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Learning to Reference Inserted and Deleted Tables
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Understanding how to use the inserted and deleted tables is critical for the effective use of triggers. Therefore, the coverage of triggers commences with a demonstration that shows how to populate these tables with values and how to recover values from the tables. You populate the inserted and deleted tables with values by invoking DML statements that generate values for the tables. You can read values from the inserted and deleted tables with a SELECT statement inside a trigger. The samples in this section and the next two sections reside in Triggers.sql. The samples for the fourth section on triggers reside in TriggerForView.sql. If you are going to run any of the sample code for triggers, you should first refresh the ProSSEAppsCh07 database by running the script in ViewForDataModificationSampleDB.sql. Before you can use a trigger you must create one. As with any database object, it is always wise to check for the existence of a previous object of the same type with the name of the new object that you want to create. The following T-SQL statement uses sys.objects and sys.schemas to check for an existing instance of a trigger named trStudentsEcho in the dbo schema. If the trigger exists, a DROP TRIGGER statement removes it. IF EXISTS(SELECT * FROM sys.objects o JOIN sys.schemas s ON o.schema_id = s.schema_id WHERE type = 'TR' AND s.name = 'dbo' AND o.name = 'trStudentsEcho') DROP TRIGGER trStudentsEcho GO The following CREATE TRIGGER statement for the trStudentsEcho trigger demonstrates some basic syntax for specifying an AFTER trigger. The specification of a trigger in the header section of a CREATE TRIGGER statement tells you a lot about what it does. The ON clause designates the object to which a trigger applies. In the following example, the trigger applies to the Students table in the current database context. A prior USE statement set this context to the ProSSEAppsCh07 database. The FOR clause designates the type of trigger (FOR indicates this is an AFTER trigger) as well as the types of events to which the trigger applies. The following code sample applies to UPDATE, INSERT, and DELETE statements that reference the Students table. You can designate a trigger for any combination of one, two, or three of these statements.
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CHAPTER 7 LEVERAGING DATABASE OBJECTS THAT ENCAPSULATE T-SQL
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The body of the trigger starts immediately after the AS keyword. The body of a trigger can often be complex though, in general, it will be as complex as necessary for the business rules and other tasks that it implements. This initial trigger sample merely displays the contents of the inserted and deleted tables. The contents of these tables will be displayed whenever an INSERT, UPDATE, or DELETE statement operates on the Students table in the ProSSEAppsCh07 database. CREATE TRIGGER trStudentsEcho ON Students FOR UPDATE, INSERT, DELETE AS SELECT * FROM inserted SELECT * FROM deleted GO The trStudentsEcho trigger fires in response to DML statements for the Students table. After each type of statement, you can see the contents of the inserted table followed by the contents of the deleted table. The following T-SQL statements display a heading (with Before INSERT) before invoking the INSERT statement to add a new student named Rick Dobson with a StudentID of 3 to the Students table. After the INSERT statement completes, the trStudentsEcho trigger fires and displays the contents of the inserted table followed by the contents of the deleted table. The first row of output shows the data added to the inserted table by the INSERT statement. The last row displays the data in the deleted table. This row is empty because an INSERT statement does not populate the deleted table. SELECT 'Before INSERT' INSERT Students (StudentID, FirstName, LastName) VALUES (3, 'Rick', 'Dobson') ------------Before INSERT StudentID FirstName LastName FullName ----------- --------- --------- ------------------3 Rick Dobson Rick Dobson StudentID FirstName LastName FullName ----------- --------- --------- ------------------The next two T-SQL statements illustrate the impact of an UPDATE statement on the inserted and deleted tables. In this case, the heading for the output is Before UPDATE. An UPDATE statement populates both the inserted and deleted tables. The new column values are in the inserted table. Notice that Rickie is the FirstName column value in the output for the inserted table. The old column values (the ones being updated) are in the deleted table. The FirstName column value being replaced (Rick) is in the deleted table. SELECT 'Before UPDATE' UPDATE Students SET FirstName = 'Rickie' WHERE StudentID = 3
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