How to Correctly Use CASE Statements in SQL Server 2008 WHERE Clauses?

SQL Server 2008 WHERE Clause: Proper CASE Statement Usage

Employing CASE statements within SQL Server 2008 WHERE clauses demands precise syntax for correct execution. The following illustrates common errors and their solutions.

An example of incorrect CASE statement placement 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 
    AND cc.ccnum = CASE LEN('TestFFNum')
        WHEN 0 THEN cc.ccnum 
        ELSE 'TestFFNum' 
    END </code>

The problem is the attempt to assign values within the CASE statement using = or LIKE. The CASE statement should be used for conditional value generation, not assignment. The corrected approach uses Boolean logic:

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

This revised query correctly evaluates the conditions based on the length of 'TestPerson'.

Date comparisons also require careful consideration. Instead of multiple nested CASE statements, using separate OR conditions is often clearer and more efficient:

<code class="language-sql">WHERE (LEN('2011-01-09 11:56:29.327') = 0 AND co.DTEntered = co.DTEntered)
   OR (LEN('2011-01-09 11:56:29.327') > 0 AND LEN('2012-01-09 11:56:29.327') = 0 AND co.DTEntered >= '2011-01-09 11:56:29.327')
   OR (LEN('2011-01-09 11:56:29.327') > 0 AND LEN('2012-01-09 11:56:29.327') > 0 AND co.DTEntered BETWEEN '2011-01-09 11:56:29.327' AND '2012-01-09 11:56:29.327')</code>

By employing these corrected syntaxes, your SQL Server 2008 queries incorporating CASE statements within WHERE clauses will execute as intended.

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