Toolkit

11-1

Developing PL/SQL Web Applications: Overview

Figure 11–1 PL/SQL Web Application

Stored
Procedure

3

4 2
Web
Browser

PL/SQL
Web
Toolkit

5
1

Web
Server

The process includes the following steps:
1.

A user visits a Web page, follows a hypertext link, or submits data in a form,
which causes the browser to send a HTTP request for a URL to an HTTP server.

2.

The HTTP server invokes a stored procedure on an Oracle database according to
the data encoded in the URL. The data in the URL takes the form of parameters to
be passed to the stored procedure.

3.

The stored procedure calls subprograms in the PL/SQL Web Toolkit. Typically,
subprograms such as HTP.Print generate Web pages dynamically. A generated
Web page varies depending on the database contents and the input parameters.

4.

The subprograms pass the dynamically generated page to the Web server.

5.

The Web server delivers the page to the client.

Implementing a PL/SQL Web Application
You can implement a Web browser-based application entirely in PL/SQL with the
Oracle Database components described in this section.

PL/SQL Web Toolkit
This set of PL/SQL packages is a generic interface that enables you to use stored
procedures called by mod_plsql at runtime.
In response to a browser request, a PL/SQL procedure updates or retrieves data from
Oracle Database according to the user input. It then generates an HTTP response to the
browser, typically in the form of a file download or HTML to be displayed. The Web
Toolkit API enables stored procedures to perform actions such as the following:
■

Obtain information about an HTTP request

■

Generate HTTP headers such as content-type and mime-type

■

Set browser cookies

■

Generate HTML pages

Table 11–1 describes commonly used PL/SQL Web Toolkit packages.

11-2 Oracle Database Application Developer’s Guide - Fundamentals

Using the mod_plsql Gateway

Table 11–1

Commonly Used Packages in the PL/SQL Web Toolkit

Package

Description of Contents

HTF

Function versions of the procedures in the htp package. The function
versions do not directly generate output in a Web page. Instead, they pass
their output as return values to the statements that invoke them. Use these
functions when you need to nest function calls.

HTP

Procedures that generate HTML tags. For instance, the procedure
htp.anchor generates the HTML anchor tag, .

OWA_CACHE

Functions and procedures that enable the PL/SQL gateway cache feature to
improve performance of your PL/SQL Web application.
You can use this package to enable expires-based and validation-based
caching with the PL/SQL gateway file system.

OWA_COOKIE

Subprograms that send and retrieve HTTP cookies to and from a client Web
browser. Cookies are strings a browser uses to maintain state between HTTP
calls. State can be maintained throughout a client session or longer if a cookie
expiration date is included.

OWA_CUSTOM

The authorize function used by cookies.

OWA_IMAGE

Subprograms that obtain the coordinates where a user clicked an image. Use
this package when you have an image map whose destination links invoke a
PL/SQL gateway.

OWA_OPT_LOCK Subprograms that impose database optimistic locking strategies to prevent
lost updates. Lost updates can otherwise occur if a user selects, and then
attempts to update, a row whose values have been changed in the meantime
by another user.
OWA_PATTERN

Subprograms that perform string matching and string manipulation with
regular expressions.

OWA_SEC

Subprograms used by the PL/SQL gateway for authenticating requests.

OWA_TEXT

Subprograms used by package OWA_PATTERN for manipulating strings. You
can also use them directly.

OWA_UTIL

The following types of utility subprograms:
■

■

■

WPG_DOCLOAD

Dynamic SQL utilities to produce pages with dynamically generated
SQL code.
HTML utilities to retrieve the values of CGI environment variables and
perform URL redirects.
Date utilities for correct date-handling. Date values are simple strings in
HTML, but must be properly treated as an Oracle Database datatype.

Subprograms that download documents from a document repository that
you define using the DAD configuration.

See Also: Oracle Database PL/SQL Packages and Types Reference for
syntax, descriptions, and examples for the PL/SQL Web Toolkit
packages

Using the mod_plsql Gateway
As explained in detail in the Oracle Application Server mod_plsql User's Guide, mod_plsql
maps Web client requests to PL/SQL stored procedures over HTTP. Refer to this
documentation for instructions.

Developing Applications with the PL/SQL Web Toolkit

11-3

Generating HTML Output with PL/SQL

See Also:
■

■

Oracle Application Server mod_plsql User's Guide to learn how to
configure and use mod_plsql
Oracle HTTP Server Administrator's Guide to obtain mod_plsql
reference material

Generating HTML Output with PL/SQL
Traditionally, PL/SQL Web applications use function calls to generate each HTML tag
for output. These functions are part of the PL/SQL Web Toolkit packages that come
with Oracle Database. Example 11–1 illustrates how to generate a simple HTML page
by calling the HTP functions that correspond to each HTML tag.
Example 11–1

Displaying HTML Tags with HTP Functions

CREATE OR REPLACE PROCEDURE html_page
IS
BEGIN
HTP.HTMLOPEN;
HTP.HEADOPEN;
HTP.TITLE('Title');
HTP.HEADCLOSE;

-----

generates
generates
generates
generates






-- generates 
HTP.BODYOPEN( cattributes => 'TEXT="#000000" BGCOLOR="#FFFFFF"');
-- generates 
Heading in the HTML File

HTP.HEADER(1, 'Heading in the HTML File');
HTP.PARA;
-- generates 

HTP.PRINT('Some text in the HTML file.');
HTP.BODYCLOSE;
-- generates 
HTP.HTMLCLOSE;
-- generates 
END;

An alternative to making function calls that correspond to each tag is to use the
HTP.PRINT function to print the text and tags together. Example 11–2 illustrates this
technique.
Example 11–2

Displaying HTML Tags with HTP.PRINT

CREATE OR REPLACE PROCEDURE html_page2
IS
BEGIN
HTP.PRINT('');
HTP.PRINT('');
HTP.PRINT('');
HTP.PRINT('');
HTP.PRINT('');
HTP.PRINT('');
HTP.PRINT('
Heading in the HTML File
');
HTP.PRINT('
Some text in the HTML file.');
HTP.PRINT('');
HTP.PRINT('');
END;

11-4 Oracle Database Application Developer’s Guide - Fundamentals

Passing Parameters to a PL/SQL Web Application

Chapter 12, "Developing PL/SQL Server Pages" describes an additional method for
delivering using PL/SQL to generate HTML content. PL/SQL server pages enables
you to build on your knowledge of HTML tags and avoid learning a new set of
function calls. In an application written as a set of PL/SQL server pages, you can still
use functions from the PL/SQL Web toolkit to do the following:
■

Simplify the processing involved in displaying tables

■

Store persistent data (cookies)

■

Work with CGI protocol internals

Passing Parameters to a PL/SQL Web Application
To be useful in a wide variety of situations, a Web application must be interactive
enough to allow user choices. To keep the attention of impatient Web surfers, you
should streamline the interaction so that users can specify these choices very simply,
without excessive decision-making or data entry.
The main methods of passing parameters to PL/SQL Web applications are:
■

■

Using HTML form tags. The user fills in a form on one Web page, and all the data
and choices are transmitted to a stored procedure when the user clicks the Submit
button on the page.
Hard-coded in the URL. The user clicks on a link, and a set of predefined
parameters are transmitted to a stored procedure. Typically, you would include
separate links on your Web page for all the choices that the user might want.

This section contains the following topics:
■

Passing List and Dropdown List Parameters from an HTML Form

■

Passing Radio Button and Checkbox Parameters from an HTML Form

■

Passing Entry Field Parameters from an HTML Form

■

Passing Hidden Parameters from an HTML Form

■

Uploading a File from an HTML Form

■

Submitting a Completed HTML Form

■

Handling Missing Input from an HTML Form

■

Maintaining State Information Between Web Pages

Passing List and Dropdown List Parameters from an HTML Form
List boxes and drop-down lists are implemented with the HTML tag .
Use a list box for a large number of choices or to allow multiple selections. List boxes
are good for showing items in alphabetical order so that users can find an item quickly
without reading all the choices.
Use a drop-down list in the following situations:
■

There are a small number of choices

■

Screen space is limited.

■

Choices are in an unusual order.

