NOT IN vs. NOT EXISTS: Which Subquery Conditional Should You Use?

Comparison of NOT IN and NOT EXISTS subquery conditions

When using subquery conditions, a common question is: Should I use NOT IN or NOT EXISTS? While both can achieve the same results, their performance characteristics and preferred usage scenarios may differ.

Execution Plan Comparison

In some cases, the execution plans of NOT IN and NOT EXISTS queries may look the same. However, certain data modifications may trigger additional processing of NOT IN.

NOT IN with nullable columns

NO IN will generally perform more operations than NOT EXISTS if either column in the comparison is nullable. This is because NOT IN must handle the possibility of NULL values, resulting in additional anti-semi-joins and more complex planning logic.

Consequences of additional processing

The additional processing required by NOT IN can have a significant impact on performance, especially when working with large data sets. This is because SQL Server's cardinality estimation becomes more difficult when NULL values ​​are present, potentially resulting in inefficient query execution.

Recommended form

Based on these considerations, it is usually recommended to use NOT EXISTS by default for subquery condition judgment. This is especially true if there is a possibility of NULL values ​​in any of the columns being compared, as NOT EXISTS will always perform more efficiently in these cases.

Other considerations

While it is generally preferred to use NOT EXISTS, it is important to consider the specific context of the query. If you are certain that NULL values ​​will never occur in the relevant columns, NOT IN may perform just as well, and in some cases may even be slightly faster. However, for most practical use cases, using NOT EXISTS is a more robust and reliable option.

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