How Can I Optimize `CASE` Statements in SQL Server 2008's `WHERE` Clause?

Optimizing CASE Statements in SQL Server 2008 WHERE Clauses

Employing CASE statements within WHERE clauses in SQL Server 2008 can sometimes lead to performance issues or incorrect results. The key is understanding how to correctly integrate the CASE statement into the overall conditional logic.

Correct Usage:

A CASE statement should always be part of a larger comparison, not the sole comparison itself. The correct structure involves using the CASE statement to produce a value that is then compared:

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

Incorrect Usage (and why it fails):

The following example is flawed because the CASE statement doesn't produce a value for comparison; it attempts to control the entire comparison operation directly, which is not how CASE works in a WHERE clause:

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

A Better Approach: Using OR Statements

When you find yourself needing a CASE statement within a WHERE clause, refactoring into OR statements often provides a more efficient and readable solution:

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

Performance Implications:

Both CASE statements and complex OR conditions in WHERE clauses can hinder query optimization and index usage. Always strive for the most straightforward and efficient WHERE clause possible. Consider alternative query structures or indexing strategies to improve performance if necessary.

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