How Can a WHERE Clause Optimize CASE Statement Updates in SQL Server 2005?

Utilizing a WHERE Clause to Optimize CASE Statement Updates in SQL Server 2005

In SQL Server 2005, the CASE statement can be employed to update records based on specified conditions. While using the ELSE clause can handle unaffected rows, it may result in unnecessary scans of the entire table. This can be addressed by employing a WHERE clause.

Consider the following statement:

UPDATE dbo.TestStudents  
SET LASTNAME = 
( CASE  
    WHEN (LASTNAME = 'AAA') THEN 'BBB' 
    WHEN (LASTNAME = 'CCC') THEN 'DDD' 
    WHEN (LASTNAME = 'EEE') THEN 'FFF' 
    ELSE  (LASTNAME)
END )

This statement will update records in the 'dbo.TestStudents' table, changing 'LASTNAME' values as per the specified conditions. However, the ELSE clause will evaluate every record in the table, even those not affected by the conditions.

To optimize this statement, a WHERE clause can be incorporated to filter the records that should be updated:

UPDATE dbo.TestStudents  
SET     LASTNAME =  CASE  
                        WHEN LASTNAME = 'AAA' THEN 'BBB' 
                        WHEN LASTNAME = 'CCC' THEN 'DDD' 
                        WHEN LASTNAME = 'EEE' THEN 'FFF' 
                        ELSE LASTNAME
                    END 
WHERE   LASTNAME IN ('AAA', 'CCC', 'EEE')

By adding the WHERE clause, the statement only evaluates records where the 'LASTNAME' value is 'AAA', 'CCC', or 'EEE'. This improves performance and avoids unnecessary scans of unaffected rows. Therefore, incorporating a WHERE clause is a recommended technique to optimize CASE statement updates in SQL Server 2005.

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