NOT IN vs. NOT EXISTS: When Should You Use Which SQL Operator?

NOT IN and NOT EXISTS: performance and semantic pitfalls

In SQL queries, the choice of using NOT IN or NOT EXISTS affects performance and query semantics. This article explores the differences between these two operators and provides guidance on when to use which operator.

Query speed: Execution plan

While the execution plan may indicate that NOT IN and NOT EXISTS perform similarly when both columns participating in the comparison are non-null, this is not always the case.

Semantic Difference: Handling NULL values

The main difference between NOT IN and NOT EXISTS is how they handle NULL values. The semantics of NOT IN can be misleading if any of the columns participating in the comparison allow NULL values. Specifically, NOT IN returns TRUE if either column is NULL, regardless of the value of the other column.

In contrast, NOT EXISTS always returns TRUE if either column is NULL, but also adds an additional condition to ensure that there are no other rows where both columns are non-NULL and match. This ensures that even if the first EXISTS check returns NULL, the query continues to search for matching values.

Performance impact of NULL values

This difference in behavior can have a significant impact on performance. The NOT IN version requires additional checks if there are NULL values ​​in any column and may result in a more expensive query plan. Furthermore, the introduction of NULL values ​​makes cardinality estimation difficult, resulting in inefficient execution plans.

Recommended form: Always use NOT EXISTS

Given the potential performance and semantic pitfalls of NOT IN, it is recommended to use NOT EXISTS as the first choice. By default, NOT EXISTS handles NULL values ​​correctly and is less prone to performance degradation when NULL values ​​are present.

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