CBSE Class 12 Informatics Practices Triggers Notes

,sans-serif;">syntax :

ALTER TRIGGER trigger_name DISABLE;

For example:

ALTER TRIGGER orders_before_insert DISABLE;

Drop a Trigger

syntax :

DROP TRIGGER trigger_name;

For example:

DROP TRIGGER orders_before_insert;

Example:

create or replace trigger check_budget_EMP

after insert or update of SAL, DEPTNO on EMP

declare

cursor DEPT_CUR is select DEPTNO, BUDGET from DEPT;

DNO DEPT.DEPTNO%TYPE;

ALLSAL DEPT.BUDGET%TYPE;

DEPT_SAL number;

begin

open DEPT_CUR;

loop

fetch DEPT_CUR into DNO, ALLSAL;

exit when DEPT_CUR%NOTFOUND;

select sum(SAL) into DEPT_SAL from EMP where DEPTNO = DNO;

if DEPT_SAL > ALLSAL then

raise_application_error(-20325, 'Total of salaries in the department '|| to_char(DNO) || ' exceeds

budget');

end if;

end loop;

close DEPT_CUR;

end; /

More about triggers :

Triggers are not exclusively used for integrity maintenance. They can also be used for

• Monitoring purposes, such as the monitoring of user accesses and modifications on certain sensitive

tables.

• Logging actions, e.g., on tables: Contd..

create trigger LOG EMP

after insert or update or delete on EMP

begin

if inserting then

insert into EMP LOG values(user, ’INSERT’, sysdate); end if ;

if updating then

insert into EMP LOG values(user, ’UPDATE’, sysdate); end if ;

if deleting then

insert into EMP LOG values(user, ’DELETE’, sysdate); end if ;

end;

By using a row trigger, even the attribute values of the modified tuples can be stored in the table EMP LOG.

• automatic propagation of modifications. For example, if a manager is transferred to another department, a trigger can be defined that automatically transfers the manager’s employees to the new department.

More about Triggers

If a trigger is specified within the SQL*Plus shell, the definition must end with a point “.” in the last line. Issuing the command run causes SQL*Plus to compile this trigger definition.

A trigger definition can be loaded from a file using the command @. Note that the last line in the file must consist of a slash “/”.

A trigger definition cannot be changed, it can only be re-created using the or replace clause.

The command drop deletes a trigger.

After a trigger definition has been successfully compiled, the trigger automatically is enabled.

The command alter trigger disable is used to deactivate a trigger. All

triggers defined on a table can be (de)activated using the command

alter table enable | disable all trigger;

The data dictionary stores information about triggers in the table USER TRIGGERS. The information includes the trigger name, type, table, and the code for the PL/SQL block.

Difference b/w For and Do Loops: When No. of repetitions are known then For loop is used, and if the No. of iterations are unknown then do loops are used.

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