SQL Server 2000 Stored Procedure & XML Programming in VS .NET

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SQL Server 2000 Stored Procedure & XML Programming
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Triggers are a unique type of procedure. Triggers are very similar to events a type of procedure in certain programming languages such as Visual Basic. Events in Visual Basic are initiated by the system when certain actions occur (for instance, a form is loaded, a text box receives focus, or a key is pressed). Triggers are associated with a table in a database and executed by SQL Server when a specific change occurs in the table. The change could be the result of the following modification statements: Insert Update Delete SQL Server 7.0 and earlier versions recognized only one type of trigger. In SQL Server 2000, this type is called an After trigger. SQL Server 2000 introduces a new type an Instead-of trigger. In the following sections, we first examine the standard (After) triggers and then introduce the new Instead-of trigger.
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SQL Server 2000 Stored Procedure & XML Programming
Physical Design of After Triggers
The following is the simplified syntax for implementing the core functionality of triggers:
Create Trigger trigger_name On table {After { [Delete] [,] [Insert] [,] [Update] } As sql_statement [...n]
As a stored procedure, a trigger logically consists of A header, which is a Transact-SQL statement for creating a trigger. It consists of three components: The name of the trigger The name of the table with which the trigger will be associated A modification statement (that is, an event) that will initiate the trigger A body, which contains Transact-SQL statement(s) to be executed at runtime. The following example first creates a new table called MyEquipment, and then populates it with Make and Model information from the Equipment table, and finally creates a trigger. The trigger is named trMyEquipment_D and is associated with the MyEquipment table. It is fired after a Delete statement is executed against the table. Its function is very simple it notifies the user regarding actions and the number of records that have been deleted.
Create Table MyEquipment (Id int identity, Description varchar(500)) GO -- populate table Insert MyEquipment(Description) Select Make + ' ' + Model from Equipment GO Create Trigger trMyEquipment_D On dbo.MyEquipment After Delete -- For Delete
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As Print 'You have just deleted ' + Cast(@@rowcount as varchar) + ' record(s)!' Go
To execute the trigger, you need to execute the Delete statement:
Delete MyEquipment Where Id = 2
SQL Server returns the following:
You have just deleted 1 record(s)! (1 row(s) affected)
You can also execute the Delete statement to delete multiple records:
Delete MyEquipment
Even in this case, the trigger will not be fired once for each record. You will receive just one message:
You have just deleted 4 record(s)! (4 row(s) affected)
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: Deleted and Inserted. These tables contain 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 does not create both tables in all cases. For example, in a trigger fired during a Delete statement, only a Deleted virtual table is accessible. A reference to an Inserted virtual table will cause an error.
SQL Server 2000 Stored Procedure & XML Programming
The following table summarizes the presence of virtual tables in the relevant Transact-SQL statements: Modification Statement
Insert Update Delete
Deleted
N/A Old version of updated records Deleted records
Inserted
New records New version of updated records N/A
The following modifies the trigger from the previous section to display which records are deleted:
Alter Trigger trMyEquipment_D On dbo.MyEquipment After Delete -- For Delete As Select 'You have just deleted following ' + Cast(@@rowcount as varchar) + ' record(s)!' Select * from deleted go
When you delete all records from the MyEquipment table, SQL Server returns the following:
--------------------------------------------------------------You have just deleted following 5 record(s)! (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
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