The drop-down captures the attention of first-time users and makes them read the
items. If you keep the choices and order consistent, then users can memorize the

Developing Applications with the PL/SQL Web Toolkit

11-5

Passing Parameters to a PL/SQL Web Application

motion of selecting an item from the drop-down list, allowing them to make selections
quickly as they gain experience. Example 11–3 shows a simple drop-down list.
Example 11–3

HTML Drop-Down List



Winter
Spring
Summer
Fall


Passing Radio Button and Checkbox Parameters from an HTML Form
Radio buttons pass either a null value (if none of the radio buttons in a group is
checked), or the value specified on the radio button that is checked.
To specify a default value for a set of radio buttons, you can include the CHECKED
attribute in one of the INPUT tags, or include a DEFAULT clause on the parameter
within the stored procedure. When setting up a group of radio buttons, be sure to
include a choice that indicates "no preference", because once the user selects a radio
button, they can still select a different one, but they cannot clear the selection
completely. For example, include a "Don't Care" or "Don't Know" selection along with
"Yes" and "No" choices, in case someone makes a selection and then realizes it was
wrong.
Checkboxes need special handling, because your stored procedure might receive a null
value, a single value, or multiple values:
All the checkboxes with the same NAME attribute make up a checkbox group. If none of
the checkboxes in a group is checked, the stored procedure receives a null value for the
corresponding parameter.
If one checkbox in a group is checked, the stored procedure receives a single
VARCHAR2 parameter.
If more than one checkbox in a group is checked, the stored procedure receives a
parameter with the PL/SQL type TABLE OF VARCHAR2. You must declare a type like
this, or use a predefined one like OWA_UTIL.IDENT_ARR. To retrieve the values, use a
loop:
CREATE OR REPLACE PROCEDURE handle_checkboxes ( checkboxes owa_util.ident_arr )
AS
BEGIN
...
FOR i IN 1..checkboxes.count
LOOP
htp.print('
Checkbox value: ' || checkboxes(i));
END LOOP;
...
END;
/
SHOW ERRORS;

Passing Entry Field Parameters from an HTML Form
Entry fields require the most validation, because a user might enter data in the wrong
format, out of range, and so on. If possible, validate the data on the client side using

11-6 Oracle Database Application Developer’s Guide - Fundamentals

Passing Parameters to a PL/SQL Web Application

dynamic HTML or Java, and format it correctly for the user or prompt them to enter it
again.
For example:
■

■

■

You might prevent the user from entering alphabetic characters in a numeric entry
field, or from entering characters once a length limit is reached.
You might silently remove spaces and dashes from a credit card number if the
stored procedure expects the value in that format.
You might inform the user immediately when they type a number that is too large,
so that they can retype it.

Because you cannot always rely on such validation to succeed, code the stored
procedures to deal with these cases anyway. Rather than forcing the user to use the
Back button when they enter wrong data, display a single page with an error message
and the original form with all the other values filled in.
For sensitive information such as passwords, a special form of the entry field, , hides the text as it is typed in.
For example, the following procedure accepts two strings as input. The first time it is
called, the user sees a simple form prompting for the input values. When the user
submits the information, the same procedure is called again to check if the input is
correct. If the input is OK, the procedure processes it. If not, the procedure prompts for
new input, filling in the original values for the user.
-- Store a name and associated zip code in the database.
CREATE OR REPLACE PROCEDURE associate_name_with_zipcode
(
name VARCHAR2 DEFAULT NULL,
zip VARCHAR2 DEFAULT NULL
)
AS
booktitle VARCHAR2(256);
BEGIN
-- Both entry fields must contain a value. The zip code must be 6 characters.
-- (In a real program you would perform more extensive checking.)
IF name IS NOT NULL AND zip IS NOT NULL AND length(zip) = 6 THEN
store_name_and_zipcode(name, zip);
htp.print('
The person ' || name || ' has the zip code ' || zip || '.');
-- If the input was OK, we stop here and the user does not see the form again.
RETURN;
END IF;
-- If some data was entered, but it is not correct, show the error message.
IF (name IS NULL AND zip IS NOT NULL)
OR (name IS NOT NULL AND zip IS NULL)
OR (zip IS NOT NULL AND length(zip) != 6)
THEN
htp.print('
Please re-enter the data. Fill in all fields, and use a
6-digit zip code.');
END IF;
-- If the user has not entered any data, or entered bad data, prompt for
-- input values.
-- Make the form call the same procedure to check the input values.
htp.formOpen( 'scott.associate_name_with_zipcode', 'GET');
htp.print('
Enter your name:');
htp.print('');
htp.print('
Enter your zip code:');
htp.print('');
htp.formSubmit(NULL, 'Submit');
htp.formClose;
END;
/
SHOW ERRORS;

Passing Hidden Parameters from an HTML Form
One technique for passing information through a sequence of stored procedures,
without requiring the user to specify the same choices each time, is to include hidden
parameters in the form that calls a stored procedure. The first stored procedure places
information, such as a user name, into the HTML form that it generates. The value of
the hidden parameter is passed to the next stored procedure, as if the user had entered
it through a radio button or entry field.
Other techniques for passing information from one stored procedure to another
include:
■

■

Sending a "cookie" containing the persistent information to the browser. The
browser then sends this same information back to the server when accessing other
Web pages from the same site. Cookies are set and retrieved through the HTTP
headers that are transferred between the browser and the Web server before the
HTML text of each Web page.
Storing the information in the database itself, where later stored procedures can
retrieve it. This technique involves some extra overhead on the database server,
and you must still find a way to keep track of each user as multiple users access
the server at the same time.

Uploading a File from an HTML Form
You can use an HTML form to choose a file on a client system, and transfer it to the
server. A stored procedure can insert the file into the database as a CLOB, BLOB, or
other type that can hold large amounts of data.
The PL/SQL Web toolkit and the PL/SQL gateway have the notion of a "document
table" that holds uploaded files.
See Also:

mod_plsql User's Guide

Submitting a Completed HTML Form
By default, an HTML form must have a Submit button, which transmits the data from
the form to a stored procedure or CGI program. You can label this button with text of
your choice, such as "Search", "Register", and so on.
You can have multiple forms on the same page, each with its own form elements and
Submit button. You can even have forms consisting entirely of hidden parameters,
where the user makes no choice other than clicking the button.
Using JavaScript or other scripting languages, you can do away with the Submit
button and have the form submitted in response to some other action, such as selecting
from a drop-down list. This technique is best when the user only makes a single
selection, and the confirmation step of the Submit button is not essential.

11-8 Oracle Database Application Developer’s Guide - Fundamentals

Passing Parameters to a PL/SQL Web Application

Handling Missing Input from an HTML Form
When an HTML form is submitted, your stored procedure receives null parameters for
any form elements that are not filled in. For example, null parameters can result from
an empty entry field, a set of checkboxes, radio buttons, or list items with none
checked, or a VALUE parameter of "" (empty quotation marks).
Regardless of any validation you do on the client side, always code stored procedures
to handle the possibility that some parameters are null:
■

■

■

■

Use a DEFAULT clause in all parameter declarations, to prevent an exception when
the stored procedure is called with a missing form parameter. You can set the
default to zero for numeric values (when that makes sense), and use DEFAULT
NULL when you want to check whether or not the user actually specifies a value.
Before using an input parameter value that has a DEFAULT NULL declaration,
check if it is null.
Make the procedure generate sensible results even when not all input parameters
are specified. You might leave some sections out of a report, or display a text string
or image in a report to indicate where parameters were not specified.
Provide a way to fill in the missing values and run the stored procedure again,
directly from the results page. For example, you could include a link that calls the
same stored procedure with an additional parameter, or display the original form
with its values filled in as part of the output.

