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C h a p t e r 8 : S p e c i a l Ty p e s o f P r o c e d u r e s
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issue a regular Transact-SQL statement against the original table. In the Where or From clause, you can reference the virtual table (Inserted) and in that way limit the subset of the original table that you are targeting. In the following example, the trigger calculates a SOUNDEX code for the Make and Model of the Equipment records affected by the Insert or Update statement that has fired the trigger:
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Alter Trigger trEquipment_IU On dbo.Equipment After Insert, Update -- For Insert, Update As -- precalculate ModelSDX and MakeSDX field -- to speed up use of SOUNDEX function update Equipment Set ModelSDX = SOUNDEX(Model), MakeSDX = SOUNDEX(Make) where EquipmentId IN (Select EquipmentId from Inserted)
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What Triggers a Trigger
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A trigger is executed once for each modification statement (Insert, Update, or Delete). An After trigger is fired after the modification statement finishes successfully. If a statement fails for another reason (for example, foreign key or check constraints), the trigger is not invoked. For example, the Equipment table has the following Delete trigger:
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Alter Trigger Equipment_DeleteTrigger On dbo.Equipment After Delete -- For Delete As Print 'One or more rows are deleted in Equipment table!'
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If you attempt to delete all records from the table:
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delete Equipment
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SQL Server aborts the execution because there is a foreign key relationship with the Inventory table. The execution is aborted before the trigger is invoked:
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Server: Msg 547, Level 16, State 1, Line 1 DELETE statement conflicted with COLUMN REFERENCE constraint 'FK_Inventory_Equipment'. The conflict occurred in database 'Asset', table 'Inventory', column 'EquipmentId'. The statement has been terminated.
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SQL Server 2000 Stored Procedure & XML Programming
A trigger and developer might have different definitions of what is a successfully finished modification to a table. The trigger will fire even when a modification statement affected zero records. The following example is based on the assumption that the record with EquipmentId set to 77777 does not exist in the database:
Delete Equipment Where EquipmentId = 77777
SQL Server nonchalantly prints from the trigger:
One or more rows are deleted in Equipment table!
Full Syntax of After Triggers
After triggers were the only type of triggers before SQL Server 2000. After triggers in SQL Server 2000 have the same syntax as before except that the keyword For is replaced with After:
Create Trigger trigger_name On table [With Encryption] { {After { [Delete] [,] [Insert] [,] [Update] } [With Append] [Not For Replication] As sql_statement [...n] } | {After { [Insert] [,] [Update] } [With Append] [Not For Replication] As { If Update (Column) [{And | Or} Update (Column)] [...n] | If (Columns_Updated() {bitwise_operator} updated_bitmask) { comparison_operator} column_bitmask [...n] } sql_statement [ ...n] } }
C h a p t e r 8 : S p e c i a l Ty p e s o f P r o c e d u r e s
If a trigger is defined with the With Encryption clause, SQL Server encrypts it so that its code remains concealed. Keep in mind that you need to preserve the source code in a script outside SQL Server if you plan to modify it later. The Not For Replication clause indicates that SQL Server should not fire a trigger during replication of the table. The With Append clause is used only when the compatibility mode of SQL Server is set to a value less than 70. For more details, refer to SQL Server Books Online. It is possible to determine which columns were updated during the Update operation. Transact-SQL includes two functions that you can use within the trigger Update() and Columns_Updated():
If Update (column) sql_statement [ ...n] If (Columns_Updated() {bitwise_operator} updated_bitmask) {comparison_operator} column_bitmask [...n] sql_statement [ ...n]
You can now modify your previously used trigger to update only the fields that were changed:
Alter Trigger trEquipment_IU On dbo.Equipment After Insert, Update -- For Insert, Update As -- precalculate ModelSDX and MakeSDX field -- to speed up use of SOUNDEX function if Update(Model) update Equipment Set ModelSDX = SOUNDEX(Model) where EquipmentId IN (Select EquipmentId from Inserted) if Update(Make) update Equipment Set MakeSDX = SOUNDEX(Make) where EquipmentId IN (Select EquipmentId from Inserted) go
The Update() function might not perform exactly as you expect. In fact, it returns True for columns that were referenced during the Transact-SQL statement rather than for columns that were actually changed. For example, if you issue the following
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