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Special Types of Procedures
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Select Make + ' ' + Model from Equipment GO Create Trigger trMyEquipment_D On dbo.MyEquipment After Delete As Print 'You have just deleted ' + Cast(@@rowcount as varchar) + ' record(s)!' Go -- For Delete
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To execute the trigger, we need to execute the Delete statement:
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Delete MyEquipment Where Id = 2
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SQL Server will return the following:
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You have just deleted 1 record(s)! (1 row(s) affected)
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You can also execute the Delete statement to delete multiple records:
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Delete MyEquipment
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Even in this case, the trigger will not be fired once for each record. We will receive just one message:
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You have just deleted 4 record(s)! (4 row(s) affected)
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For this reason, it is important to design your trigger to handle actions against multiple records. You will see more reasons in following paragraphs.
Inserted and Deleted Virtual Tables
SQL Server maintains two temporary virtual tables during the execution of a trigger. They are called Deleted and Inserted. These tables contain
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all the records inserted or deleted during the operation that fired the trigger. You can use this feature to perform additional verification or additional activities on affected records. You are probably wondering if there is an Updated table. No. Because an Update can be performed as a combination of the Delete and Insert statements, records that were updated will appear in both the Deleted and Inserted tables. SQL Server will not create both tables in all cases. For example, in a trigger fired during a Delete statement, only Deleted items will be accessible. A reference to an Inserted item will cause an error. The following table will summarize the presence of virtual tables in the relevant Transact-SQL statements:
Modification Statement Deleted Inserted
INSERT UPDATE DELETE
N/A Old version of updated records Deleted records
New records New version of updated records N/A
Let s modify the trigger from the previous section to display which records are deleted:
Alter Trigger trMyEquipment_D On dbo.MyEquipment After Delete As Select 'You have just deleted following ' + Cast(@@rowcount as varchar) + ' record(s)!' Select * from deleted go -- For Delete
When you delete all records from the MyEquipment table, SQL Server returns the following:
--------------------------------------------------------------You have just deleted following 5 record(s)!
9:
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(1 row(s) affected) Id 1 2 3 4 5 Description Toshiba Portege 7020CT Sony Trinitron 17XE NEC V90 HP LaserJet 4 HP LaserJet 4
----------- --------------------------------------------------
(5 row(s) affected)
You can use values from these tables, but you cannot modify them directly. If you need to perform some operation on records that were inserted, for example, you should not try to change them in the Inserted table. The proper method would be to 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:
Alter Trigger trEquipment_IU On dbo.Equipment After 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) -- For Insert, Update
What Triggers a Trigger
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
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reason (for example: foreign key or check constraints), the trigger is not invoked. For example, the Equipment table has the following Delete trigger:
Alter Trigger Equipment_DeleteTrigger On dbo.Equipment After Delete As Print 'One or more rows are deleted in Equipment table!' -- For Delete
Attempt to delete all records from the table:
delete Equipment
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:
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.
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:
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