Maintaining State Information Between Web Pages
Web applications are particularly concerned with the idea of state, the set of data that
is current at a particular moment in time. It is easy to lose state information when
switching from one Web page to another, which might result in asking the user to
make the same choices over and over.
You can pass state information between dynamic Web pages using HTML forms. The
information is passed as a set of name-value pairs, which are turned into stored
procedure parameters for you.
If the user has to make multiple selections, or one selection from many choices, or it is
important to avoid an accidental selection, use an HTML form. After the user makes
and reviews all the choices, they confirm the choices with the Submit button.
Subsequent pages can use forms with hidden parameters (
tags) to pass these choices from one page to the next.
If the user is only considering one or two choices, or the decision points are scattered
throughout the Web page, you can save the user from hunting around for the Submit
button by representing actions as hyperlinks and including any necessary name-value
pairs in the query string (the part following the ? within a URL).
An alternative way to main state information is to use Oracle Application Server
and its mod_ose module. This approach lets you store state information in package
variables that remain available as a user moves around a Web site.
See Also: The Oracle Application Server documentation set at
http://www.oracle.com/technology/documentation

Developing Applications with the PL/SQL Web Toolkit

11-9

Performing Network Operations within PL/SQL Stored Procedures

Performing Network Operations within PL/SQL Stored Procedures
While built-in PL/SQL features are focused on traditional database operations and
programming logic, Oracle Database provides packages that open up Internet
computing to PL/SQL programmers.
This section contains the following topics:
■

Sending E-Mail from PL/SQL

■

Getting a Host Name or Address from PL/SQL

■

Working with TCP/IP Connections from PL/SQL

■

Retrieving the Contents of an HTTP URL from PL/SQL

■

Working with Tables, Image Maps, Cookies, and CGI Variables from PL/SQL

Sending E-Mail from PL/SQL
You can send e-mail from a PL/SQL program or stored procedure with the UTL_SMTP
package. You can read about this package in the Oracle Database PL/SQL Packages and
Types Reference.
The following code example illustrates how the SMTP package might be used by an
application to send e-mail. The application connects to an SMTP server at port 25 and
sends a simple text message.
PROCEDURE send_test_message
IS
mailhost
VARCHAR2(64) := 'mailhost.fictional-domain.com';
sender
VARCHAR2(64) := 'me@fictional-domain.com';
recipient
VARCHAR2(64) := 'you@fictional-domain.com';
mail_conn utl_smtp.connection;
BEGIN
mail_conn := utl_smtp.open_connection(mailhost, 25);
utl_smtp.helo(mail_conn, mailhost);
utl_smtp.mail(mail_conn, sender);
utl_smtp.rcpt(mail_conn, recipient);
-- If we had the message in a single string, we could collapse
-- open_data(), write_data(), and close_data() into a single call to data().
utl_smtp.open_data(mail_conn);
utl_smtp.write_data(mail_conn, 'This is a test message.' || chr(13));
utl_smtp.write_data(mail_conn, 'This is line 2.' || chr(13));
utl_smtp.close_data(mail_conn);
utl_smtp.quit(mail_conn);
EXCEPTION
WHEN OTHERS THEN
-- Insert error-handling code here
NULL;
END;

Getting a Host Name or Address from PL/SQL
You can determine the host name of the local machine, or the IP address of a given
host name from a PL/SQL program or stored procedure using the UTL_INADDR
package. You can find details about this package in the Oracle Database PL/SQL
Packages and Types Reference. You use the results in calls to the UTL_TCP package.

11-10 Oracle Database Application Developer’s Guide - Fundamentals

Performing Network Operations within PL/SQL Stored Procedures

Working with TCP/IP Connections from PL/SQL
You can open TCP/IP connections to machines on the network, and read or write to
the corresponding sockets, using the UTL_TCP package. You can find details about this
package in the Oracle Database PL/SQL Packages and Types Reference.

Retrieving the Contents of an HTTP URL from PL/SQL
You can retrieve the contents of an HTTP URL using the UTL_HTTP package. The
contents are typically in the form of HTML-tagged text, but may be plain text, a JPEG
image, or any sort of file that is downloadable from a Web server. You can find details
about this package in the Oracle Database PL/SQL Packages and Types Reference.
The UTL_HTTP package lets you:
■

■

■

Control the details of the HTTP session, including header lines, cookies, redirects,
proxy servers, IDs and passwords for protected sites, and CGI parameters through
the GET or POST methods.
Speed up multiple accesses to the same Web site using HTTP 1.1 persistent
connections.
Construct and interpret URLs for use with UTL_HTTP through the ESCAPE and
UNESCAPE functions in the UTL_URL package.

Typically, developers have used Java or Perl to perform these operations; this package
lets you do them with PL/SQL.
CREATE OR REPLACE PROCEDURE show_url
(
url
IN VARCHAR2,
username IN VARCHAR2 DEFAULT NULL,
password IN VARCHAR2 DEFAULT NULL
) AS
req
utl_http.req;
resp
utl_http.resp;
name
VARCHAR2(256);
value
VARCHAR2(1024);
data
VARCHAR2(255);
my_scheme VARCHAR2(256);
my_realm VARCHAR2(256);
my_proxy BOOLEAN;
BEGIN
-- When going through a firewall, pass requests through this host.
-- Specify sites inside the firewall that don't need the proxy host.
utl_http.set_proxy('proxy.my-company.com', 'corp.my-company.com');
-- Ask UTL_HTTP not to raise an exception for 4xx and 5xx status codes,
-- rather than just returning the text of the error page.
utl_http.set_response_error_check(FALSE);
-- Begin retrieving this Web page.
req := utl_http.begin_request(url);
-- Identify ourselves. Some sites serve special pages for particular browsers.
utl_http.set_header(req, 'User-Agent', 'Mozilla/4.0');
-- Specify a user ID and password for pages that require them.
IF (username IS NOT NULL) THEN
utl_http.set_authentication(req, username, password);
END IF;

Developing Applications with the PL/SQL Web Toolkit 11-11

Performing Network Operations within PL/SQL Stored Procedures

BEGIN
-- Start receiving the HTML text.
resp := utl_http.get_response(req);
-- Show the status codes and reason phrase of the response.
dbms_output.put_line('HTTP response status code: ' || resp.status_code);
dbms_output.put_line('HTTP response reason phrase: ' || resp.reason_phrase);
-- Look for client-side error and report it.
IF (resp.status_code >= 400) AND (resp.status_code <= 499) THEN
-- Detect whether the page is password protected, and we didn't supply
-- the right authorization.
IF (resp.status_code = utl_http.HTTP_UNAUTHORIZED) THEN
utl_http.get_authentication(resp, my_scheme, my_realm, my_proxy);
IF (my_proxy) THEN
dbms_output.put_line('Web proxy server is protected.');
dbms_output.put('Please supply the required ' || my_scheme ||
' authentication username/password for realm ' || my_realm ||
' for the proxy server.');
ELSE
dbms_output.put_line('Web page ' || url || ' is protected.');
dbms_output.put('Please supplied the required ' || my_scheme ||
' authentication username/password for realm ' || my_realm ||
' for the Web page.');
END IF;
ELSE
dbms_output.put_line('Check the URL.');
END IF;
utl_http.end_response(resp);
RETURN;
-- Look for server-side error and report it.
ELSIF (resp.status_code >= 500) AND (resp.status_code <= 599) THEN
dbms_output.put_line('Check if the Web site is up.');
utl_http.end_response(resp);
RETURN;
END IF;
-- The HTTP header lines contain information about cookies, character sets,
-- and other data that client and server can use to customize each session.
FOR i IN 1..utl_http.get_header_count(resp) LOOP
utl_http.get_header(resp, i, name, value);
dbms_output.put_line(name || ': ' || value);
END LOOP;
-- Keep reading lines until no more are left and an exception is raised.
LOOP
utl_http.read_line(resp, value);
dbms_output.put_line(value);
END LOOP;
EXCEPTION
WHEN utl_http.end_of_body THEN
utl_http.end_response(resp);
END;
END;

11-12 Oracle Database Application Developer’s Guide - Fundamentals

Performing Network Operations within PL/SQL Stored Procedures

/
SET serveroutput ON
-- The following URLs illustrate the use of this procedure,
-- but these pages do not actually exist. To test, substitute
-- URLs from your own Web server.
exec show_url('http://www.oracle.com/no-such-page.html')
exec show_url('http://www.oracle.com/protected-page.html')
exec show_url('http://www.oracle.com/protected-page.html', 'scott', 'tiger')

Working with Tables, Image Maps, Cookies, and CGI Variables from PL/SQL
Packages for all of these functions are supplied with Oracle8i and higher. You use
these packages in combination with the mod_plsql plug-in of Oracle HTTP Server
(OHS). You can format the results of a query in an HTML table, produce an image
map, set and get HTTP cookies, check the values of CGI variables, and combine other
typical Web operations with a PL/SQL program.
Documentation for these packages is not part of the database documentation library.
The location of the documentation depends on the particular application server you
are running. To get started with these packages, look at their procedure names and
parameters using the SQL*Plus DESCRIBE command:
DESCRIBE HTP;
DESCRIBE HTF;
DESCRIBE OWA_UTIL;

Developing Applications with the PL/SQL Web Toolkit 11-13

Performing Network Operations within PL/SQL Stored Procedures

11-14 Oracle Database Application Developer’s Guide - Fundamentals

12
Developing PL/SQL Server Pages
This section contains the following topics:
■

PL/SQL Server Pages: Overview

■

Writing a PL/SQL Server Page

■

Loading a PL/SQL Server Page into the Database

■

Executing a PL/SQL Server Page Through a URL

■

Examples of PL/SQL Server Pages

■

Debugging PL/SQL Server Page Problems

■

Putting PL/SQL Server Pages into Production

PL/SQL Server Pages: Overview
This section contains the following topics:
■

What Are PL/SQL Server Pages and Why Use Them?

