Session7 Trigger

8/7/2019 Session7 Trigger

1/23

Implementing Trigger

Vu Tuyet [email protected]

Hanoi University of Technology

1


2/23

Microsoft

Microsoft

Introduction to Triggers

What Is a Trigger?

Uses of Triggers

Considerations for Using Triggers


3/23

Microsoft

Microsoft

What Is a Trigger?

Associated with a Table

Invoked Automatically

Cannot Be Called Directly

Is Part of a Transaction


4/23

Microsoft

Microsoft

Outline


Creating, Altering, and Dropping Triggers

Working with Triggers Uses of Trigger

Performance Consideration


5/23

Microsoft

Microsoft

Creating and Managing Triggers

Creating Triggers

Altering and Dropping Triggers


6/23

Microsoft

Microsoft

Creating Triggers

Requires Appropriate Permissions

Cannot Contain Certain Statements

Use NorthwindGOCREATE TRIGGER Empl_Delete ON EmployeesFOR DELETEASIF (SELECT COUNT(*) FROM Deleted) > 1BEGIN

RAISERROR('You cannot delete more than one employee at a time.', 16, 1)

ROLLBACK TRANSACTIONEND


7/23

MicrosoftMicrosoft

Altering and Dropping Triggers Altering a Trigger

Changes the definition without dropping the trigger Can disable or enable a trigger

Dropping a Trigger

USE NorthwindGOALTER TRIGGER Empl_Delete ON EmployeesFOR DELETEASIF (SELECT COUNT(*) FROM Deleted) > 6BEGIN

RAISERROR(

'You cannot delete more than six employees at a time.', 16, 1)ROLLBACK TRANSACTION

END


8/23

MicrosoftMicrosoft

How Triggers Work

How an INSERT TriggerWorks

How a DELETE TriggerWorks

How an UPDATE TriggerWorks How an INSTEAD OF TriggerWorks


9/23


10/23

MicrosoftMicrosoft

Activating Insert Trigger

Step 1

INSERT statement to a table with an INSERT triggerdefined

Step 2

INSERT Statement Logged

Step 3

TriggerActions Executed


11/23


12/23

MicrosoftMicrosoft

Activating UPDATE Trigger

Step 1:

DELETE Statement to a Table with a DELETEStatement Defined

Step 2

DELETE Statement Logged

Step 3



13/23


14/23

MicrosoftMicrosoft

Activating DELETE Trigger

Step 1

UPDATE Statement to a Table with an UPDATE Trigger

Defined

Step 2

UPDATE Statement Logged as INSERT and DELETE

Statements

Step 3



15/23

MicrosoftMicrosoft

Outline


Creating, Altering, and Dropping Triggers

Working with Triggers Uses of Trigger

Performance Consideration


16/23

MicrosoftMicrosoft

Uses of Triggers

Cascade changes through related tables in

a database

Enforce more complex data integrity than aCHECK Constraint

Define custom error messages

Maintain denormalized data

Compare before and after states of data undermodification


17/23

MicrosoftMicrosoft

Considerations for Using Triggers

Triggers Are Reactive; Constraints Are Proactive

Constraints Are Checked First

Tables Can Have Multiple Triggers forAny Action

Table Owners Can Designate the First and LastTrigger to Fire

You Must Have Permission to Perform AllStatements That Define Triggers

Table Owners Cannot Create AFTER Triggers on

Views or Temporary Tables


18/23

MicrosoftMicrosoft

Examples of Triggers

Enforcing Data Integrity

Enforcing Business Rules


19/23

MicrosoftMicrosoft

Enforcing Data Integrity

CREATE TRIGGER BackOrderList_DeleteON Products FOR UPDATEASIF (SELECT BO.ProductID FROM BackOrders AS BO JOIN

Inserted AS I ON BO.ProductID = I.Product_ID) > 0

BEGIN

DELETE BO FROM BackOrders AS BOINNER JOIN Inserted AS ION BO.ProductID = I.ProductID

END

ProductsProducts

ProductIDUnitsInStock

1

3

4

15

10

65

20

2 15 Updated

BackOrdersBackOrders

ProductID UnitsOnOrder

1

12

3

15

10

65

2 15Trigger Deletes Row


20/23


21/23

MicrosoftMicrosoft

Performance Considerations

Triggers Work Quickly Because the Inserted and

Deleted Tables Are in Cache

Execution TimeIs Determined by:

Number of tables that are referenced

Number of rows that are affected

Actions Contained in Triggers Implicitly Are Part of

a Transaction


22/23

MicrosoftMicrosoft

Summary

Use Triggers Only When Necessary

Keep Trigger Definition Statements as Simple as

Possible

Include Recursion Termination Check Statements in

Recursive Trigger Definitions

Minimize Use ofROLLBACK Statements in Triggers


23/23

MicrosoftMicrosoft

Documents

Session7 Trigger