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Introduction to SQL Trigger
A SQL trigger is a set of SQL statements stored in the database catalog. A SQL trigger
is executed or fired whenever an event associated with a table occurs e.g., insert,
update or delete.
A SQL trigger is a special type of stored procedure. It is special because it is not called
directly like a stored procedure. The main difference between a trigger and a stored
procedure is that a trigger is called automatically when a data modification event is made
against a table whereas a stored procedure must be called explicitly.
In MySQL, a trigger is a set of SQL statements that is invoked automatically when a
change is made to the data on the associated table. You can to define maximum six
triggers for each table.
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BEFORE INSERT – activated before data is inserted into the table.
AFTER INSERT – activated after data is inserted into the table.
BEFORE UPDATE – activated before data in the table is updated.
AFTER UPDATE – activated after data in the table is updated.
BEFORE DELETE – activated before data is removed from the table.
AFTER DELETE – activated after data is removed from the table.

When you use a statement that does not use INSERT , DELETE or UPDATE statement to
change data in a table, the triggers associated with the table are not invoked. For
example, the TRUNCATE statement removes all data of a table but does not invoke the
trigger associated with that table.
There are some statements that use the INSERT statement behind the scenes such
as REPLACE statement or LOAD DATA statement. If you use these statements, the
corresponding triggers associated with the table are invoked.
You must use a unique name for each trigger associated with a table. However, you can
have the same trigger name defined for different tables though it is a good practice.
You should name the triggers using the following naming convention:
(BEFORE/ AFTER)_TABLENAME_(INSERT/UPDATE/DELETE)
For example, before_order_update is a trigger invoked before a row in the order table
is updated.
The following naming convention is as good as the one above.
TABLENAME_(BEFORE/ AFTER)_ (INSERT/UPDATE/DELETE)
For example, order_before_update is the same as before_update_update trigger
above.

MySQL trigger syntax
In order to create a new trigger, you use the CREATE TRIGGER statement. The following
illustrates the syntax of the CREATE TRIGGER statement:

Let’s examine the syntax above in more detail.
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You put the trigger name after the CREATE TRIGGER statement. The trigger name
should follow the naming convention [trigger time]_[table name]_[trigger
event], for example before_employees_update.
Trigger activation time can be BEFORE or AFTER. You must specify the activation
time when you define a trigger. You use the BEFORE keyword if you want to
process action prior to the change is made on the table and AFTER if you need to
process action after the change is made.
The trigger event can be INSERT, UPDATE or DELETE. This event causes the trigger
to be invoked. A trigger only can be invoked by one event. To define a trigger that
is invoked by multiple events, you have to define multiple triggers, one for each
event.
A trigger must be associated with a specific table. Without a table trigger would not
exist therefore you have to specify the table name after the ON keyword.
You place the SQL statements between BEGIN and END block. This is where you
define the logic for the trigger.

MySQL trigger example
Let’s start creating a trigger in MySQL to log the changes of the employees table.

First, create a new table named employees_audit to keep the changes of
the employee table. The following statement creates the employee_audit table.

Next, create a BEFORE UPDATE trigger that is invoked before a change is made to
the employees table

Inside the body of the trigger, we used the OLD keyword to
access employeeNumber and lastnamecolumn of the row affected by the trigger.
Notice that in a trigger defined for INSERT, you can use NEW keyword only. You cannot
use the OLDkeyword. However, in the trigger defined for DELETE, there is no new row
so you can use the OLDkeyword only. In the UPDATE trigger, OLD refers to the row
before it is updated and NEW refers to the row after it is updated.
Then, to view all triggers in the current database, you use SHOW TRIGGERS statement as
follows:

After that, update the employees table to check whether the trigger is invoked.

As you see, the trigger was really invoked and it inserted a new row into
the employees_audit table.
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Author                          : Sakshi Sharma
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Create Date                     : 2017:03:23 09:03:51
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