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:
- create trigger [trigger_name]:
Creates or replaces an existing trigger with the trigger_name.
- [before | after]: This specifies
when the trigger will be executed.
- {insert | update | delete}: This
specifies the DML operation.
- on [table_name]: This specifies the
name of the table associated with the trigger.
- [for each row]: This specifies a
row-level trigger, i.e., the trigger will be executed for each row being
affected.
- [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.
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