trigger dbms

SHREE GANESH

SEMINAR ON:-

DATABASE TRIGGERS

SUBMMITED TO:- SUBMMITED BYKuldeep Kumar

PRIYANKA Ma’m Mca 3rd sem

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Contents-

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INTRODUCTION

A database trigger is

procedural code that isautomatically executed inresponse to certain eventson a particular table or view

in a database.

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The events that fire a trigger

Event 1

Event 2

Event 3

Event 4

• DML statements

• DDL statements

• System events

• User events

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The need and the usage

Audit changes

Enforce business rules

Execute business rules.

Enhance performance

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Major Features

triggers do not accept parametersor arguments .

triggers cannot perform commit orrollback operations .

triggers are normally slow

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Type of trigger

RowTriggers

andStatement

Trigger

BEFOREand

AFTERTriggers

SystemEvents

and UserEvents

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Row Triggers and Statement Trigger

• A row trigger is firedRow

Triggerseach time the tableis affected by the

triggering statement.

• A statement trigger isStatement fired once on behalf

Triggers of the triggeringstatement.

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BEFORE and AFTER Triggers

BEFORE triggers run the triggeraction before the triggering

Before

AFTER

statement is run.situations:

To eliminate unnecessaryprocessing

To derive specific column values.

• AFTER triggers run the triggeraction after the triggering statement

is run.

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Triggers on System Events and Userevents

SystemEvents

Databasestartup andshutdown

Data Guardrole

transitions

Server errormessage

events

UserEvents

User logonand logoff

DDLstatements

DMLstatements

Exp. DBMS_AQ Package database level15

Parts of a Trigger

Atriggering A trigger A triggerevent or restriction action

statement

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THE TRIGGERING EVENT OR STATMENT

A CREATE, ALTER, or DROP statementon any schema object

An INSERT, UPDATE, or DELETEstatement on a specific table

A database startup or instanceshutdown

A specific error message or any errormessage

A user logon or logoff

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TriggerRestriction

Return true and false

Exmp :-

new.parts_on_hand <new.reorder_point

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Trigger Action

Depends of true and falseof trigger restriction.

A trigger action is the procedure (PL/SQL block, Javaprogram, or C callout) that contains the SQL

statements and code to be run when the followingevents occur.

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Trigger Mode

• An enabled trigger runs its trigger actionif a triggering statement is issued and the

Enabledtrigger restriction (if any) evaluates to

true

• A disabled trigger does not run its triggeraction, even if a triggering statement isissued and the trigger restriction (if any)

Disabled would evaluate to false.

ALTER TRIGGER <Trigger Name> DISABLE|ENABLE21

Creating a Database Trigger

Name of the trigger

Table to be associated with

When trigger is to be fired - before orafter

Command that invokes the trigger -UPDATE, DELETE, or INSERT

Whether row-level trigger or not

Condition to filter rows.

PL/SQL block that is to be executedwhen trigger is fired.

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General Structure

CREATE [OR REPLACE]

TRIGGER trigger_name

BEFORE (or AFTER)

INSERT OR UPDATE [OF COLUMNS] OR DELETEON tablename

[FOR EACH ROW [WHEN (condition)]]

BEGIN

END;23

Trigger SyntaxCREATE TRIGGER <triggerName>

BEFORE|AFTER INSERT|DELETE|UPDATE[OF <columnList>] ON <tableName>|<viewName>

[REFERENCING [OLD AS <oldName>] [NEW AS<newName>]]

[FOR EACH ROW] (default is “FOR EACHSTATEMENT”)

[WHEN (<condition>)]<PSM body>;

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Example 1:

CREATE OR REPLACE TRIGGERPERSON_INSERT_BEFORE

BEFORE INSERT

ON EMP

BEGINDBMS_OUTPUT.PUT_LINE(’BEFORE INSERT OF

’ || :NEW.NAME);END;

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Example 2:

CREATE OR REPLACE TRIGGER Print_salary_changesBEFORE DELETE OR INSERT OR UPDATE ON emp

WHEN (EMPNO > 0)

DECLAREsal_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(' Difference ' ||sal_diff);

END; /27

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