Module 11: Implementing Triggers

Transcription

1 Module 11: Implementing Triggers

2 Overview Introduction Defining Create, drop, alter triggers How Triggers Work Examples Performance Considerations Analyze performance issues related to triggers

3 Introduction to Triggers What Is a Trigger? Uses Considerations for Using Triggers

4 What Is a Trigger? Associated with a Table Invoked Automatically Cannot Be Called Directly Is Part of a Transaction Along with the statement that calls the trigger Can ROLLBACK transactions (use with care)

5 Uses of Triggers Cascade Changes Through Related Tables in a Database A delete or update trigger can cascade changes to related tables: Soda name change to change in soda name in Sells table Enforce More Complex Data Integrity Than a CHECK Constraint Change prices in case of price rip-offs. Define Custom Error Messages Maintain Denormalized Data Automatically update redundant data. Compare Before and After States of Data Under Modification

6 Considerations for Using Triggers Triggers Are Reactive; Constraints Are Proactive Constraints Are Checked First Tables Can Have Multiple Triggers for Any Action Table Owners Can Designate the First and Last Trigger to Fire You Must Have Permission to Perform All Statements That Define Triggers Table Owners Cannot Create AFTER Triggers on Views or Temporary Tables

7 Defining Triggers Creating Triggers Altering and Dropping Triggers

8 Creating Triggers Requires Appropriate Permissions Cannot Contain Certain Statements Use Northwind GO CREATE TRIGGER Empl_Delete ON Employees FOR DELETE AS IF (SELECT COUNT(*) FROM Deleted) > 1 BEGIN RAISERROR( 'You cannot delete more than one employee at a time.', 16, 1) ROLLBACK TRANSACTION END

9 Altering and Dropping Triggers Altering a Trigger Changes the definition without dropping the trigger Can disable or enable a trigger USE Northwind GO ALTER TRIGGER Empl_Delete ON Employees FOR DELETE AS IF (SELECT COUNT(*) FROM Deleted) > 6 BEGIN RAISERROR( 'You cannot delete more than six employees at a time.', 16, 1) ROLLBACK TRANSACTION END Dropping a Trigger

10 How Triggers Work How an INSERT Trigger Works How a DELETE Trigger Works How an UPDATE Trigger Works How an INSTEAD OF Trigger Works How Nested Triggers Work Recursive Triggers

11 How an INSERT Trigger Works INSERT statement to a table with an INSERT Trigger Defined TRIGGER Actions Execute INSERT [Order Trigger Details] Code: VALUES (10525, 1 2, 19.00, 5, 0.2) INSERT Statement USE Northwind to a Table with an INSERT CREATE TRIGGER OrdDet_Insert Trigger Defined ON [Order Details] FOR Order INSERT Details AS OrderID ProductID UnitPrice Quantity Discount 2 INSERT Statement UPDATE P SET Logged UnitsInStock = (P.UnitsInStock I.Quantity) FROM 10523Products AS P INNER 9 JOIN 0.15 Inserted AS I 3 ON P.ProductID = I.ProductID Trigger Actions Executed Order Details OrderID ProductID UnitPrice Quantity Discount Products ProductID UnitsInStock Insert statement logged inserted

12 How a DELETE Trigger Works DELETE Statement to a table with a DELETE Trigger Defined Trigger Actions Execute Categories 1 DELETE Statement CategoryIDto CategoryNamea Table with a Description DELETE Picture DELETE Categories 1 Beverages Products Statement Defined Soft drinks, coffees 0x15 WHERE ProductID Discontinued 2 Condiments Sweet and savory 0x15 CategoryID = Confections Desserts, 1 candies, 0 0x15 DELETE Statement 4 Logged Dairy Products Cheeses x15 USE Northwind CREATE TRIGGER Category_Delete ON Categories Trigger Actions Executed 4 0 FOR DELETE AS UPDATE P SET Discontinued = 1 FROM Products AS P INNER JOIN deleted AS d ON DELETE P.CategoryID statement = d.categoryid logged Deleted 4 Dairy Products Cheeses 0x15

