How Can I Correctly Use Nested CASE Statements in SQL Server 2008 WHERE Clauses?

Using nested CASE statements in the WHERE clause in SQL Server 2008

When using SQL Server 2008, users may encounter challenges when using CASE statements in the WHERE clause. Error messages often result from incorrect placement of these conditional expressions.

The fundamental problem is syntax. The "CASE" statement should be used as part of an expression, not as the expression itself. For example, you can write:

<code class="language-sql">WHERE co.DTEntered = CASE 
                          WHEN LEN('blah') = 0 
                               THEN co.DTEntered 
                          ELSE '2011-01-01' 
                     END </code>

However, the following syntax is incorrect:

<code class="language-sql">WHERE 
    CASE LEN('TestPerson')
        WHEN 0 THEN co.personentered  = co.personentered
   ELSE co.personentered LIKE '%TestPerson'
    END </code>

To solve this problem, a more appropriate approach is to use a combined OR statement. The modified query looks like this:

<code class="language-sql">WHERE (
        (LEN('TestPerson') = 0 
             AND co.personentered = co.personentered
        ) 
        OR 
        (LEN('TestPerson') <> 0 
             AND co.personentered LIKE '%TestPerson')
      )</code>

While this approach can mitigate syntax errors, be aware that heavily nested CASE statements within the WHERE clause can hinder query optimization. The presence of these conditional expressions prevents the index from being used, potentially resulting in poor query performance.

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