■

Prerequisites for Developing and Deploying PL/SQL Server Pages

■

PSP and the HTP Package

■

PSP and Other Scripting Solutions

What Are PL/SQL Server Pages and Why Use Them?
PL/SQL Server Pages (PSP) are server-side scripts that include dynamic content,
including the results of SQL queries, inside Web pages. You can author the Web pages
in an HTML authoring tool and insert blocks of PL/SQL code.
Example 12–1 shows a simple PL/SQL server page called simple.psp.
Example 12–1

simple.psp

<%@ page language="PL/SQL" %>
<%@ page contentType="text/html" %>
<%@ plsql procedure="show_employees" %>
<%-- This example displays the last name and first name of every
employee in the hr.employees table. --%>
<%!
CURSOR emp_cursor IS
SELECT last_name, first_name
FROM hr.employees
ORDER BY last_name;
%>
Developing PL/SQL Server Pages 12-1

PL/SQL Server Pages: Overview







List of Employees

<% FOR emp_record IN emp_cursor LOOP %>

<% END LOOP; %>


Last Name
First Name

 <%= emp_record.last_name %> 
 <%= emp_record.first_name %> 




You can compile and load this script into an Oracle database with the loadpsp
command-line utility. The following example loads this server page into the hr
schema, replacing the show_employees procedure if it already exists:
loadpsp -replace -user hr/hr simple.psp

Browser users can execute the show_employees procedure through a URL. An
HTML page that displays the last and first names of employees in the hr.employees
table is returned to the browser through the PL/SQL gateway.
Deploying content through PL/SQL Server Pages has the following advantages:
■

■

■

■

■

For developers familiar with PL/SQL, the server pages are the easiest way to
create professional Web pages that included database-generated content. You can
develop Web pages as normal and then embed PL/SQL code in the HTML.
PSP can be more convenient than using the HTP and HTF packages to write out
HTML content line by line.
Because processing is performed on the database server, the client browser
receives a plain HTML page with no special script tags. You can support all
browsers and browser levels equally.
Network traffic is efficient because use of PSP minimizes the number of database
round-trips.
You can write content quickly and follow a rapid, iterative development process.
You maintain central control of the software, with only a Web browser required on
the client machine.

Prerequisites for Developing and Deploying PL/SQL Server Pages
To develop and deploy PL/SQL server pages, you must meet the following
prerequisites:
■

■

To write a PL/SQL server page you need access to a text editor or HTML
authoring tool for writing the script. No other development tool is required.
To load a PL/SQL server page you need:

12-2 Oracle Database Application Developer’s Guide - Fundamentals

PL/SQL Server Pages: Overview

■

–

An account on an Oracle database in which to load the server pages.

–

Execution rights to the loadpsp command-line utility, which is located in
$ORACLE_HOME/bin.

To deploy the server pages you must use mod_plsql. As explained in "PL/SQL
Web Toolkit" on page 11-2, the gateway makes use of the PL/SQL Web Toolkit.
See Also:
■

"Using the mod_plsql Gateway" on page 11-3

PSP and the HTP Package
You can enable browser users to execute PL/SQL program units through HTTP in the
following ways:
■

■

By writing an HTML page with embedded PL/SQL code and compiling it as a
PL/SQL server page. You may call procedures from the PL/SQL Web Toolkit, but
not to generate every line of HTML output.
By writing a complete stored procedure that produces HTML by calling the HTP
and OWA_* packages in the PL/SQL Web Toolkit. This technique is described in
"Generating HTML Output with PL/SQL" on page 11-4.

Thus, you must choose which technique to use when writing your Web application.
The key factors in choosing between these techniques are:
■

■

What source are you using as a starting point?
–

If you have a large body of HTML, and if you want to include dynamic
content or make it the front end of a database application, then use PSP.

–

If you have a large body of PL/SQL code that produces formatted output,
then you may find it more convenient to produce HTML tags by changing
your print statements to call the HTP package of the PL/SQL Web Toolkit.

What is the fastest and most convenient authoring environment for your group?
–

If most work is done using HTML authoring tools, then use PSP.

–

If you use authoring tools that produce PL/SQL code, then it might be less
convenient to use PSP.

PSP and Other Scripting Solutions
Scripting solutions can be client-side or server-side. JavaScript is one of the most
popular client-side scripting language. PSP fully supports JavaScript. Because any
kind of tags can be passed unchanged to the browser through a PL/SQL server page,
you can include JavaScript or other client-side script code in a PL/SQL server page.
Java Server Pages (JSP) and Active Server Pages (ASP) are two of the most popular
server-side scripting solutions. Note the following points of comparison with PSP:
■

■

Java server pages are loosely analogous to PSP pages; Java servlets are analogous
to PL/SQL packages. PSP uses the same script tag syntax as JSP to make it easy to
switch back and forth.
PSP uses syntax that is similar to ASP, although not identical. Typically, you must
translate from VBScript or JScript to PL/SQL. The best candidates for migration
are pages that use the Active Data Object (ADO) interface to perform database
operations.

Developing PL/SQL Server Pages 12-3

Writing a PL/SQL Server Page

You cannot mix PL/SQL server pages with other server-side
script features, such as server-side includes. In many cases, you can
get the same results by using the corresponding PSP features.

Note:

Writing a PL/SQL Server Page
To write a PL/SQL server page, you can start with an existing Web page or with an
existing stored procedure. Either way, with a few additions and changes you can create
dynamic Web pages that perform database operations and display the results.
The file for a PL/SQL server page must have the extension .psp. It can contain
whatever content you choose, with text and tags interspersed with PSP directives,
declarations, and scriptlets. A server page can take the following forms:
■

■

■

In the simplest case, it is an HTML file. Compiling it as a PL/SQL server page
produces a stored procedure that outputs exactly the same HTML file.
In the most complex case, it is a PL/SQL procedure that generates all the content
of the Web page, including the tags for title, body, and headings.
In the typical case, it is a mixture of HTML (providing the static parts of the page)
and PL/SQL (providing the dynamic content).

The order and placement of the PSP directives and declarations is usually not
significant. It becomes significant only when another file is included. For ease of
maintenance, it is recommended that you place the directives and declarations
together near the beginning of the file.
Table 12–1 lists the PSP elements and directs you to the section that discusses how to
use them. The section "Quoting and Escaping Strings in a PSP Script" on page 12-11
describes how to quote strings that are used in various PSP elements.
Table 12–1

PSP Elements

PSP Element

Name

Specifies . . .

Section

<%@ page ... %>

Page Directive

Characteristics of the PL/SQL server
page.

"Specifying Basic Server Page
Characteristics" on page 12-5

<%@ parameter ... %> Parameter Directive The name, and optionally the type and "Accepting User Input" on page 12-7
default, for each parameter expected
by the PSP stored procedure.
<%@ plsql ... %>

Procedure Directive The name of the stored procedure
produced by the PSP file.

"Naming the PL/SQL Stored
Procedure" on page 12-8

<%@ include ... %>

Include Directive

The name of a file to be included at a
specific point in the PSP file.

"Including the Contents of Other
Files" on page 12-8

<%! ... %>

Declaration Block

The declaration for a set of PL/SQL
variables that are visible throughout
the page, not just within the next
BEGIN/END block.

"Declaring Global Variables in a PSP
Script" on page 12-9

<% ... %>

Code Block

A set of PL/SQL statements to be
executed when the procedure is run.

"Specifying Executable Statements in
a PSP Script" on page 12-9

<%= ... %>

Expression Block

A single PL/SQL expression, such as a "Substituting an Expression Result in
string, arithmetic expression, function a PSP Script" on page 12-10
call, or combination of these.

