Trigger

A trigger is a statement that is executed upon modification to the database. A trigger is triggered automatically when an associated DML statement is executed.

A trigger is a stored procedure in database which automatically invokes whenever a special event in the database occurs. 

A trigger description contains three parts:

Event: A change to database that activates the trigger

Condition: A query or test that is run when the trigger is executed

Action: A procedure that is executed when the trigger is activated and its condition is true.

Users may not be aware that a trigger was executed as a side effect of the procedure.

Syntax:

create trigger [trigger_name]

[before | after] 

{insert | update | delete} 

on [table_name] 

[for each row] 

[trigger_body]

Explanation of syntax:

1. create trigger [trigger_name]: Creates or replaces an existing trigger with the trigger_name.
2. [before | after]: This specifies when the trigger will be executed.
3. {insert | update | delete}: This specifies the DML operation.
4. on [table_name]: This specifies the name of the table associated with the trigger.
5. [for each row]: This specifies a row-level trigger, i.e., the trigger will be executed for each row being affected.
6. [trigger_body]: This provides the operation to be performed as trigger is fired

BEFORE and AFTER of Trigger:

BEFORE triggers run the trigger action before the triggering statement is run.
AFTER triggers run the trigger action after the triggering statement is run.