What's the Best Technique: `NOT EXISTS`, `NOT IN`, or `LEFT JOIN WITH IS NULL` for Efficient Database Queries?

Differences and performance comparisons of

NOT EXISTS, NOT IN and LEFT JOIN WITH IS NULL

In

database query, NOT EXISTS, NOT IN and LEFT JOIN WHERE IS NULL can achieve similar functions, but their subtle differences will affect query efficiency.

NOT IN vs. NOT EXISTS

NOT INCompares a value to a list of other values, and if the value is not in the list, the condition becomes true. Unlike NOT IN, NOT EXISTS is a subquery that checks if any row in the subquery matches the condition and returns true if there is no match.

LEFT JOIN WITH IS NULL

LEFT JOINCombine data from two tables by matching rows based on common columns. If there are no matching rows in the right table, a NULL value is added to the result. WHERE IS NULL filters out rows with a value of NULL in the join column, effectively producing results similar to NOT IN or NOT EXISTS.

Performance Difference

The performance of these techniques depends on the specific implementation of the database:

• SQL Server: LEFT JOIN WHERE IS NULL is less efficient than NOT IN or NOT EXISTS.
• PostgreSQL: NOT IN is less efficient than NOT EXISTS or LEFT JOIN WHERE IS NULL.
• Oracle: The performance of the three methods is roughly the same.
• MySQL: NOT EXISTS is slightly less efficient than NOT IN or LEFT JOIN WHERE IS NULL.

Choose the right technology

Which technology to choose depends on the specific query needs:

• If the subquery is small and does not significantly impact performance, NOT IN should be preferred.
• More efficient when the subquery is large and may return a large number of rows. NOT EXISTS
It is recommended to use
• if additional information from the right table is required (even for unmatched rows). LEFT JOIN WHERE IS NULL

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