<%-- ... --%>

Comment

A comment in a PSP script.

12-4 Oracle Database Application Developer’s Guide - Fundamentals

"Including Comments in a PSP
Script" on page 12-11

Writing a PL/SQL Server Page

If you are familiar with dynamic HTML and want to start
coding right away, you can jump forward to"Examples of PL/SQL
Server Pages" on page 12-15.

Note:

Specifying Basic Server Page Characteristics
Use the <%@ page ... %> directive to specify characteristics of the PL/SQL server
page such as the following:
■

What scripting language it uses.

■

What type of information (MIME type) it produces.

■

What code to run to handle all uncaught exceptions. This might be an HTML file
with a friendly message, renamed to a .psp file. You must specify this same file
name in the loadpsp command that compiles the main PSP file. You must specify
exactly the same name in both the errorPage directive and in the loadpsp
command, including any relative path name such as ../include/.

The following code shows the syntax of the page directive (note that the attribute
names contentType and errorPage are case-sensitive):
<%@ page [language="PL/SQL"] [contentType="content type string"] charset="encoding" [errorPage="file.psp"] %>

Specifying the Scripting Language
To identify a file as a PL/SQL server page, include the following directive somewhere
in the file:
<%@ page language="PL/SQL" %>

This directive is for compatibility with other scripting environments. Example 12–1
shows an example of a simple PL/SQL server page that includes the language
directive.

Returning Data to the Client
You have the following basic options when specifying the type of data to return to the
client browser:
■

Returning HTML

■

Returning XML, Text, or Other Document Types

■

Returning Pages Containing Different Character Sets

Returning HTML The PL/SQL parts of a PL/SQL server page are enclosed within special
delimiters. All other content is passed along verbatim—including any whitespace—to
the browser. To display text or HTML tags, write it as you would a typical Web page.
You do not need to call any output functions. As illustration, the server page in
Example 12–1 returns the HTML page shown in Example 12–2, except that it includes
the table rows for the queried employees.
Example 12–2

Sample Returned HTML Page








Developing PL/SQL Server Pages 12-5

Writing a PL/SQL Server Page

List of Employees


Last Name
First Name






Sometimes you might want to display one line of output or another, or change the
value of an attribute, based on a condition. You can include control structures and
variable substitution inside the PSP delimiters, as shown in the following code
fragment from Example 12–1:
<% FOR emp_record IN emp_cursor LOOP %>

 <%= emp_record.last_name %> 
 <%= emp_record.first_name %> 

<% END LOOP; %>

Returning XML, Text, or Other Document Types By default, the PL/SQL gateway transmits
files as HTML documents so that the browser interprets the HTML tags. If you want
the browser to interpret the document as XML, plain text (with no formatting), or
some other document type, then include the following directive:
<%@ page contentType="MIMEtype" %>

The attribute name is case-sensitive, so be sure to capitalize it as contentType. Insert
text/html, text/xml, text/plain, image/jpeg, or some other MIME type that
the browser or other client program recognizes. Users may have to configure their
browsers to recognize some MIME types. The following shows an example of a
directive for an Excel spreadsheet:
<%@ page contentType="application/vnd.ms-excel" %>

Typically, a PL/SQL server page is intended to be displayed in a Web browser. It can
also be retrieved and interpreted by a program that can make HTTP requests, such as a
a Java or Perl client.
Returning Pages Containing Different Character Sets By default, the PL/SQL gateway
transmits files with the character set defined by the PL/SQL gateway. To convert the
data to a different character set for browser display, include the following directive:
<%@ page charset="encoding" %>

Specify Shift_JIS, Big5, UTF-8, or another encoding that the client program
recognizes.
You must also configure the character set setting in the database accessor descriptor
(DAD) of the PL/SQL gateway. Users may have to select the same encoding in their
browsers to see the data displayed properly. For example, a database in Japan might
have a database character set that uses the EUC encoding, but the Web browsers are
configured to display Shift_JIS encoding.

12-6 Oracle Database Application Developer’s Guide - Fundamentals

Writing a PL/SQL Server Page

Handling Script Errors
When writing PL/SQL server pages, be mindful of the following types of errors:
■

■

■

HTML syntax errors. Any errors in HTML markup are handled by the browser.
The loadpsp utility does not check for them.
PL/SQL syntax errors. If you make a syntax error in the PL/SQL code, the
loadpsp utility stops and displays the line number, column number, and a brief
message. You must fix the error before continuing. Note that any previous version
of the stored procedure can be erased when you attempt to replace it with a script
that contains a syntax error. You might want to use one database for prototyping
and debugging, then load the final stored procedure into a different database for
production. You can switch databases using a command-line flag without
changing any source code.
Runtime errors. To handle database errors that occur when the script runs, you can
include PL/SQL exception-handling code within a PSP file and have any
unhandled exceptions bring up a special PL/SQL server page. Use the
errorPage attribute (note that the name is case-sensitive) of the <%@ page ...
%> directive to specify the page name.
The page for unhandled exceptions is a PL/SQL server page with extension .psp.
The error procedure does not receive any parameters, so to determine the cause of
the error, it can call the SQLCODE and SQLERRM functions. You can also display a
standard HTML page without any scripting when an error occurs, but you must
still give it the extension .psp and load it into the database as a stored procedure.

The following example shows a directive that specifies errors.psp as the page to
run when errors are encountered:
<%@ page language="PL/SQL" contentType="text/html" errorPage="errors.psp" %>

Accepting User Input
To set up parameter passing for a PL/SQL server page, include a directive with the
following syntax:
<%@ plsql parameter="parameter name" [type="PL/SQL type"] [default="value"] %>

Example 12–9 shows an example of a script that includes the parameter directive.
By default, parameters are of type VARCHAR2. To use a different type, include a
type="PL/SQL type" attribute within the directive, as in the following example:
<%@ plsql parameter="p_employee_id" type="NUMBER" %>

To set a default value, so that the parameter becomes optional, include a
default="expression" attribute in the directive. The values for this attribute are
substituted directly into a PL/SQL statement, so any strings must be single-quoted,
and you can use special values such as null, as in the following example:
<%@ plsql parameter="p_last_name" default="null" %>

User input comes encoded in the URL that retrieves the HTML page. You can generate
the URL by hard-coding it in an HTML link, or by calling your page as the action of an
HTML form. Your page receives the input as parameters to a PL/SQL stored
procedure. For example, assume that you change the first few lines of Example 12–1 to
include a parameter directive as follows, and then load it into the database:
<%@ page language="PL/SQL" %>
<%@ page contentType="text/html" %>

Developing PL/SQL Server Pages 12-7

Writing a PL/SQL Server Page

<%@ plsql parameter="p_employee_id" default="null" type="NUMBER" %>
<%@ plsql procedure="show_employees" %>
<%!
CURSOR emp_cursor IS
SELECT last_name, first_name
FROM hr.employees
WHERE employee_id = p_employee_id
ORDER BY last_name;
%>

If the PL/SQL gateway were configured so that you could execute procedures by
calling http://www.host.com/pls/proc_name, where proc_name is the name of
a procedure, then you could pass 200 for parameter p_employee_id as follows:
http://www.host.com/pls/show_employees?p_employee_id=200

Naming the PL/SQL Stored Procedure
Each top-level PL/SQL server page corresponds to a stored procedure within the
server. When you load the page with loadpsp, the utility creates a PL/SQL stored
procedure. By default, the procedure is given the same name as the PSP script, except
with the .psp extension removed. Thus, if your script is named hello_world.psp,
then by default the utility creates a procedure named hello_world.
To give the procedure a name that is different from the script name, include the
following directive, where procname is the name of a procedure:
<%@ plsql procedure="procname" %>

Example 12–1 includes the following directive, which gives the stored procedure the
name show_employees:
<%@ plsql procedure="show_employees" %>

Thus, you could name the file empnames.psp or anything else that ends with *.psp,
but the procedure is created as show_employees. Note that it is the name of the
procedure, not the name of the PSP script, that you include in the URL.

