How Can I Optimize SQL Server Updates Using CASE Statements and WHERE Clauses?

Updating Selective Records with CASE Statement in SQL Server 2005

The given SQL statement employs a CASE statement to update specific records based on their LASTNAME values. However, a concern arises from the ELSE condition encompassing all rows, leading to unnecessary processing. To address this, an optimized solution is proposed.

To minimize the unneeded scanning, one can incorporate a WHERE clause into the statement as follows:

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');

In this revised code, the WHERE clause confines the update operation only to the LASTNAME values of interest ('AAA', 'CCC', and 'EEE'). This ensures that the unaffected rows remain unchanged while the intended updates are performed efficiently.

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