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Trigger

V. Saranya AP/CSE,

Sri Vidya College of Engg & Tech, virudhunagar

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Example: 1

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Example : 2

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Example : 3

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Trigger

A procedure that starts automatically if

specified changes occur to the DBMS

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Triggers (Active database)

Three parts: Event (activates the trigger)

Condition (tests whether the triggers should run)[Optional]

Action(what happens if the trigger runs)

Semantics:

When event occurs, and condition is satisfied, theaction is performed.

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TriggersEvent,Condition,Action

Events could be :BEFORE|AFTER INSERT|UPDATE|DELETE ON

e.g.: BEFORE INSERT ON Professor

Condition is SQL expression or even an SQLquery (query with non-empty result meansTRUE)

Action can be many different choices :

SQL statements , even DDL and transaction-oriented statements likecommit.

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Example Trigger

Assume our DB has a relation schema :

Professor (pNum, pName, salary)

We want to write a trigger that :

Ensures that any new professorinserted has salary >= 60000

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Example Trigger

CREATE TRIGGER minSalary BEFOREINSERTONProfessor

for what context ?

BEGIN

check for violation here ?

END;

Event

Condition

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Example Trigger

CREATE TRIGGER minSalary BEFOREINSERT ON Professor

FOR EACH ROW

BEGIN

Violation of Minimum ProfessorSalary?

END;

trigger is performedfor each row inserted

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Example TriggerCREATE TRIGGER minSalary BEFORE INSERT ON

Professor

FOR EACH ROW

BEGIN

IF (:new.salary < 60000)

THEN RAISE_APPLICATION_ERROR (-20004,

Violation of MinimumProfessor Salary);

END IF;

END;

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Example trigger

CREATE TRIGGER minSalary BEFORE INSERT ON ProfessorFOR EACH ROW

DECLARE temp int; -- dummy variable notneeded

BEGIN

IF (:new.salary < 60000)

THEN RAISE_APPLICATION_ERROR (-20004,Violation of Minimum Professor

Salary);END IF;

temp := 10; -- to illustrate declaredvariables

END;

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Details of Trigger Example

BEFORE INSERT ON Professor

This trigger is checked before the tuple is inserted

FOR EACH ROW

specifies that trigger is performed for each rowinserted

:new

refers to the new tuple inserted

If (:new.salary < 60000) then an application error is raised and hence the

row is not inserted; otherwise the row is inserted.

Use error code: -20004;

this is in the valid range

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Example Trigger Using

ConditionCREATE TRIGGER minSalary BEFORE INSERT ON Professor

FOR EACH ROW

WHEN (new.salary < 60000)

BEGINRAISE_APPLICATION_ERROR (-20004,

Violationof Minimum Professor Salary);

END;

Conditions can refer to old/new values of tuplesmodified by the statement activating the trigger.

Condition

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Triggers: REFERENCING

CREATE TRIGGER minSalary BEFORE INSERT ON Professor

REFERENCING NEW as newTuple

FOR EACH ROW

WHEN (newTuple.salary < 60000)

BEGIN

RAISE_APPLICATION_ERROR (-20004,

Violation of Minimum Professor Salary);

END;

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Example Trigger

CREATE TRIGGER minSalary

BEFORE UPDATEON Professor

REFERENCING OLD AS oldTuple NEW as newTuple

FOR EACH ROW

WHEN (newTuple.salary < oldTuple.salary)

BEGIN

RAISE_APPLICATION_ERROR (-20004, Salary

Decreasing !!);END;

Ensure that salary does not decrease

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Another Trigger Example (SQL:99)

CREATE TRIGGER youngSailorUpdate

AFTER INSERT ON SAILORS

REFERENCING NEW TABLEASNewSailorsFOR EACH STATEMENT

INSERT

INTO YoungSailors(sid, name, age, rating)

SELECT sid, name, age, ratingFROM NewSailors N

WHERE N.age

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Types of Triggers

This section describes the different types of triggers:

Row level triggers

Trigger once foreach row in a transaction.

Statement level triggers

Triggers execute once foreach transaction.

Before and after triggers Triggers can be executed immediately before and

after INSERT, UPDATE and DELETE.

Defined by triggeringtransaction & level at which thetrigger is executed

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Row vs Statement Level Trigger

Example: Consider a relation schema

Account (num, amount)

where we will allow creation of new accounts

only during normal business hours.

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Example: Statement level trigger

CREATE TRIGGER MYTRIG1

BEFORE INSERTON AccountFOR EACH STATEMENT --- is default

BEGIN

IF (TO_CHAR(SYSDATE,dy) IN (sat,sun))

OR

(TO_CHAR(SYSDATE,hh24:mi) NOT BETWEEN 08:00

AND 17:00)

THEN

RAISE_APPLICATION_ERROR(-20500,Cannotcreate new account now !!);

END IF;

END;

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When to use BEFORE/AFTER

Based on efficiency considerations orsemantics.

Suppose we perform statement-level afterinsert, then all the rows are inserted first,then if the condition fails,and all the inserted rows must berolledback

Not very efficient !!

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Combining multiple events into one

triggerCREATE TRIGGER salaryRestrictions

AFTER INSERT OR UPDATEON Professor

FOR EACH ROW

BEGINIF (INSERTING AND :new.salary < 60000) THEN

RAISE_APPLICATION_ERROR (-20004, 'below minsalary'); END IF;

IF (UPDATING AND :new.salary < :old.salary)

THEN RAISE_APPLICATION_ERROR (-20004,Salary Decreasing !!'); END IF;

END;

1

2

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Summary : Trigger Syntax

CREATE TRIGGER

BEFORE|AFTER INSERT|DELETE|UPDATE

[OF ] ON |

[REFERENCING [OLD AS ] [NEW AS]]

[FOR EACH ROW] (default is FOR EACH STATEMENT)

[WHEN ()]

;

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Some Points about Triggers

Check the system tables :

user_triggers

user_trigger_cols user_errors

ORA-04091: mutating relation problem

In a row level trigger, you cannothave thebody refer to the table specified in the event

Also INSTEAD OFtriggers can be specified on

views

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To Show Compilation Errors

SELECT line, position, text

FROM user_errors

WHERE name = 'MY_TRIGGER'AND TYPE = 'TRIGGER

In SQL*Plus, you can also use the following

shortcut:

SQL> SHOW ERRORS TRIGGER MY_TRIGGER

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Mutating Trigger

Trigger that triggers itself

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Constraints versus Triggers

Constraints are useful for database consistency More opportunity for optimization

Not restricted into insert/delete/update

Triggers are flexible and powerful Alerts

Event logging for auditing

Security enforcement

Analysis of table accesses (statistics)

Workflow and business intelligence

But can be hard to understand Several triggers (Arbitrary order unpredictable !?)

Chain triggers (When to stop ?)

Recursive triggers (Termination?)

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Purpose of Triggers

To generate data automatically

To enforce complex integrity constraints

To customize complex security authorization

To maintain replicate tables

To audit data modifications.