Week5 Triggers

8/8/2019 Week5 Triggers

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Week 5Week 5

Triggers


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CP2503/5503 Entreprise Database System - Oracle J

CU-SM

PIT2

In this lecture, you will learn:

Introduction toTriggers Partsofa Trigger

TypesofTriggers

TriggerExecution ExampleofTriggerUses

Viewing Trigger Information in the Data

Dictionary


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Readings

RequiredReadings Oracle Database Concepts:

Chapter22 Triggers

Oracle Database Application Developer's Guide -

Fundamentals Chapter9 CodingTriggers

FurtherReadings

Oracle Database PL/SQL User's Guide and

Reference Oracle Database SQL Reference


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Triggers

Trigger: a PL/SQL unit implicitlyfired by Oraclewhen atriggeringevent occurs.

Youcan write triggers that firewheneveroneof the

followingoperationsoccurs,

DML statements (INSERT, UPDATE, DELETE) on a particular

tableorview,issued by anyuser

DDL statements (CREATE orALTERprimarily) issuedeitherby

a particularschema/userorby anyschema/userin thedatabase

Databaseevents,such aslogon/logoff,errors,or

startup/shutdown, alsoissued


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Firing Triggers

Figure 22-1 Triggersin Oracledocumentation


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How Triggers Are Used

Automaticallygeneratederivedcolumn values Prevent invalid transactions

Enforcecomplex security authorizations

Enforcereferentialintegrity across nodesin adistributeddatabase

Enforcecomplex businessrules

Providetransparent event logging


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How Triggers Are Used

Provideauditing Maintain synchronous tablereplicates

Gatherstatisticson table access

Modify tabledata when DML statements areissued against views

Publish information about databaseevents,user

events, and SQL statements tosubscribing

applications


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A Simple Trigger

Thefollowingstatement creates a triggerforthe Emp_tab table:

CREATE OR REPLACE TRIGGERPrint_salary_changes

BEFORE DELETE OR INSERT OR UPDATE ON Emp_tabFOR EACH ROW

WHEN (new.Empno > 0)DECLAREsal_diff number;

BEGINsal_diff := :new.sal - :old.sal;

dbms_output.put('Old salary: ' || :old.sal);dbms_output.put(' New salary: ' || :new.sal);dbms_output.put_line(' Difference ' || sal_diff);

END;/


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Parts of a Trigger

A trigger has three basicparts: A triggeringevent orstatement

A triggerrestriction

A trigger action

Figure 22-3 The

REORDERTriggerinOracledocumentation


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Triggering Event or Statement

A triggeringevent orstatement is the SQL statement,databaseevent,oruserevent that causes a trigger to

fire. A triggeringevent can beoneormoreof the

following:

An INSERT, UPDATE,orDELETE statement on a specific table

(orview,in somecases)

A CREATE, ALTER,orDROP statement on anyschema object

A databasestartuporinstanceshutdown

A specificerrormessageoranyerrormessage

A userlogon orlogoff

Forexample:... UPDATE OF parts_on_hand ON inventory ...


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Trigger Restriction

A triggerrestriction specifies a Booleanexpression that must be trueforthe trigger to

fire.

The triggeraction will not run if the trigger

restriction evaluates tofalseorunknown.

In theexample, the triggerrestriction is:

new.parts_on_hand < new.reorder_point

The triggerdoes not fireunless the numberofavailablepartsis

less than a preset orderamount


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CP

2503/5503 Entreprise Database System - Oracle JCU

-SMPIT 1

2

Trigger Action

A trigger action is theprocedure (PL/SQL block,Java program,orC callout) that contains theSQL statements andcode to berun when thefollowingeventsoccur: A triggeringstatement isissued

The triggerrestriction evaluates totrue

Likestoredprocedures, a trigger action can: Contain SQL, PL/SQL,orJava statements

Define PL/SQL languageconstructssuch asvariables,constants,cursors,exceptions

Define Java languageconstructs

Callstoredprocedures


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CP 2503/5503 Entreprise Database System - Oracle JCU-SMPIT 13

Designing Triggers

Limit thesizeofthe trigger action to no morethan 60 lines (~35Kbytes)

ifmorecodingis needed,placeinside a stored

procedure, then create a trigger that calls the

procedure

Becarefulofrecursive triggers

a recursivetriggerthat fires after theupdateofEMP

table,which the triggeritselfalsoissue an update tothe EMP table.

it willloopuntilout ofmemory


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Designing Triggers

Becarefulofcascading triggers Excessiveuseoftriggerscan result in complex

interdependencies

Thiscan bedifficult to maintain in a large application

a recursivetriggerthat fires after theupdateofEMPcascadingtrigger causingchain fireoftriggers

Cascading triggerscan causeunintendedresults

Difficult tounderstand

Couldresult in a recursive triggercall


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Cascading Triggers

Figure 22-2

Cascading

Triggersin

Oracle

documentation


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Triggers Compared with DeclarativeIntegrity Constraints

Youcan use both triggers andintegrityconstraints todefine andenforce any typeofintegrityrule

However,onlyuse triggers toconstrain data input in the

followingsituations: Toenforcereferentialintegritywhen thechild andparent tables

areon different nodesofa distributeddatabase

Toenforcecomplex businessrules not definableusingintegrity

constraints

When a requiredreferentialintegrityrulecannot beenforcedusing thefollowingintegrityconstraints:

NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK,

DELETE CASCADE, DELETE SET NULL


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Types ofTriggers

RowTriggers and Statement Triggers BEFORE and AFTERTriggers

INSTEAD OF Triggers

Triggerson System Events and UserEvents


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Row Triggers and Statement Triggers

