TriggersIn Oracle

A PL/SQL block stored in the database and fired in response to a specified event◦ DML statements : insert , update, delete◦ DDL statements : create, alter, drop◦ Startup or shutdown◦ An error◦ A user logon or logoff

What is a trigger?

Create or replace trigger <trigger_name>{before|after|Instead of}{Insert | Update | Delete}[of <col_name>]On <table_name>[referencing old as o new as n][for each row]When (<condition>)BeginSql statementsEnd;

Syntax for creating a trigger

Triggering SQL statement Trigger action Trigger restriction

Components of a trigger

DML triggers◦ Defined on a table and fired in response to an

insert, delete or update DDL triggers

◦ Defined in response to events such as logon or logoff, create alter drop etc.

Instead of triggers◦ Used for non updateable views

Types of triggers

DML trigger definition consists of:◦ Event that fires the trigger◦ Database table on which the event occurs◦ Optional condition controlling when trigger

executes Types:

◦ Row level Trigger fired or each row updated , inserted or

deleted◦ Statement level

Trigger fired for each sql statement executed

DML Triggers

create or replace trigger before_update_empbefore update on empbegininsert into empcheck values('Before update','Statement', sysdate);dbms_output.put_line('Some updation done on Emp');end;/Update emp set sal=sal+100;Some updation done on emp14 rows updatedSelect * from empcheck;EVENT LEVEL_CHECK DATE_CHEC-------------------- -------------------------------- ------------------------------Before update Statement 13-SEP-13

Statement level before update

create or replace trigger before_update_row_emp_salbefore update of sal on empFor each rowbegininsert into empcheck values('Before update',’Row', sysdate);dbms_output.put_line('Some updation done on Emp Sal column');end;/SQL> update emp set sal=sal+10 where deptno=10;Some updation done on EmpSome updation done on Emp Sal columnSome updation done on Emp Sal columnSome updation done on Emp Sal column

3 rows updated.

SQL> select * from empcheck;

EVENT LEVEL_CHECK DATE_CHEC-------------------- ------------------------------ ---------Before update Statement 13-SEP-13Before update Row 13-SEP-13Before update Row 13-SEP-13Before update Row 13-SEP-13

Row level before update

create or replace trigger display_salary_changesbefore delete or update or insert on empfor each rowdeclaresal_diff number;beginsal_diff:=:new.sal-:old.sal;dbms_output.put_line('Old salary: '||:old.sal);dbms_output.put_line('New salary: '||:new.sal);dbms_output.put_line('Salary Difference: '||sal_diff);end;/

Using :old and :new

create trigger trig_alter_scottbefore alter on scott.schemabeginraise_application_error(-20000,'Cannot alter

any table in scott');end;/alter table emp drop primary key;ERROR at line 1:ORA-00604: error occurred at recursive SQL level 1ORA-20000: Cannot alter any table in scott

DDL trigger

Help to make non updateable views updateable

Called so because unlike other types of triggers, oracle fires the trigger instead of executing the triggering statement

Exist only on views

Instead of Triggers

Create a viewcreate or replace view v as select empno,ename ,sal from emp

where sal>2000;Create a trigger(Instead of) create or replace trigger emp_triginstead of insert on vbeginif :new.sal >2000 theninsert into emp (empno,ename,sal)

values(:new.empno,:new.ename,:new.sal);elseraise_application_error(-20001,'Insertion from this view

invalid');end if;end;/

Instead of Triggers

Creation of a non updateable viewCreate view vtest as select deptno,sum(sal)

as total from emp group by deptno; Updation through the view not possibleupdate vtest set deptno=20 where

deptno=10;ORA-01732: data manipulation operation not

legal on this view

Instead of trigger example

Creation of Instead of triggercreate or replace trigger emp_trig2 instead of update on vtest begin if updating then update emp set deptno=:new.deptno; end if; end; /Updation using triggerupdate vtest set deptno=20 where deptno=10;1 row updated.

Instead of trigger

Mutating table exceptions occur triggering table is referenced from within the row level trigger code

This error is raised when a trigger attempts to execute a DML statement on the same table that the trigger is defined on

Triggers and Mutating table

create or replace trigger trig_up_empafter update on empfor each rowdeclarei number;beginselect sum(sal) into i from emp;dbms_output.put_line('The sum of salary of employees is '||i);end;/ERROR at line 1:ORA-04091: table SCOTT.EMP is mutating, trigger/function may not

see itORA-06512: at "SCOTT.TRIG_UP_EMP", line 4ORA-04088: error during execution of trigger 'SCOTT.TRIG_UP_EMP'

Triggers and Mutating table

create or replace trigger trig_up_emp after update on emp for each row declare i number; pragma autonomous_transaction; begin select sum(sal) into i from emp; dbms_output.put_line('The sum of salary of employees is

'||i); commit; end; /

Solving Mutating table error

View ALL_TRIGGERS, USER_TRIGGERS

Alter trigger tigger_name enable;Alter trigger tigger_name disable;

Drop trigger trigger_name;

Alter table table_name disable all triggers;Alter table table_name enable all triggers;

Listing, Enabling,Disabling,Dropping

Refers to SQL statements that are constructed and executed at run time

Execute DDL statements Manage Ad hoc query and update requirements of

web based applications

create or replace procedure dynsql_eg(ddl_string varchar2)is beginexecute immediate ddl_string;

end;

Dynamic SQL