Including the Contents of Other Files
You can set up an include mechanism to pull in the contents of other files, typically
containing either static HTML content or more PL/SQL scripting code. Insert the
following directive at the point where the content of the other file should appear,
replacing filename with the name of the file to be included:
<%@ include file="filename" %>

The included file must have an extension other than .psp. You must specify exactly
the same name in both the include directive and in the loadpsp command,
including any relative path name such as ../include/.
Because the files are processed when you load the stored procedure into the database,
the substitution is performed only once, not whenever the page is served. Therefore,
changes to the included files that occur after the page is loaded into the database are
not displayed when the procedure is executed.
You can use the include feature to pull in libraries of code, such as a navigation
banners, footers, tables of contents, and so forth into multiple files. Alternatively, you
can use this feature as a macro capability to include the same section of script code in
more than one place in a page. The following example includes an HTML footer:

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<%@ include file="footer.htm" %>

Note the following characteristics of included files:
■

■

■

You can use any names and extensions for the included files. For example, you
could include a file called products.txt.
If the included files contain PL/SQL scripting code, then they do not need their
own set of directives to identify the procedure name, character set, and so on.
When specifying the names of files to the loadpsp utility, you must include the
names of all included files also. Specify the names of included files before the
names of any .psp files.

Declaring Global Variables in a PSP Script
You can use the <%! ... %> directive to define a set of PL/SQL variables that are
visible throughout the page, not just within the next BEGIN/END block. This element
typically spans multiple lines, with individual PL/SQL variable declarations ended by
semicolons. The syntax for this directive is as follows:
<%! PL/SQL declaration;
[ PL/SQL declaration; ] ... %>

The usual PL/SQL syntax is allowed within the block. The delimiters server as
shorthand, enabling you to omit the DECLARE keyword. All declarations are available
to the code later in the file. Example 12–1 includes the following cursor declaration:
<%!
CURSOR emp_cursor IS
SELECT last_name, first_name
FROM hr.employees
ORDER BY last_name;
%>

You can specify multiple declaration blocks; internally, they are all merged into a
single block when the PSP file is created as a stored procedure.
You can also use explicit DECLARE blocks within the <% ... %> delimiters that are
explained in "Specifying Executable Statements in a PSP Script" on page 12-9. These
declarations are only visible to the following BEGIN/END block.
To make things easier to maintain, keep all your directives and
declarations together near the beginning of a PL/SQL server page.

Note:

Specifying Executable Statements in a PSP Script
You can use the <% ... %> code block directive to execute a set of PL/SQL
statements when the stored procedure is run. The following code shows the syntax for
executable statements:
<% PL/SQL statement;
[ PL/SQL statement; ] ... %>

This element typically spans multiple lines, with individual PL/SQL statements ended
by semicolons. The statements can include complete blocks, as in the following
example, which calls the OWA_UTIL.TABLEPRINT procedure:
<% OWA_UTIL.TABLEPRINT(CTABLE => 'hr.employees', CATTRIBUTES => 'border=2',
CCOLUMNS => 'last_name,first_name', CCLAUSES => 'WHERE employee_id > 100'); %>

Developing PL/SQL Server Pages 12-9

Writing a PL/SQL Server Page

The statements can also be the bracketing parts of IF/THEN/ELSE or BEGIN/END
blocks. When a code block is split into multiple directives, you can put HTML or other
directives in the middle, and the middle pieces are conditionally executed when the
stored procedure is run. The following code from Example 12–10 provides an
illustration of this technique:
<% FOR ITEM IN (SELECT product_name, list_price, catalog_url
FROM product_information
WHERE list_price IS NOT NULL
ORDER BY list_price DESC) LOOP
IF item.list_price > p_minprice THEN
v_color := '#CCCCFF';
ELSE
v_color := '#CCCCCC';
END IF;
%>

<%= item.product_name %>
<%= item.list_price %>

<% END LOOP; %>

All the usual PL/SQL syntax is allowed within the block. The delimiters server as
shorthand, letting you omit the DECLARE keyword. All the declarations are available
to the code later on in the file.
To share procedures, constants, and types across different
PL/SQL server pages, compile them into a package in the database by
using a plain PL/SQL source file. Although you can reference
packaged procedures, constants, and types from PSP scripts, the PSP
scripts can only produce standalone procedures, not packages.

Note:

Substituting an Expression Result in a PSP Script
An expression directive outputs a single PL/SQL expression, such as a string,
arithmetic expression, function call, or combination of these things. The result is
substituted as a string at that spot in the HTML page that is produced by the stored
procedure. The expression result must be a string value or be able to be cast to a string.
For any types that cannot be implicitly cast, such as DATE, pass the value to the
PL/SQL TO_CHAR function.
The syntax of an expression directive is as follows, where the expression
placeholder is replaced by the desired expression:
<%= expression %>

Note that you do not need to end the PL/SQL expression with a semicolon.
Example 12–1 includes a directive to print the value of a variable in a row of a cursor:
<%= emp_record.last_name %>

Compare the preceding example to the equivalent htp.print call in the following
example (note especially the semicolon that ends the statement):
<% HTP.PRN (emp_record.last_name); %>

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Writing a PL/SQL Server Page

The content within the <%= ... %> delimiters is processed by the HTP.PRN function,
which trims leading or trailing whitespace and requires that you quote literal strings.
Note that you can use concatenation by using the twin pipe symbol (||) as you would
in PL/SQL. The following directive shows an example of concatenation:
<%= 'The employee last name is ' || emp_record.last_name %>

Quoting and Escaping Strings in a PSP Script
PSP attributes use double quotes to delimit data. When values specified in PSP
attributes are used for PL/SQL operations, they are passed exactly as you specify them
in the PSP file. Thus, if PL/SQL requires a single-quoted string, then you must specify
the string with the single quotes around it—and surround the whole thing with
double quotes.
For example, your PL/SQL procedure may use the string Babe Ruth as the default
value for a variable. Because the string will be used in PL/SQL, you must enclose it in
single quotes as 'Babe Ruth'. If you specify this single-quoted string in the
default attribute of a PSP directive, then you must surround it in double quotes as in
the following example:
<%@ plsql parameter="in_players" default="'Babe Ruth'" %>

You can also nest single-quoted strings inside single quotes. In this case, you must
escape the nested single quotes by specifying the sequence \'. For example:
<%@ plsql parameter="in_players" default="'Walter \'Big Train\' Johnson'" %>

You can include most characters and character sequences in a PSP file without having
them changed by the PSP loader. To include the sequence %>, specify the escape
sequence %\>. To include the sequence <%, specify the escape sequence <\%. For
example:
<%= 'The %\> sequence is used in scripting language: ' || lang_name %>
<%= 'The <\% sequence is used in scripting language: ' || lang_name %>

Including Comments in a PSP Script
To put a comment in the HTML portion of a PL/SQL server page for the benefit of
those reading the PSP source code, use the following syntax:
<%-- PSP comment text --%>

Comments in the preceding form do not appear in the HTML output from the PSP and
also do not appear when you query the PL/SQL source code in USER_OBJECTS.
To create a comment that is visible in the HTML output and in the USER_OBJECTS
source, place the comment in the HTML and use the normal HTML comment syntax:


To include a comment inside a PL/SQL block within a PSP, and to make the comment
invisible in the HTML output but visible in USER_OBJECTS, use the normal PL/SQL
comment syntax, as in the following example:
-- Comment in PL/SQL code

Example 12–3 shows a fragment of a PSP file with the three types of comments.

Developing PL/SQL Server Pages 12-11

Loading a PL/SQL Server Page into the Database

Example 12–3

Sample Comments in a PSP File

Today we introduce our new model XP-10.
<%-This is the project with code name "Secret Project".
Users viewing the HTML page will not see this PSP script comment.
The comment is not visible in the USER_OBJECTS source code.
--%>

<%
FOR image_file IN (SELECT pathname, width, height, description
FROM image_library WHERE model_num = 'XP-10')
-- Comments interspersed with PL/SQL statements.
-- Users viewing the HTML page source will not see these PL/SQL comments.
-- These comments are visible in the USER_OBJECTS source code.
LOOP
%>

height=<% image_file.height %> alt="<% image_file.description %>">


<% END LOOP; %>

Loading a PL/SQL Server Page into the Database
Use the loadpsp utility, which is located in $ORACLE_HOME/bin, to load one or more
PSP files into the database as stored procedures. Each .psp file corresponds to one
stored procedure. The pages are compiled and loaded in one step, to speed up the
development cycle. The syntax of the loadpsp utility as follows:
loadpsp [ -replace ] -user username/password[@connect_string]
[ include_file_name ... ] [ error_file_name ] psp_file_name ...

