How Do NULL Values Affect the Results of NOT IN Clauses in SQL?

Understanding NULLs in SQL's NOT IN Clause

The behavior of NOT IN clauses in SQL is significantly affected by the presence of NULL values. This subtle difference can lead to unexpected results if not carefully considered. The following examples illustrate this behavior.

Example Query 1:

<code class="language-sql">select 'true' where 3 in (1, 2, 3, null)</code>

This query evaluates to:

<code>3 = 1 OR 3 = 2 OR 3 = 3 OR 3 = NULL</code>

Because 3 = 3 is true, the query returns a result.

Example Query 2:

<code class="language-sql">select 'true' where 3 not in (1, 2, null)</code>

This is equivalent to:

<code class="language-sql">3 <> 1 AND 3 <> 2 AND 3 <> NULL</code>

With ANSI_NULLS enabled (the standard setting in most SQL databases), 3 <> NULL evaluates to UNKNOWN. An AND condition with UNKNOWN results in UNKNOWN, and UNKNOWN is treated as false in a WHERE clause. Consequently, this query returns no rows.

Disabling ANSI_NULLS alters this behavior. Without ANSI_NULLS, 3 <> NULL might evaluate to true, causing the query to return a result. This highlights the inconsistency introduced by NULL values.

Conclusion:

When using NOT IN, always account for potential NULL values within the list. The presence of NULL significantly impacts the query's outcome due to the three-valued logic of SQL (TRUE, FALSE, UNKNOWN). It's recommended to avoid NOT IN with NULL values and use alternative approaches like NOT EXISTS for more predictable and reliable results.

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