Tài liệu Introducing Triggers ppt

Listing 4.7: INSERTPRODUCTTRIGGER.SQL /* InsertProductTrigger.sql creates a trigger that fires after an INSERT statement is performed on the Products table */ CREATE TRIGGER InsertPro

Introducing Triggers

A database trigger is a special kind of stored procedure that is run automatically by the database-or in trigger terms, fired-after a specified INSERT, UPDATE, or DELETE

statement is run against a specified database table Triggers are very useful for doing things such as auditing changes made to column values in a table

A trigger can also fire instead of an INSERT, UPDATE, or DELETE For example, instead of performing an INSERT to add a row to the Products table, a trigger could raise

an error if a product with the same ProductID already existed in the table

As mentioned, triggers are very useful for auditing changes made to column values In this section, you'll see an example of a trigger that will audit changes made to the

Products table

Also, when an UPDATE statement modifies the UnitPrice column of a row in the

Products table, a row will be added to the ProductAudit table Finally, when a DELETE statement removes a row from the Products table, a row will be added to the

ProductAudit table

Before you see the triggers, you'll need to create the ProductAudit table Listing 4.6

shows the ProductAudit.sql script that creates the ProductAudit table

Listing 4.6: PRODUCTAUDIT.SQL

/*

ProductAudit.sql creates a table that is used to

store the results of triggers that audit modifications

to the Products table

*/

USE Northwind

CREATE TABLE ProductAudit (

ID int IDENTITY(1, 1) PRIMARY KEY,

Action nvarchar(100) NOT NULL,

PerformedBy nvarchar(15) NOT NULL DEFAULT User,

TookPlace datetime NOT NULL DEFAULT GetDate()

)

The IDENTITY clause creates an identity for the ID primary key column of the

ProductAudit table An identity automatically generates values for a column The identity

for the ID column starts with the value 1, which is incremented by 1 after each INSERT The Action column stores a string that records the action performed, for example,

'Product added with ProductID of 80' The PerformedBy column stores the name of the user who performed the action; this is defaulted to User, which returns the current user The TookPlace column stores the date and time when the action took place; this is

defaulted using the GetDate() function, which returns the current date and time

In the following sections, you'll learn how to create and use the following triggers:

• InsertProductTrigger Fires after an INSERT statement is performed on the Products table

• UpdateUnitPriceProductTrigger Fires after an UPDATE statement is performed

on the Products table

• DeleteProductTrigger Fires after a DELETE statement is performed on the

Products table

First off, let's examine InsertProductTrigger

Creating InsertProductTrigger

You create a trigger using the CREATE TRIGGER statement Listing 4.7 shows the InsertProductTrigger.sql script that creates the trigger InsertProductTrigger, which audits the addition of new rows to the Products table

Listing 4.7: INSERTPRODUCTTRIGGER.SQL

/*

InsertProductTrigger.sql creates a trigger that fires

after an INSERT statement is performed on the

Products table

*/

CREATE TRIGGER InsertProductTrigger

ON Products

AFTER INSERT

AS

don't return the number of rows affected

SET NOCOUNT ON

declare an int variable to store the new

ProductID

DECLARE @NewProductID int

get the ProductID of the new row that

was added to the Products table

SELECT @NewProductID = ProductID

FROM inserted

add a row to the ProductAudit table

INSERT INTO ProductAudit (

Action

) VALUES (

'Product added with ProductID of ' +

CONVERT(nvarchar, @NewProductID)

)

There are several things you should notice about this CREATE TRIGGER statement:

• The AFTER INSERT clause specifies that the trigger is to fire after an INSERT statement is performed

• SET NOCOUNT ON prevents the trigger from returning the number of rows affected This improves performance of the trigger

• You can retrieve column values for the INSERT statement that caused the trigger

to fire by performing a SELECT against the special inserted table For example, you can retrieve all the columns of a newly added row using SELECT * FROM inserted The trigger code retrieves the ProductID column of the new row from the inserted table

• The INSERT statement that adds a row to the ProductAudit table supplies a value only for the Action column This is because the ID, PerformedBy, and TookPlace column values are set automatically by SQL Server

You can also create, edit, and delete triggers using Enterprise Manager You do this by first clicking the Tables node in the Databases folder, then clicking the right mouse

button on the table you want to modify, and then selecting All Tasks ➣ Manage

Triggers Figure 4.8 shows InsertProductTrigger in Enterprise Manager You'll notice I've added some comments to the start of the code that indicates what the trigger does

Figure 4.8: Using Enterprise Manager to view a trigger

The Object Browser of Query Analyzer also allows you to view, modify, and delete triggers

Tip You can delete a trigger using the DROP TRIGGER statement, and you can modify a trigger using the ALTER TRIGGER statement

Testing InsertProductTrigger

To test InsertProductTrigger, all you have to do is to add a row to the Products table using an INSERT statement For example:

INSERT INTO Products (

ProductName, SupplierID, UnitPrice

) VALUES (

'Widget', 1, 10

)

You can check that InsertProductTrigger fired by retrieving the rows from the

ProductAudit table using the following SELECT statement:

SELECT *

FROM ProductAudit

The row added to the ProductAudit table by InsertProductTrigger as a result of

performing the previous INSERT statement is shown in Table 4.8

Table 4.8: ROW ADDED TO THE ProductAudit TABLE BY InsertProductTrigger

ID ACTION PERFORMEDBY TOOKPLACE

1 Product added with

ProductID of 80

Creating and Testing UpdateUnitPriceProductTrigger

The UpdateUnitPriceProductTrigger trigger fires after an UPDATE statement is

performed on the UnitPrice column of the Products table If the reduction of the unit price

of a product is greater than 25 percent, then a row is added to the ProductAudit table to audit the change Listing 4.8 shows the UpdateUnitPriceProductTrigger.sql script

Listing 4.8: UPDATEUNITPRICEPRODUCTTRIGGER.SQL

/*

UpdateUnitPriceProductTrigger.sql creates a trigger

that fires after an UPDATE statement is performed on the

the UnitPrice column of the Products table

If the reduction of the unit price of a product is

greater than 25% then a row is added to the ProductAudit table

to audit the change

*/

CREATE TRIGGER UpdateUnitPriceProductTrigger

ON Products

AFTER UPDATE

AS

don't return the number of rows affected

SET NOCOUNT ON

only run the code if the UnitPrice column

was modified

IF UPDATE(UnitPrice)

BEGIN

declare an int variable to store the

ProductID

DECLARE @MyProductID int

declare two money variables to store the

old unit price and the new unit price

DECLARE @OldUnitPrice money

DECLARE @NewUnitPrice money

declare a float variable to store the price

reduction percentage

DECLARE @PriceReductionPercentage float

get the ProductID of the row that

was modified from the inserted table

SELECT @MyProductID = ProductID

FROM inserted

get the old unit price from the deleted table

SELECT @OldUnitPrice = UnitPrice

FROM deleted

WHERE ProductID = @MyProductID

get the new unit price from the inserted table

SELECT @NewUnitPrice = UnitPrice

FROM inserted

calculate the price reduction percentage

SET @PriceReductionPercentage =

((@OldUnitPrice -@NewUnitPrice) / @OldUnitPrice) * 100

if the price reduction percentage is greater than 25%

then audit the change by adding a row to the PriceAudit table

IF (@PriceReductionPercentage > 25)

BEGIN

add a row to the ProductAudit table

INSERT INTO ProductAudit (

Action

) VALUES (

'UnitPrice of ProductID #' +

CONVERT(nvarchar, @MyProductID) +

' was reduced by ' +

CONVERT(nvarchar, @PriceReductionPercentage) +

'%'

)

END

END

There are a couple of things you should notice about this CREATE TRIGGER statement:

• The AFTER UPDATE clause specifies that the trigger is to fire after an UPDATE statement is performed

• You can retrieve the old column values before the UPDATE was applied from the deleted table, and you can retrieve the new column values after the UPDATE was applied from the inserted table

To test UpdateUnitPriceProductTrigger, all you have to do is to reduce the value of the UnitPrice column for a row in the Products table using an UPDATE statement For example, the following UPDATE statement multiplies the UnitPrice by 0.70 for the row with a ProductID of 80 (this reduces the UnitPrice of that row by 30 percent):

UPDATE Products

SET UnitPrice = UnitPrice * 0.70

WHERE ProductID = 80

The row added to the ProductAudit table as a result of performing this UPDATE

statement is shown in Table 4.9 This row is added by UpdateUnitPriceProductTrigger

Table 4.9: ROW ADDED TO THE ProductAudit TABLE BY

UpdateUnitPriceProductTrigger

2 UnitPrice of ProductID #80

was reduced by 30%

Creating and Testing DeleteProductTrigger

The DeleteProductTrigger trigger fires after a DELETE statement is performed on the Products table This trigger adds a row to the ProductAudit table to audit the change

Listing 4.9: DELETEPRODUCTTRIGGER.SQL

/*

DeleteProductTrigger.sql creates a trigger that fires

after a DELETE statement is performed on the

Products table

*/

CREATE TRIGGER DeleteProductTrigger

ON Products

AFTER DELETE

AS

don't return the number of rows affected

SET NOCOUNT ON

declare an int variable to store the

ProductID

DECLARE @NewProductID int

get the ProductID of the row that

was removed from the Products table

SELECT @NewProductID = ProductID

FROM deleted

add a row to the ProductAudit table

INSERT INTO ProductAudit (

Action

) VALUES (

'Product #' +

CONVERT(nvarchar, @NewProductID) +

' was removed'

)

To test DeleteProductTrigger, all you have to do is to remove a row from the Products table using a DELETE statement For example, the following DELETE statement removes the row with the ProductID of 80:

DELETE FROM Products

WHERE ProductID = 80

The row added to the ProductAudit table as a result of performing this DELETE statement is shown in Table 4.10 This row is added by DeleteProductTrigger

Table 4.10: ROW ADDED TO THE ProductAudit TABLE BY DeleteProductTrigger

Table 4.10: ROW ADDED TO THE ProductAudit TABLE BY DeleteProductTrigger

3 Product #80 was removed dbo 2002-07-18 17:35:53.510