How Do NULL Values Affect the Outcome of a NOT IN Clause?

Understanding NULLs and the NOT IN Clause

Inconsistencies often arise when using NOT IN clauses compared to LEFT JOINs, especially when dealing with NULL values. Let's examine this with two example queries:

Query A:

<code class="language-sql">SELECT 'true' WHERE 3 IN (1, 2, 3, NULL);</code>

Query B:

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

Query A produces a result, while Query B doesn't. This difference stems from how SQL handles comparisons involving NULL values.

Dissecting the Queries

Let's break down each query to understand the behavior:

Query A:

This query can be simplified to:

<code class="language-sql">SELECT 'true' WHERE 3 = 1 OR 3 = 2 OR 3 = 3 OR 3 = NULL;</code>

Because 3 = 3 evaluates to TRUE, the query returns a result.

Query B:

This query simplifies to:

<code class="language-sql">SELECT 'true' WHERE 3 != 1 AND 3 != 2 AND 3 != NULL;</code>

With ANSI_NULLS enabled (the default in most SQL databases), 3 != NULL evaluates to UNKNOWN. An AND condition with UNKNOWN results in the entire predicate being UNKNOWN, leading to no rows being returned.

If ANSI_NULLS is disabled, 3 != NULL evaluates to TRUE, potentially returning a row.

Conclusion

The presence of NULL values within a NOT IN clause significantly impacts query results due to the three-valued logic of SQL (TRUE, FALSE, UNKNOWN) and the setting of ANSI_NULLS. It's crucial to be aware of this behavior when working with NULL values to avoid unexpected outcomes. Using LEFT JOIN and IS NULL checks often provides a more reliable alternative for handling NULL values in such scenarios.

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