Download Triggers Writeup

Aim – Implement Triggers
Theory
A trigger is a statement that is executed automatically by the system as a side
effect of a modification to the database. Triggers are fired implicitly and not
called by user like procedure and function. To design a trigger mechanism, we
must specify two things:
 Specify the conditions under which the trigger is to be executed
 Specify the actions to be taken when the trigger executes
Use of Database Triggers
 To access table during regular business hours or on predetermined weekdays
 To keep track of modification of data along with the user name, the
operation performed and the time when the operation was performed
 To prevent invalid transaction
 Enforces complex security authorization
Database Triggers Vs Procedures
 Triggers do not accept parameters whereas procedures can have parameters
 Triggers are executed (fired) automatically upon modification of the table or
it’s data whereas to execute a procedure it has to be explicitly called by the
user
How To apply Database Triggers
A trigger has three parts
 A triggering event or statement – An SQL statement that causes a trigger to
be fired. It can be insert, update or delete statement for a specific table
 A trigger restriction – It specifies a Boolean expression that must be TRUE
for the trigger to fire. It conditionally controls the execution of trigger.
Specified using WHEN clause
 Trigger Action – PL/SQL block to be executed when triggering statement is
encountered and trigger restriction evaluates to TRUE
Types of Triggers
 Row Triggers – A row trigger is fired each time a row in the table is affected
by triggering statement. If the triggering statement affects no rows, the
trigger is not executed at all
 Statement Triggers – A statement trigger is fired once on behalf of the
triggering statement, independent of number of rows affected by the
triggering statement
Trigger Timing (Before Vs After Triggers)
When defining a trigger it is necessary to specify the trigger timing i. e. when
trigger action is to be executed in relation to the triggering Statement. Before
and After apply to both row and statement trigger
 Before Triggers – Trigger action is executed before triggering statement
 After Triggers – Trigger action is executed after triggering statement
Creating a Trigger
Syntax :
CREATE OR REPLACE TRIGGER [ schema. ]
< trigger_name >
{ BEFORE, AFTER }
{ DELETE, INSERT, UPDATE [ OF column1, . . . ]
ON [schema.] < table_name >
[ REFERENCING { OLD AS old, NEW AS new} ]
[ FOR EACH ROW [ WHEN condition ] ]
DECLARE
<variable declarations>;
<constant declarations>;
BEGIN
< PL/SQL sub-program body >;
Exception
< exception PL/SQL block >;
End;
Example :
CREATE OR REPLACE TRIGGER t_Audit_trail
BEFORE DELETE OR UPDATE ON
FOR EACH ROW
DECLARE
oper varchar2(8);
BEGIN
If updating then
oper :=‘Update’
end if;
If deleting then
oper :=‘Delete’
end if;
insert into audit_cust values (:OLD.custno, :OLD.fname,
:OLD.lname, :OLD.address, oper, user, sysdate);
End;
Conclusion – Thus we have implemented trigger to protect data of database.