How Can I Correctly Use a CASE Statement in a SQL Server 2008 WHERE Clause?

SQL Server 2008: Correctly Using CASE Statements in WHERE Clauses

Using CASE statements within WHERE clauses in SQL Server 2008 can lead to errors if the syntax isn't precisely correct. This guide clarifies proper usage and offers alternatives.

Correct CASE Statement Placement

The CASE statement should be part of a larger expression within the WHERE clause, not a standalone condition. For instance:

WHERE co.DTEntered = CASE 
                          WHEN LEN('blah') = 0 
                               THEN co.DTEntered 
                          ELSE '2011-01-01' 
                     END

Common Syntax Errors

The following example demonstrates an incorrect usage that will result in errors:

WHERE 
    CASE LEN('TestPerson')
        WHEN 0 THEN co.personentered  = co.personentered
        ELSE co.personentered LIKE '%TestPerson'
    END 

A More Efficient Alternative: Using OR

For simpler scenarios, combining OR statements often provides a more efficient and readable solution than using CASE within the WHERE clause:

WHERE (
        (LEN('TestPerson') = 0 
             AND co.personentered = co.personentered
        ) 
        OR 
        (LEN('TestPerson') <> 0 
             AND co.personentered LIKE '%TestPerson')
      )

Performance Implications

Complex WHERE clauses, regardless of whether they use CASE statements or other intricate logic, can negatively impact query performance and hinder the query optimizer's ability to utilize indexes effectively. Always carefully analyze and optimize your query structure for best results.

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