Module 15Responding to Data

Manipulation via Triggers

Module Overview• Designing DML Triggers• Implementing DML Triggers• Advanced Trigger Concepts

Lesson 1: Designing DML Triggers• What are DML Triggers?• AFTER Triggers vs. INSTEAD OF Triggers• inserted and deleted Virtual Tables• SET NOCOUNT ON• Trigger Performance Considerations

What are DML Triggers?

Other types of triggers:

DDL triggers fire on statements like CREATE, ALTER, DROP

Logon triggers fire when a session is established

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Triggers are special stored procedures that execute when events occur. DML triggers fire on INSERT, UPDATE, DELETE.

AFTER Triggers vs. INSTEAD OF Triggers

• AFTER triggers Fire after the event that they relate to Are treated as part of the same transaction as the statement

that triggered them Can roll back the statement that triggered them (and any

transaction that statement was part of)• INSTEAD OF triggers

Allow executing alternate code – unlike a BEFORE trigger in other database engines

Are often used to create updatable views with more than one base table

• Both can be implemented in managed code or T-SQL

Inserted and Deleted Virtual Tables• inserted and deleted Virtual Tables

Allow us to access the state of the data before and after the modification began

Virtual tables are often joined to the modified table data Available in both AFTER and INSTEAD OF triggers

Statement inserted deletedINSERT rows just insertedDELETE rows just deletedUPDATE modified row contents original row contents

SET NOCOUNT ON

Triggers should not return rows of dataü

Client applications often check the number of rows affected by data modification statementsü

Triggers should generally not change that countü

SET NOCOUNT ON avoids affecting outer statementü

The configuration setting ‘disallow results from triggers’ can be used to prevent triggers from returning resultsets.

Trigger Performance Considerations

Constraints are preferred to triggersü

Constraints avoid data modification overhead on violationü

Triggers use a rowversion store in tempdb databaseü

Excess trigger usage can impact tempdb performanceü

Triggers are complex to debugü

Triggers can increase the duration of transactionsü

Lesson 2: Implementing DML Triggers• AFTER INSERT Triggers• Demonstration 2A: AFTER INSERT Triggers• AFTER DELETE Triggers• Demonstration 2B: AFTER DELETE Triggers• AFTER UPDATE Triggers• Demonstration 2C: AFTER UPDATE Triggers

AFTER INSERT Triggers

CREATE TRIGGER TR_Opportunity_InsertON Sales.OpportunityAFTER INSERT AS BEGIN SET NOCOUNT ON; INSERT INTO Sales.OpportunityAudit (OpportunityID, ActionPerformed, ActionOccurredAt) SELECT i.OpportunityID, 'I', SYSDATETIME() FROM inserted AS i;END;

• INSERT statement is executed• AFTER INSERT trigger then fires• Ensure multi-row INSERTs are supported

Demonstration 2A: AFTER INSERT TriggersIn this demonstration you will see how to:• Create an AFTER INSERT trigger• Test the trigger action• Drop the trigger

AFTER DELETE Triggers

CREATE TRIGGER TR_Category_Delete ON Product.CategoryAFTER DELETE AS BEGIN SET NOCOUNT ON; UPDATE p SET p.Discontinued = 1 FROM Product.Product AS p INNER JOIN deleted as d ON p.CategoryID = d.CategoryID;END;GO

• DELETE statement is executed• AFTER DELETE trigger then fires

Demonstration 2B: AFTER DELETE TriggersIn this demonstration you will see how to:• Create an AFTER DELETE trigger• Test the trigger• Drop the trigger

AFTER UPDATE Triggers

CREATE TRIGGER TR_ProductReview_UpdateON Product.ProductReviewAFTER UPDATE AS BEGIN SET NOCOUNT ON; UPDATE pr SET Product.ProductReview.ModifiedDate = SYSDATETIME() FROM Product.ProductReview AS pr INNER JOIN inserted AS i ON i.ProductReviewID = pr.ProductReviewID;END;