To create procedures with CREATE OR REPLACE syntax, use the -replace flag.
When you load a PSP file, the loader performs the following actions:
1.

Logs on to the database with the specified user name, password, and net service
name

2.

Creates the stored procedures in the user schema

Include the names of all the include files before the names of the PL/SQL server pages.
Also include the name of the file specified in the errorPage attribute of the page
directive. These filenames on the loadpsp command line must match exactly the
names specified within the PSP include and page directives, including any relative
path name such as ../include/. Example 12–4 shows a sample PSP load command.
Example 12–4

Loading a PSP

loadpsp -replace -user hr/hr@orcl banner.inc error.psp display_order.psp

Note the following characteristics of Example 12–4:
■

The stored procedure is created in the database orcl. The database is accessed as
user hr with password hr, both to create the stored procedure and when the
stored procedure is executed.

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■

■

■

banner.inc is a file containing boilerplate text and script code that is included
by the .psp file. The inclusion occurs when the PSP is loaded into the database,
not when the stored procedure is executed.
error.psp is a file containing code, text, or both that is processed when an
unhandled exception occurs, to present a friendly page rather than an internal
error message.
display_order.psp contains the main code and text for the Web page. By
default, the corresponding stored procedure is named display_order.

Querying PSP Source Code
After you have loaded a PSP file, you can view the source code in the USER_SOURCE
or DBA_SOURCE tables in the data dictionary. For example, suppose that you load the
script in Example 12–1 with the following command:
loadpsp -replace -user hr/hr simple.psp

If you log on to the database as user hr, then you can execute the following query in
SQL*Plus to view the source code of the PSP:
SET HEADING OFF
SELECT TEXT
FROM
USER_SOURCE
WHERE NAME = 'SHOW_EMPLOYEES'
ORDER BY LINE;

Sample output is shown in Example 12–5. Note that the code generated by loadpsp is
different from the code in the source file. The loadpsp utility has added extra code,
mainly calls to the HTP package, to the PSP code. The HTP package generates the
HTML tags for the web page.
Example 12–5

Output from Query of USER_SOURCE

PROCEDURE show_employees

AS

CURSOR emp_cursor IS
SELECT last_name, first_name
FROM hr.employees
ORDER BY last_name;
BEGIN NULL;
owa_util.mime_header('text/html'); htp.prn('
');
htp.prn('
');
htp.prn('
');
htp.prn('






List of Employees


Developing PL/SQL Server Pages 12-13

Executing a PL/SQL Server Page Through a URL


');
FOR emp_record IN emp_cursor LOOP
htp.prn('

');
END LOOP;
htp.prn('

Last Name
First Name

 ');
htp.prn( emp_record.last_name );
htp.prn(' 
 ');
htp.prn( emp_record.first_name );
htp.prn(' 



');
END;

Executing a PL/SQL Server Page Through a URL
After the PL/SQL server page has been turned into a stored procedure, you can run
the procedure by retrieving an HTTP URL through a Web browser or other
Internet-aware client program. The virtual path in the URL depends on the way the
PL/SQL gateway is configured.
The parameters to the stored procedure are passed through either the POST method or
the GET method of the HTTP protocol. With the POST method, the parameters are
passed directly from an HTML form and are not visible in the URL. With the GET
method, the parameters are passed as name-value pairs in the query string of the URL,
separated by & characters, with most non-alphanumeric characters in encoded format
(such as %20 for a space). You can use the GET method to call a PSP page from an
HTML form, or you can use a hard-coded HTML link to call the stored procedure with
a given set of parameters.
Using METHOD=GET, the syntax of the URL looks something like the following:
http://sitename/schemaname/procname?parmname1=value1&parmname2=value2

For example, the following URL includes a p_lname and p_fname parameter:
http://www.host.com/pls/show_employees?p_lname=Ashdown&p_fname=Lance

Using METHOD=POST, the syntax of the URL does not show the parameters:
http://sitename/schemaname/procname

For example, the following URL specifies a procedure name but does not pass
parameters:
http://www.host.com/pls/show_employees

The METHOD=GET format is more convenient for debugging and allows visitors to pass
exactly the same parameters when they return to the page through a bookmark.

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Examples of PL/SQL Server Pages

The METHOD=POST format allows a larger volume of parameter data, and is suitable
for passing sensitive information that should not be displayed in the URL. (URLs
linger on in the browser's history list and in the HTTP headers that are passed to the
next-visited page.) It is not practical to bookmark pages that are called this way.

Examples of PL/SQL Server Pages
This section shows how you might start with a very simple PL/SQL server page, and
produce progressively more complicated versions as you gain more confidence.
As you go through each step, you can follow the instructions in "Loading a PL/SQL
Server Page into the Database" on page 12-12 and "Executing a PL/SQL Server Page
Through a URL" on page 12-14 to test the examples.
This section contains the following topics:
■

Setup for PL/SQL Server Pages Examples

■

Printing the Sample Table with a Loop

■

Allowing a User Selection

■

Using an HTML Form to Call a PL/SQL Server Page

Setup for PL/SQL Server Pages Examples
These examples use the product_information table in the oe schema, which is
described as follows:
Table PRODUCT_INFORMATION
Name
Null?
----------------------------------------- -------PRODUCT_ID
NOT NULL
PRODUCT_NAME
PRODUCT_DESCRIPTION
CATEGORY_ID
WEIGHT_CLASS
WARRANTY_PERIOD
SUPPLIER_ID
PRODUCT_STATUS
LIST_PRICE
MIN_PRICE
CATALOG_URL

Type
---------------------------NUMBER(6)
VARCHAR2(50)
VARCHAR2(2000)
NUMBER(2)
NUMBER(1)
INTERVAL YEAR(2) TO MONTH
NUMBER(6)
VARCHAR2(20)
NUMBER(8,2)
NUMBER(8,2)
VARCHAR2(50)

The examples assume the following:
■

■
■

Youn have set up mod_plsql as described in "Using the mod_plsql Gateway" on
page 11-3.
You have created a DAD for static authentication of the oe user.
You can access PL/SQL stored procedures created in the oe schema through the
following URL, where proc_name is the name of a stored procedure:
http://www.host.com/pls/proc_name

For debugging purposes, you can display the complete contents of an SQL table. You
can do this with a single call to OWA_UTIL.TABLEPRINT as illustrated in
Example 12–6. In subsequent iterations, we use other techniques to gain more control
over the presentation.

Developing PL/SQL Server Pages 12-15

Examples of PL/SQL Server Pages

Example 12–6

show_prod_simple.psp

<%@ plsql procedure="show_prod_simple" %>



<%
DECLARE
dummy BOOLEAN;
BEGIN
dummy := OWA_UTIL.TABLEPRINT('oe.product_information','border');
END;
%>



Load the PSP in Example 12–6 at the command line as follows:
loadpsp -replace -user oe/oe show_prod_simple.psp

Access the PSP through the following URL:
http://www.host.com/pls/show_prod_simple

Printing the Sample Table with a Loop
Example 12–6 loops through the items in the product_information table and
adjusts the SELECT statement to retrieve only a subset of the rows or columns. In this
example, we pick a very simple presentation, a set of list items, to avoid any problems
from mismatched or unclosed table tags.
Example 12–7

show_catalog_raw.psp

<%@ plsql procedure="show_prod_raw" %>




<% FOR item IN (SELECT product_name, list_price, catalog_url
FROM product_information
WHERE list_price IS NOT NULL
ORDER BY list_price DESC) LOOP %>

Item = <%= item.product_name %>

Price = <%= item.list_price %>

URL = <%= item.catalog_url %>

<% END LOOP; %>




Example 12–8 shows a more sophisticated variation of Example 12–7 in which
formatting is added to the HTML to improve the presentation.
Example 12–8

show_catalog_pretty.psp

<%@ plsql procedure="show_prod_pretty" %>





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Examples of PL/SQL Server Pages

<% FOR item IN (SELECT product_name, list_price, catalog_url
FROM product_information
WHERE list_price IS NOT NULL
ORDER BY list_price DESC) LOOP %>

Item = ><%= item.product_name %>

Price = <%= item.list_price %>

<% END LOOP; %>




Allowing a User Selection
In the previous examples, the HTML page remains the same unless the
product_information table is updated. Example 12–9 livens up the page by:
■

■

Making it accept a minimum price, and present only the items that are more
expensive. (Your customers' buying criteria may vary.)
Setting the default minimum price to 100 units of the appropriate currency. Later,
we see how to allow the user to pick a minimum price.