When youdefine a trigger,youcan specify thenumberoftimes the trigger action is to berun:

Onceforeveryrow affected by the triggering

statement,such as a triggerfired by an UPDATEstatement that updates manyrows

Oncefor the triggeringstatement, no matterhow

manyrowsit affects


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RowTriggers (BEFORE /AFTER)

triggerfiresonceforeach row affected by thetriggeringstatement

e.g., an updatestatement updates manyrowsin atablewillfire the trigger many times

R

ow triggers areusefulif thecodein the triggeraction dependson data provided by thetriggeringstatement orrows that are affected bythe triggeringstatement


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Statement Triggers (BEFORE /AFTER) triggerfiresonceforeach triggeringstatement,

regardlessof the numberofrows affected,even ifnorows are affected

e.g., a deletestatement deleteseveralrowsfrom a

table, a statement level DELETE triggerwillfireonlyonce

Statement triggers areusefulif thecodein thetriggeraction does not dependon data provided

by the triggeringstatement orrows affected: Make a complex securitycheck on thecurrent timeor

user

Generate a single audit record


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BEFORE and AFTER Triggers

When defining a trigger,youcan specify thetriggertiming

BEFOREtriggersrun the triggeraction before

the triggeringstatement isrun:

When the trigger action determineswhether the

triggershould be allowed tocomplete

Toderivespecificcolumn values beforecompleting a

triggering INSERTorUPDATE statement

AFTER triggersrun the triggeraction afterthe

triggeringstatement isrun


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INSTEAD ofTriggers

When you need toexecute DML on a non-updateableview,you need tocreate an INSTEAD of trigger

The triggerwill beexecuted,insteadofthe triggeringstatement

Modifyingviewscan have ambiguousresults

Deleting a rowin a viewcouldeithermean deletingit from the

base tableor

updatingsomevaluesso that it is nolongerselected by theview

Inserting a rowin a viewcouldeithermean inserting a newrow

into the base table

orupdating an existingrowso that it isprojected by theview

Updating a column in a view that involves joins might thesemanticsof

othercolumns that are not projected by theview


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Triggers on System Events and User Events

System events Databasestartup andshutdown

Data Guardrole transitions

Servererrormessageevents

Userevents

Userlogon andlogoff

DML statements (INSERT, DELETE, & UPDATE)

DDL statements (CREATE, ALTER, & DROP)


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Triggers on System Events and User Events

Triggerson system eventscan bedefined at thedatabaselevelorschema level

CREATE OR REPLACE TRIGGER On_Logon

AFTER LOGONON The_user.Schema

BEGINDo_Something;

END;


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Enabling and Disabling Triggers

You might temporarilydisable a triggerif: an object the triggerreferencesis not available

you need toperform a largedata load, andyouwant it toproceed

quicklywithout firing triggers

you arereloadingdata

ALTER TRIGGER trigger_name ENABLE;

ALTER TRIGGER trigger_name DISABLE;

ALTER TABLE table_name ENABLE ALL

TRIGGERS;

ALTER TABLE table_name DISABLE ALL

TRIGGERS;


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The Execution Model for Triggers andIntegrity Constraint Checking

A single SQL statement can potentiallyfireup tofourtypesoftriggers:

BEFORE statement trigger

BEFORE row trigger

AFTERstatement trigger

AFTERrow trigger


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The Execution Model for Triggers andIntegrity Constraint Checking

BEFOR

E statement trigger Beforeexecuting the triggeringstatement, the triggeraction is

run

BEFORE row trigger Before modifyingeach row affected by the triggeringstatement

and beforechecking appropriateintegrityconstraints, the triggeraction isrun,ifthe triggerrestriction is not violated

AFTERstatement trigger Afterexecuting the triggeringstatement and applying any

deferredintegrityconstraints, the triggeraction isrun

AFTERrow trigger Aftermodifyingeach row affected by the triggeringstatement and

possibly applying appropriateintegrityconstraints, the triggeraction isrun forthecurrent rowprovided the triggerrestrictionwas not violated. AFTERrow triggerslock rows


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Execution Model for Triggers

1.Run allBEFORE statement triggers that apply to thestatement.

2.Loopforeach row affected by the SQL statement.

a.Run allBEFORE row triggers that apply to thestatement.

b.Lock andchangerow, andperform integrityconstraint

checking. (Thelock is not releaseduntil the transaction iscommitted.)

c.Run allAFTERrow triggers that apply to thestatement.

3.Completedeferredintegrityconstraint checking.

4.Run allAFTERstatement triggers that apply to thestatement.

What willhappenifthere aremultiplebeforestatementtriggerstriggered by

thesame DML statement?


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Triggers Information

Thefollowing Data Dictionaryviews haveinformation about triggers: USER_TRIGGERS

ALL_TRIGGERS

DBA_TRIGGERS

SELECT Trigger_type, Triggering_event, Table_nameFROM USER_TRIGGERS

WHERE Trigger_name = 'REORDER';

TYPE TRIGGERING_STATEMENT TABLE_NAME------------ -------------------- --------

AFTER EACH ROW UPDATE INVERTORY

SELECT Trigger_body

FROM USER_TRIGGERSWHERE Trigger_name = 'REORDER';


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Trigger Example (Source codes provided inreadings)

Auditing

Integrity

Complex Check

Complex Security

Transparent Logging

Tracking System Events


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Revision Questions

What is a trigger? What are thedifferences between row triggers

andstatement triggers?

What is an insteadoftrigger?

Which Data Dictionaryviews haveinformationabout a trigger?

How triggers areused?

Howis the triggerexecution modelin Oracle

work? How tocreate,enable anddisable triggers?

When we maydisable triggers?


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