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ECA Model : Triggers.
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  • A trigger is a statement that is executed automatically by the system as a side effect of a modification to the database.

  • Triggers are procedures that are stored in the database and are implicitly run, or fired, when something happens.

  • To design a trigger mechanism we must:

  • Specify the conditions under which the trigger is to be executed.

  • Specify the actions to be taken when the trigger executes.

Generalized Syntax:

Create Trigger < trigger name>

After/Before <insert OR delete OR update>

OF <columnname> ON <tablename>

FOR EACH ROW

WHEN <condition>

<trigger actions>

  • Here triggering event can be insert, delete or update.

  • Triggers on update can be restricted to a specific attributes.

  • Values of attributes before and after an update can be referenced.

Consider two relations as:

emp(id, Name, sal, dno)

Dept(dno, dname, totalsal, managerid)

Consider trigger as:

Create Trigger $T_2$

After Insert on emp

For each row

When (New, dno IS NOT NULL)

Update Dept

Set total sal + = New.sal

Where dno = NEW.dno

  • This trigger wil be activated when a new employee entry is done in emp table.

  • The salary of new employee gets added to the total salary of department and that value is updated in department (dept) table.

  • Deletion of trigger can be done using:

DROP TRIGGER < trigger name>

  • Triggers are useful in many cases when it is convenient to specify action to be taken when certain event occurs and certain conditions are satisfied.
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