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Triggers

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IF-2305 Pemrograman Basis Data

Tahun Ajaran 2011-2012

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Triggers

Triggers is a procedure that is automatically invoked by the DBMSin response to specified changes to the database

A trigger description contains three part:

Event: A change to the database that activates the trigger

The sorts of events allowed are usually insert, delete or

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up ate to a part cu ar re at on Condition: A query or test that is run when the trigger is

activated

If the condition does not hold, then nothing else associated

with the trigger happens in response to this event. Action: A procedure that is executed when the trigger is

activated and its condition is true

The action could be any sequence of database operations,

perhaps even operations not connected in any way to thetriggering event

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Triggers vs Constraints

Triggers are only awakened when certain events, specified by thedatabase programmer, occur.

Instead of immediately preventing the event that awakened it, atrigger tests a condition.

If the condition of the trigger is satisfied, theactionassociated with

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.

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Principal Features

The action may be executed either before or after the triggeringevent.

The action can refer to both old and/or new values of tuples thatwere inserted, deleted, or updated in the event that triggered theaction.

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attributes.

A condition may be specified by a WHEN clause; the action isexecuted only if the rule is triggered and the condition holds whenthe triggering event occurs.

The programmer has an option of specifying that the action isperformed either:

Once for each modified tuple, or

Once for all the tuples that are changed in one databaseoperation.

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Example

CREATE TABLE T4 (a INTEGER, b CHAR(10));

CREATE TABLE T5 (c CHAR(10), d INTEGER);

CREATE TRIGGER trig1

AFTER INSERT ON T4

} declaration

} event

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REFERENCING NEW AS newRowFOR EACH ROW

WHEN (newRow.a

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Trigger Layout

BEFORE: the WHEN condition is tested before the triggeringevent, that is, before the modification that awakened the trigger

has been made to the database

If the condition is true, then the action of the trigger isexecuted.

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,whether the condition is true.

AFTER: the action is executed after the triggering event

The OF keyword: optional for UPDATE

defines the event to be only an update of the attribute(s) listedafter the keyword OF

The WHEN clause is optional.

If it is missing, then the action is executed whenever the trigger

is awakened.

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Trigger Layout

Action surrounded by BEGIN.. .END and separated by semicolons

FOR EACH ROW: row-level trigger

executed once for each tuple

FOR EACH STATEMENT: statement-level trigger

executed once whenever a statement of the appropriate type

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is executed no matter how many rows - zero, one, or many - itactually affects

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Trigger Syntax in Oracle

CREATE [OR REPLACE] TRIGGER

{BEFORE|AFTER} {INSERT|DELETE|UPDATE} ON

[REFERENCING [NEW AS ] [OLD AS ]

[FOR EACH ROW [WHEN ()]]

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Basic triggers syntax in Oracle is differs slightly from standard

SQL syntax

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Trigger Syntax in Oracle Basic triggers syntax in Oracle is differs slightly from standard

SQL syntax

Only for row-level triggers: The special variables NEW and OLD are available to refer to

new and old tuples respectively. Note: In the trigger body,NEW and OLD must be preceded by a colon (":"), but in the

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WHEN clause, they do not have a preceding colon! Seeexample below.

The REFERENCING clause can be used to assign aliases tothe variables NEW and OLD.

A trigger restriction can be specified in the WHEN clause,

enclosed by parentheses. The trigger restriction is a SQLcondition that must be satisfied in order for Oracle to fire thetrigger. This condition cannot contain subqueries. Without theWHEN clause, the trigger is fired for each row.

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Trigger Syntax in Oracle The restrictions on include:

You cannot modify the same relation whose modification is theevent triggering the trigger.

You cannot modify a relation connected to the triggeringrelation by another constraint such as a foreign-key constraint.

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ExampleSET SERVEROUTPUT ON

CREATE TRIGGER CheckStar

BEFORE INSERT ON StarsIn

REFERENCING NEW AS newRow

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FOR EACH ROWBEGIN

INSERT INTO MovieStar(name) VALUES (:newRow.StarName);

DBMS_OUTPUT.PUT_LINE('New Star '||:newRow.StarName||'!!');

END CheckStar;

What is the event, condition and action of this trigger?

What does this trigger do?

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ExampleSET SERVEROUTPUT ON

CREATE TRIGGER StudioProduct

AFTER INSERT ON Movie

DECLARE

m_length NUMBER := 0;

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BEGINSELECT SUM(length) INTO m_length FROM Movie;

DBMS_OUTPUT.PUT_LINE('We have '|| m_length ||' minutes ofmovies!!');

END StudioProduct;

What is the event, condition and action of this trigger?

What does this trigger do?

What differentiate this trigger from the previous?

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Aborting Triggers with ErrorCREATE TABLE Person (age INT);

CREATE TRIGGER PersonCheckAgeAFTER INSERT OR UPDATE OF age ON PersonFOR EACH ROW

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IF (:new.age < 0) THENRAISE_APPLICATION_ERROR(-20000, 'no negative age

allowed');END IF;

END;.RUN;

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Aborting Triggers with Error If we attempted to execute the insertion:

INSERT INTO Person VALUES (-3);

we would get the error message:

ERROR at line 1:

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ORA-20000: no negative age allowedORA-06512: at "MYNAME.PERSONCHECKAGE", line 3ORA-04088: error during execution of trigger'MYNAME.PERSONCHECKAGE'

and nothing would be inserted. In general, the effects of both thetrigger and the triggering statement are rolled back.

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Why Triggers Can Be Hard to Understand

The DBMS always checks whether some trigger is activated bythe statement(s) that modifies the database

If a statement activates more than one trigger, the DBMS typicallyprocesses all of them, in some arbitrary order

The execution of the action part of a trigger could in turn activate

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In particular, execution of the action part of a trigger couldagain activate the same trigger (recursive triggers)

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Recursive TriggersCREATE TRIGGER t1

AFTER INSERT ON table1

FOR EACH ROW

BEGIN

update_table_2();

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CREATE TRIGGER t2

AFTER UPDATE ON table2

FOR EACH ROW

BEGIN

insert_into_table_1();

END

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Constraints versus Triggers A constraint prevents the data from being made inconsistent by

any kind of statement

A trigger is activated by a special kind of statement (e.g., an insertor delete statement)

Triggers allow us to maintain database integrity in more flexible

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Other Uses of Triggers Triggers can alert users to unusual events (as reflected in updates

to the database)

Triggers can generate a log of events to support auditing andsecurity checks

Triggers can be used to gather statistics on table accesses and

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