How Do NULL Values Affect the Results of SQL's NOT IN Clause?

SQL's NOT IN Clause and the Nuances of NULL Values

The NOT IN clause in SQL serves to exclude rows matching values within a specified list. However, the presence of NULL values within this list introduces complexities.

The Issue: Unexpected Results with NULL

Consider these SQL queries:

<code class="language-sql">A: SELECT 'true' WHERE 3 IN (1, 2, 3, NULL);
B: SELECT 'true' WHERE 3 NOT IN (1, 2, NULL);</code>

Query A yields a result, while Query B does not. This stems from how SQL handles NULL comparisons. Query A checks if 3 is equal to any value in the list. Since 3 equals 3, it returns a result.

Query B, conversely, checks if 3 is not equal to every value. With ANSI_NULLS enabled (the default in many systems), a comparison involving NULL results in UNKNOWN. UNKNOWN is neither true nor false; therefore, Query B returns no rows.

ANSI_NULLS Setting and its Influence

Disabling ANSI_NULLS alters NULL comparison behavior. With ANSI_NULLS off, a comparison with NULL returns TRUE if the values are unequal and FALSE if they are equal. Consequently, Query B would return a result with ANSI_NULLS disabled because 3 NOT IN (1, 2, NULL) would evaluate to TRUE.

Understanding the Nature of NULL

It's vital to remember that NULL signifies an unknown value. Comparing NULL to any value always yields UNKNOWN because equality or inequality cannot be definitively determined. When employing NOT IN with NULL values, potential issues must be anticipated, and appropriate techniques—such as explicit NULL checks or using COALESCE to replace NULL with a known value—should be implemented for robust query handling.

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