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  SQL Server Tips by Gama and Naughter

 

Set default

 

The Set default action is implemented by triggers that will examine the data and then set to their default values the rows from table [Tbl B] related to changed or deleted records from table [Tbl A].

 

For the next tests it will be necessary to create a default value for table [Tbl A]:

 

ALTER TABLE [Tbl A] ADD CONSTRAINT

      [DF_Tbl A_col A] DEFAULT 'abc' FOR [col A]

 

Delete Set Default
 

Delete set Default requires one trigger for table [Tbl A] that will set to their default values the related columns from table [Tbl B].

 

CREATE TRIGGER "[Tbl A DDTrig]" ON [Tbl A] FOR DELETE AS

SET NOCOUNT ON

/* * DELETE SET DEFAULT TO '[Tbl B]' */

IF (SELECT COUNT(*) FROM deleted, [Tbl B] WHERE (deleted. [Col A]= [Tbl B]. [Col B])) > 0

       UPDATE [Tbl B]

       SET [Tbl B]. [Col B]=(SELECT SUBSTRING (COLUMN_DEFAULT, 3, LEN(COLUMN_DEFAULT)-4) FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME='Tbl A' AND COLUMN_NAME='Col A')

       FROM [Tbl B], deleted

       WHERE deleted. [Col A]= [Tbl B].[Col B]

 

Let us test the trigger:

 

DELETE [Tbl A] WHERE [col A]='value2'

 

By checking the contents of table [Tbl B] it is obvious that the trigger worked.

 


The above book excerpt is from:

Super SQL Server Systems
Turbocharge Database Performance with C++ External Procedures

ISBN: 0-9761573-2-2
Joseph Gama, P. J. Naughter

 http://www.rampant-books.com/book_2005_2_sql_server_external_procedures.htm
 

 

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