ECA Model : Triggers.
1 Answer
  • 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>


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.
Please log in to add an answer.