Why Do NULL Values Cause Unexpected Results with SQL's NOT IN Clause?

Understanding the Unexpected Behavior of NULL Values and the NOT IN Clause in SQL

SQL queries often produce surprising outcomes when NULL values interact with the NOT IN operator. This discrepancy becomes clear when comparing a NOT IN query to an equivalent query using a LEFT JOIN.

Consider these 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 returns a result, while Query B does not. The root cause lies in how different SQL databases handle NULL values.

With ANSI_NULLS enabled (the default in many systems like SQL Server 2005), any comparison involving NULL, such as 3 = NULL, evaluates to UNKNOWN. This indeterminacy stems from the inherent uncertainty of NULL.

In Query B, the NOT IN operator encounters NULL in the subquery, resulting in an UNKNOWN evaluation. Since no other values match, the entire predicate becomes UNKNOWN, yielding an empty result set.

Conversely, disabling ANSI_NULLS alters the behavior. With ANSI_NULLS off, 3 = NULL evaluates to TRUE. SQL interprets NULL as potentially equal to everything, including itself. Consequently, Query B's predicate evaluates to TRUE, returning a result.

In short, the inconsistent results stem from the varying treatment of NULL values determined by the ANSI_NULLS setting. When ANSI_NULLS is on, comparisons with NULL produce UNKNOWN, leading to unexpected outcomes in queries utilizing NOT IN and NULL values. This highlights the importance of understanding NULL handling in SQL for accurate query results.

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