Triggers Database Management System Design Saba Aamir Computing and Software McMaster University

Triggers

Database Management System Design

Saba Aamir

Computing and Software

McMaster University

Triggers - Database Managment System Design

2

Introduction

A trigger is a statement that the system executes automatically as a side effect of a modification to the database.

Triggers define a set of actions that are executed by a delete, insert or update operation on a specified table.


3

Agenda

Roles of Triggers in database processing Structure of a trigger Semantic issues in trigger handling Triggers in SQL Syntax Of SQL:1999 Triggers Syntax Of Triggers in DB2


4

Roles Of Triggers

Constraint maintenance Business rules Monitoring Maintenance of auxiliary cached data Simplified application design Generating an attribute value


5

Structure Of A Trigger

---------------------------------------------------

ON event IF precondition THEN action

--------------------------------------------------- Triggers are called event-condition-action

or ECA rules.


6

Semantic Issues

Trigger consideration Immediate Deferred

Trigger execution Before trigger After trigger Instead-of trigger


7

Semantic Issues

Trigger granularity Row-level granularity Statement-level granularity

Trigger conflicts Ordered conflict resolution Group conflict resolution

Triggers and integrity constraints


8

Triggers in SQL

Triggering events. An event can be the execution of an SQL INSERT, DELETE and UPDATE statement as a whole or a change to individual rows made by such statements.

Trigger precondition. Any condition allowed in the where clause of SQL.


9

Triggers in SQL

Triggering action. An SQL query, a Delete, INSERET, UPDATE, ROLLBACK or SIGNAL statement or a program in SQL/PSM.

Trigger conflict resolution. Ordered—SQL:1999 assumes that all triggers are ordered and executed in some implementation-specific way.


10

Triggers in SQL

Trigger consideration. Immediate—the preconditions of all triggers activated by an event are checked immediately when the event is requested.

Trigger execution. Immediate—execution can be either before or after the event.

Trigger granularity. Row-level and statement level are both available.


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Syntax Of SQL:1999 Triggers

CREATE TRIGGER trigger-name {BEFORE | AFTER} {INSERT | DELETE | UPDATE [OF column-name-list] } ON table-name [REFERENCING [OLD AS var-to-refer-to-old-tuple] [NEW AS var-to-refer-to-new-tuple] ] [OLD TABLE AS name-to-refer-to-old-table] ]

[NEW TABLE AS name-to-refer-to-new-table] ] [FOR EACH {ROW | STATEMENT} ] [WHEN (precondition)] statement-list


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Constraint Maintenance

CREATE TRIGGER CrsChangeTrigger

AFTER UPDATE OF CrsCode, Semester

ON Transcript

WHEN ( Grade ISNOTNULL)

ROLLBACK


13

Business Rule

CREATE TRIGGER LimitSalaryRaise AFTER UPDATE OF Salary ON Employee REFERENCING OLD AS O NEW AS N FOR EACH ROW WHEN (N.Salary – O.Salary > 0.05 * O.Salary) UPDATE Employee SET Salary = 1.05 * O.Salary WHERE Id = O.Id


14

Generating An Attribute Value

CREATE TRIGGER SetValue

BEFORE UPDATE ON r

REFERENCING NEW ROW AS nrow

FOR EACH ROW

WHEN nrow.phone-number = ‘ ‘

SET nrow.phone-number = NULL


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Syntax Of Triggers In DB2CREATE TRIGGER trigger-name {NO CASCADE BEFORE | AFTER} {INSERT | DELETE | UPDATE [OF column-name-list] } ON table-name [REFERENCING [OLD AS var-to-refer-to-old-tuple] [NEW AS var-to-refer-to-new-tuple] ] [OLD_TABLE AS name-to-refer-to-old-table] ]

[NEW_TABLE AS name-to-refer-to-new-table] ] FOR EACH {ROW | STATEMENT} MODE DB2SQL [WHEN (precondition)] {triggered-SQL-statement | BEGIN ATOMIC triggered-SQL-statement-list; END}


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Example: System Without Trigger


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Example: Creating Trigger


18

Example: System With Trigger


19

References

Database Systems: An Application-Oriented Approach by Kifer, Bernstein, Lewis

Database System Concepts by Silberschatz, Korth, Sudarshan

Active Rules in Database Systems by Paton DB2 Online Manual: SQL Reference, Volume 1

and 2

Documents

Triggers Database Management System Design Saba Aamir Computing and Software McMaster University