13 How an UPDATE Trigger Works UPDATE TRIGGER Statement Actions Execute to a table with an UPDATE Trigger Defined USE Northwind GO UPDATE Employees CREATE SET 1 EmployeeID TRIGGER UPDATE Employee_Update Statement = 17 to a Table with an UPDATE ON WHERE Employees EmployeeID = 2 FOR UPDATE Trigger Defined AS Employees IF UPDATE (EmployeeID) EmployeeID LastName FirstName Title HireDate BEGIN TRANSACTION RAISERROR 2 UPDATE ('Transaction Statement 1cannot Logged Davolio be as processed.\ INSERT Nancy Sales andrep. ~~~ ***** Employee ID number cannot be modified.', 10, 1) ROLLBACK DELETE TRANSACTION Statements 2 Fuller Barr Andrew Vice R Pres. ~~~ 3 Leverling Janet Sales Rep. ~~~ Transaction 3 cannot be processed. 4 Peacock MargaretSales Rep. ~~~ ***** Member Trigger number Actions cannot be modified Executed Employees UPDATE Statement logged as INSERT and DELETE Statements EmployeeID LastName FirstName Title HireDate inserted 1 Davolio Nancy Sales Rep. ~~~ 17 Fuller 2 Andrew Vice Pres. ~~~ Fuller Barr Andrew Vice R Pres. ~~~ deleted 3 Leverling Janet Sales Rep. ~~~ 2 Fuller 4AndrewPeacock Vice Pres. Margaret ~~~ Sales Rep. ~~~

14 How an INSTEAD OF Trigger Works Create a View That Combines Two or More Tables CREATE VIEW Customers 1 AS SELECT * FROM CustomersMex UNION SELECT * FROM 2 CustomersGer UPDATE is Made to the View INSTEAD OF Trigger Can Be on a Table or View INSTEAD OF trigger directs the update to the base table Customers CustomerID CompanyName Country Phone The Action That Initiates the Trigger Does NOT Occur ALFKI Alfreds Fu Germany ~~~ ANATR ANTON Ana Trujill Antonio M Mexico Mexico (5) (5) ~~~ ~~~ Original Insert to 3 CustomersMex the Allows Updates to Views Not Previously Updateable CustomerID View Does CompanyName Not Country CustomersGer Phone Occur ANATR Ana Trujill Mexico CustomerID (5) CompanyName ~~~ Country Phone ANTON Antonio M Mexico ALFKI(5) Alfreds Fu ~~~ Germany ~~~ CENTC Centro Co Mexico BLAUS(5) Blauer Se ~~~ Germany ~~~ DRACD Drachenb Germany ~~~

15 How Nested Triggers Work OrDe_Update Order_Details OrderID ProductID UnitPrice Quantity Discount InStock_Update Products ProductID UnitsInStock UnitsInStock + UnitsOnOrder is < ReorderLevel for ProductID 2 Placing an order causes the OrDe_Update trigger to execute Executes an UPDATE statement on the Products table InStock_Update trigger executes Sends message

16 Recursive Triggers Activating a Trigger Recursively Types of Recursive Triggers Direct recursion occurs when a trigger fires and performs an action that causes the same trigger to fire again Indirect recursion occurs when a trigger fires and performs an action that causes a trigger on another table to fire Determining Whether to Use Recursive Triggers

17 Examples of Triggers Enforcing Data Integrity Enforcing Business Rules

18 Enforcing Data Integrity CREATE TRIGGER BackOrderList_Delete ON Products FOR UPDATE AS IF (SELECT BO.ProductID FROM BackOrders AS BO JOIN Inserted AS I ON BO.ProductID = I.Product_ID ) > 0 BEGIN DELETE BO FROM BackOrders AS BO INNER JOIN Inserted AS I ON BO.ProductID = I.ProductID END Products ProductID UnitsInStock Updated Trigger Deletes Row BackOrders ProductID UnitsOnOrder

19 Enforcing Business Rules Products with Outstanding Orders Cannot Be Deleted IF (Select Count (*) FROM [Order Details] INNER JOIN deleted ON [Order Details].ProductID = deleted.productid ) > 0 ROLLBACK TRANSACTION DELETE statement executed on Product table Products ProductID UnitsInStock Trigger code checks the Order Details table Order Details OrderID ProductID UnitPrice Quantity Transaction rolled back Discount 'Transaction cannot be processed' 'This product has order history'

20 Performance Considerations Triggers Work Quickly Because the Inserted and Deleted Tables Are in Cache Execution Time Is Determined by: Number of tables that are referenced Number of rows that are affected Actions Contained in Triggers Implicitly Are Part of a Transaction

21 Recommended Practices Use Triggers Only When Necessary Keep Trigger Definition Statements as Simple as Possible Include Recursion Termination Check Statements in Recursive Trigger Definitions Minimize Use of ROLLBACK Statements in Triggers

22 Review Introduction Defining Create, drop, alter triggers How Triggers Work Examples Performance Considerations Analyze performance issues related to triggers