LEFT OUTER JOIN vs. NOT EXISTS: Which Performs Better in SQL Server?

LEFT OUTER JOIN vs. NOT EXISTS: Performance Considerations

When comparing data between two tables, selecting elements that exist in one but not the other can be achieved through either a LEFT OUTER JOIN or a NOT EXISTS subquery. While some may speculate that ANSI-compliant JOINs, such as LEFT OUTER JOINs, perform better on SQL Server, the actual performance depends on various factors.

Generally, if the fields involved have appropriate indexes and you anticipate filtering out a substantial portion of the data, NOT EXISTS tends to offer better performance. This is because NOT EXISTS, like EXISTS, leverages short-circuiting: as soon as a match is found for a record, it is either included or excluded, and the optimizer proceeds to the next record.

In contrast, LEFT JOINs retrieve all records regardless of whether they match, subsequently filtering out non-matching ones. This can be resource-intensive if the tables are large or multiple join criteria are specified.

Consequently, NOT EXISTS and EXISTS are generally preferred when possible. For SQL Server specifically, the operators IN and NOT IN, which are semantically equivalent to NOT EXISTS, may be more convenient to write and provide guaranteed short-circuiting.

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