This thread contains some useful tips/sample codes regarding TRIGGERS in oracle, that the forum members may find useful.
TRIGGERS:
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Database trigger is a PL/SQL block that is executed on an event in the database.
The event is related to a particular data manipulation of a table such as inserting, deleting or updating a row of a table.
Triggers may be used :
1.To implement complex business rule, which cannot be implemented using integrity constraints.
2.To audit the process. For example, to keep track of changes made to a table.
3. To automatically perform an action when another concerned action takes place. For example, updating a table whenever there is an insertion of a row into another table.
Types of Triggers:
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Depending upon when a trigger is fired it may be classified as :
a.Statement-level trigger
b.Row-level trigger
c.Before triggers
d.After triggers
e.Instead of triggers
Syntax:
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CREATE [OR REPLACE] TRIGGER triggername
{BEFORE | AFTER | INSTEAD OF}
{DELETE | INSERT | UPDATE [OF columns]}
[OR {DELETE | INSERT |UPDATE [OF columns]}]...
ON table_name|SCHEMA
[REFERENCING [OLD AS old] [NEW AS new]]
[FOR EACH ROW [WHEN condition]]
<PL/SQL block>
Trigger Sample Ex #1
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CREATE OR REPLACE TRIGGER MYTRIG AFTER INSERT OR DELETE ON EMP
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DECLARE
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MCOUNT NUMBER;
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BEGIN
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SELECT COUNT(*) INTO MCOUNT FROM EMP;
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--checks the number of records in the table.
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DBMS_OUTPUT.PUT_LINE('THE TOTAL NUMBER OF EMPLOYEE ARE:' || MCOUNT);
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--and displays the same after the event .
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END;
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Trigger Sample Ex #2
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- CREATE OR REPLACE TRIGGER ONLYPOSITIVE BEFORE INSERT OR UPDATE OF SAL ON EMP FOR EACH ROW
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BEGIN
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IF :NEW.SAL<0 THEN
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--checks of the value inserted into teh sal column is less than 0.
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RAISE_APPLICATION_ERROR(-20100,'PLEASE INSERT A POSITIVE VALUE');
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--if valus is less than 0 ,then raise the exception.
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END IF;
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END;
Note:-Trigger does not allow negative values to insert/update to the table.
trigger example #3
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- CREATE OR REPLACE TRIGGER NODELETE BEFORE INSERT OR UPDATE OR DELETE ON DEPT FOR EACH ROW
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BEGIN
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IF INSERTING THEN
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RAISE_APPLICATION_ERROR(-20020,'CAN''T INSERT NEW RECORDS TO THIS TABLE');
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ELSIF UPDATING THEN
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RAISE_APPLICATION_ERROR(-20021,'CAN''T UPDATE RECORDS OF THIS TABLE');
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ELSIF DELETING THEN
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RAISE_APPLICATION_ERROR(-20022,'YOU ARE NOT AUTHORISED TO DELETE THESE RECORDS');
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END IF;
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END;
Note:- the triggermakes the trigger readonly by not allowing any DML on it.
Trigger Sample Ex #4
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create or replace trigger mytrig
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before insert on emp
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declare
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a char(3);
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begin
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--selects the system day to the vaiable.
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select to_char(sysdate,'dy') into a from dual;
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if a in('sun','sat') then
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--restrict any transaction on saturday and sunday.
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Raise_application_error(-20004,'Cannot do manipulation today');
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end if;
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end;
Note:--This trigger restricts transactions on the table on weekends.
Also check
TRIGGERS - 2 
