LEFT OUTER JOIN vs. NOT EXISTS in SQL Server: Which Performs Better for Finding Missing Records?

SQL Server Performance: LEFT OUTER JOIN vs. NOT EXISTS

Dilemma:
When navigating a dataset, you may need to retrieve records present in table A but absent in table B. For this, two options arise: LEFT OUTER JOIN or NOT EXISTS. Given that SQL Server favors ANSI compliance, does the ANSI JOIN operator offer superior performance in this scenario? Are join operators inherently more efficient than NOT EXISTS in SQL Server?

Answer:
While Joe's and Quassnoi's insights provide valuable context, let's delve deeper into the performance differences:

NOT EXISTS vs. LEFT OUTER JOIN Performance:

• Indexing: If the fields involved are adequately indexed, NOT EXISTS often performs better, especially when a significant number of records match the subquery (i.e., exist in table B).
• Short-circuiting: Both EXISTS and NOT EXISTS short-circuit, meaning as soon as a record meets the criteria, it is included or excluded from the results, allowing the optimizer to move on to the next record.
• Resource consumption: LEFT OUTER JOIN joins all records, regardless of whether they match, and then filters out non-matches. This process can be resource-intensive for large tables or multiple join criteria.

Recommendation:

Generally, priorities should be:

1. Use NOT EXISTS (or equivalently, NOT IN) where possible due to guaranteed short-circuiting and potential performance gains.

2. Consider LEFT OUTER JOIN when:

   • You need to retain all records from both tables, even if they don't match.
   • The subquery involves complex criteria or numerous joins, making it inefficient to use NOT EXISTS.

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