• UPDATE statement is executed• AFTER UPDATE trigger then fires

Demonstration 2C: AFTER UPDATE TriggersIn this demonstration, you will see how to:• Create an AFTER UPDATE trigger• Test the trigger• Query the sys.triggers view

Lesson 3: Advanced Trigger Concepts• INSTEAD OF Triggers• Demonstration 3A: INSTEAD OF Triggers• How Nested Triggers Work• Considerations for Recursive Triggers• UPDATE Function• Trigger Firing Order• Alternatives to Using Triggers• Demonstration 3B: Replacing Triggers with Computed

Columns

INSTEAD OF Triggers

CREATE TRIGGER TR_ProductReview_Delete ON Product.ProductReviewINSTEAD OF DELETE AS BEGIN SET NOCOUNT ON; UPDATE pr SET pr.Discontinued = 1 FROM Product.ProductReview AS pr INNER JOIN deleted as d ON pr.ProductReviewID = d.ProductReviewID;END;

• INSERT, UPDATE, or DELETE statement requested to be executed• Statement does not execute• INSTEAD OF trigger code executes instead

Demonstration 3A: INSTEAD OF TriggersIn this demonstration, you will see how to:• Create an INSTEAD OF DELETE trigger• Test the trigger

How Nested Triggers Work

INSERT, UPDATE, or DELETE statement

Trigger executes INSERT, UPDATE, or DELETE on another table…

1

23…and so on…

Considerations for Recursive Triggers

Disabled by default – to enable:

• ALTER DATABASE AdventureWorks2008R2SET RECURSIVE_TRIGGERS ON

• Can usually be replaced with non-recursive logic

• Careful design and thorough testing to ensure that the 32 level nesting limit is not exceeded

• Can be difficult to control the order of table updates

Considerations:

• Option only affects direct recursion

UPDATE Function

CREATE TRIGGER TR_Product_Update_ListPriceAuditON Production.ProductAFTER UPDATE AS BEGIN IF UPDATE(ListPrice) BEGIN INSERT INTO Production.ListPriceAudit (ProductID, ListPrice, ChangedWhen) SELECT i.ProductID, i.ListPrice, SYSDATETIME() FROM inserted AS i; END;END;

• UPDATE determines if a particular column is being updated• Used in triggers AFTER INSERT or AFTER UPDATE

Trigger Firing Order

EXEC sp_settriggerorder @triggername= 'Production.TR_Product_Update_ListPriceAudit', @order='First', @stmttype = 'UPDATE';

• Multiple triggers may be created for a single event• You cannot specify the order that the triggers will fire• sp_settriggerorder allows you to specify which triggers will fire

first and last

Alternatives to Using TriggersMany developers use triggers in situations where other alternatives would be preferable• Use constraints for checking values• Use defaults for values not supplied during INSERTs• Use foreign key constraints to check for referential

integrity• Use computed and persisted computed columns• Use indexed views for pre-calculating aggregates

Demonstration 3B: Replacing Triggers with Computed ColumnsIn this demonstration you will see how a trigger could be replaced by a computed column.

Lab 15: Responding to Data Manipulation via Triggers• Exercise 1: Create the Audit Trigger • Challenge Exercise 2: Improve the Audit Trigger (Only if

time permits)

Logon information

Estimated time: 45 minutes

Virtual machine 623XB-MIA-SQLUser name AdventureWorks\AdministratorPassword Pa$$w0rd

Lab ScenarioThe Marketing.CampaignBalance table holds details of amounts of money still available for each of the marketing campaigns the company is undertaking. Changes to the balances are considered quite sensitive.You are required to audit any changes to data in the table. You have decided to implement this via DML triggers as the requirements in this case are not provided for directly by the SQL Server Audit mechanism.

Lab Review• What advantages does the use of triggers for auditing

provide over other options?• What did you need to specify as well as the trigger’s name

when altering it?

Module Review and Takeaways• Review Questions• Best Practices