Example 12–9

show_product_partial.psp

<%@ plsql procedure="show_product_partial" %>
<%@ plsql parameter="p_minprice" default="100" %>



This report shows the items whose price is greater than <%= p_minprice %>.

<% FOR ITEM IN (SELECT product_name, list_price, catalog_url
FROM product_information
WHERE list_price > p_minprice
ORDER BY list_price DESC)
LOOP %>

Item = <%= item.product_name %>

Price = <%= item.list_price %>

<% END LOOP; %>




After loading Example 12–9 into the database, you can pass a parameter to the
show_product_partial procedure through a URL. The following example specifies
a minimum price of 250:
http://www.host.com/pls/show_product_partial?p_minprice=250

This technique of filtering results is fine for some applications, such as search results,
in which users might worry about being overwhelmed by choices. But in a retail
situation, you might want to use the alternative technique illustrated in Example 12–10
so that customers can still choose to purchase other items. Note the following features
of this example:
■

Instead of filtering the results through a WHERE clause, we can retrieve the entire
result set and then take different actions for different returned rows.

Developing PL/SQL Server Pages 12-17

Examples of PL/SQL Server Pages

■

■

We can change the HTML to highlight the output that meets their criteria. In this
case, we use the background color for an HTML table row. We could also insert a
special icon, increase the font size, or use some other technique to call attention to
the most important rows.
We can present the results in an HTML table.

Example 12–10 show_product_highlighed.psp
<%@ plsql procedure="show_product_highlighted" %>
<%@ plsql parameter="p_minprice" default="100" %>
<%! v_color VARCHAR2(7); %>



This report shows all items, highlighting those whose price is
greater than <%= p_minprice %>.


<% FOR ITEM IN (SELECT product_name, list_price, catalog_url
FROM product_information
WHERE list_price IS NOT NULL
ORDER BY list_price DESC) LOOP
IF item.list_price > p_minprice THEN
v_color := '#CCCCFF';
ELSE
v_color := '#CCCCCC';
END IF;
%>

<% END LOOP; %>


Product
Price

<%= item.product_name %>
<%= item.list_price %>




Using an HTML Form to Call a PL/SQL Server Page
Example 12–11 shows a bare-bones HTML form that allows the user to enter a price.
The form calls the show_product_partial stored procedure illustrated in
Example 12–9 and passes it the entered value as the p_minprice parameter.
To avoid coding the entire URL of the stored procedure in the ACTION= attribute of the
form, we can make the form a PSP file so that it resides in the same directory as the
PSP file that it calls. Even though this HTML file contains no PL/SQL code, we can
give it a .psp extension and load it as a stored procedure into the database. When the
product_form stored procedure is executed through a URL, it displays the HTML
exactly as it appears in the file.
Example 12–11 product_form.psp



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Examples of PL/SQL Server Pages


Enter the minimum price you want to pay:






Including JavaScript in a PSP File
To produce an elaborate HTML file, perhaps including dynamic content such as
JavaScript, you can simplify the source code by implementing it as a PSP. This
technique avoids having to deal with nested quotation marks, escape characters,
concatenated literals and variables, and indentation of the embedded content.
Example 12–12 shows a version of Example 12–9 that uses JavaScript to display the
order status in the browser status bar when the user moves his or her mouse over the
product URL.
Example 12–12 show_product_javascript.psp
<%@ plsql procedure="show_product_javascript" %>
<%@ plsql parameter="p_minprice" default="100" %>






This report shows the items whose price is greater than <%= p_minprice %>.


<% FOR ITEM IN (SELECT product_name, list_price, catalog_url, product_status
FROM product_information
WHERE list_price > p_minprice
ORDER BY list_price DESC)
LOOP %>

Item =

<%= item.product_name %>

Developing PL/SQL Server Pages 12-19

Debugging PL/SQL Server Page Problems




Price = <%= item.list_price %>

<% END LOOP; %>




Debugging PL/SQL Server Page Problems
As you begin experimenting with PSP, and as you adapt your first simple pages into
more elaborate ones, keep these guidelines in mind when you encounter problems:
■

■

■

The first step is to get all the PL/SQL syntax and PSP directive syntax right. If you
make a mistake here, the file does not compile.
–

Make sure you use semicolons to terminate lines where required.

–

If a value must be quoted, quote it. You might need to enclose a single-quoted
value (needed by PL/SQL) inside double quotes (needed by PSP).

–

Mistakes in the PSP directives are usually reported through PL/SQL syntax
messages. Check that your directives use the right syntax, that directives are
closed properly, and that you are using the right element (declaration,
expression, or code block) depending on what goes inside it.

–

PSP attribute names are case-sensitive. Most are specified in all lowercase;
contentType and errorPage must be specified as mixed-case.

When using a URL to request a PSP, you may get an error that the file is not found.
In this case, note the following:
–

Make sure you are requesting the right virtual path, depending on the way the
Web gateway is configured. Typically, the path includes the host name,
optionally a port number, the schema name, and the name of the stored
procedure (with no .psp extension).

–

If you use the -replace option when compiling the file, the old version of
the stored procedure is erased. So, after a failed compilation, you must fix the
error or the page is not available. You might want to test new scripts in a
separate schema, then load them into the production schema.

–

If you copied the file from another file, remember to change any procedure
name directives in the source to match the new file name.

–

When you get one file-not-found error, make sure to request the latest version
of the page the next time. The error page might be cached by the browser. You
may need to force a page reload in the browser to bypass the cache.

When the PSP script is run, and the results come back to the browser, use standard
debugging techniques to check for and correct wrong output. The difficult part is
to configure the interface between different HTML forms, scripts, and CGI
programs so that the right values are passed into your page. The page might
return an error because of a parameter mismatch. Note the following tips:
–

To determine exactly what is being passed to your page, use METHOD=GET in
the calling form so that the parameters are visible in the URL.

–

Make sure that the form or CGI program that calls your page passes the
correct number of parameters, and that the names specified by the NAME=
attributes on the form match the parameter names in the PSP file. If the form

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Putting PL/SQL Server Pages into Production

includes any hidden input fields, or uses the NAME= attribute on the Submit
or Reset buttons, then the PSP file must declare equivalent parameters.
–

Make sure that the parameters can be cast from string into the correct PL/SQL
types. For example, do not include alphabetic characters if the parameter in
the PSP file is declared as a NUMBER.

–

Make sure that the query string of the URL consists of name-value pairs,
separated by equals signs, especially if you are passing parameters by
constructing a hard-coded link to the page.

–

If you are passing a lot of parameter data, such as large strings, you might
exceed the volume that can be passed with METHOD=GET. You can switch to
METHOD=POST in the calling form without changing your PSP file.

–

Although the loadpsp command reports line numbers correctly when there is
a syntax error in your source file, line numbers reported for runtime errors
refer to a transformed version of the source and do not match the line numbers
in the original source. When you encounter errors that produce an error trace
instead of the expected Web page, you will need to locate the error through
exception handlers and by printing debug output.

Putting PL/SQL Server Pages into Production
Before putting your PSP application into production, consider issues such as usability
and download speed:
■

■

■

■

■

Pages can be rendered faster in the browser if the HEIGHT= and WIDTH= attributes
are specified for all images. You might standardize on picture sizes, or store the
height and width of images in the database along with the data or URL.
For viewers who turn off graphics, or who use alternative browsers that read the
text out loud, include a description of significant images using the ALT= attribute.
You might store the description in the database along with the image.
Although an HTML table provides a good way to display data, a large table can
make your application seem slow. Often, the reader sees a blank page until the
entire table is downloaded. If the amount of data in an HTML table is large,
consider splitting the output into multiple tables.
If you set text, font, or background colors, test your application with different
combinations of browser color settings:
–

Test what happens if you override just the foreground color in the browser, or
just the background color, or both.

–

Generally, if you set one color (such as the foreground text color), you should
set all the colors through the  tag, to avoid hard-to-read combinations
like white text on a white background.

–

If you use a background image, specify a similar background color to provide
proper contrast for viewers who do not load graphics.

–

If the information conveyed by different colors is crucial, consider using an
alternative technique. For example, you might put an icon next to special
items in a table. Some users may see your page on a monochrome screen or on
browsers that cannot represent different colors.

Providing context information prevents users from getting lost. Include a
descriptive