Does SQL WHERE Clause Utilize Short-Circuit Evaluation?

SQL WHERE Clause and Short-Circuiting: A Closer Look

SQL WHERE clauses employ Boolean logic to filter data based on specified criteria. A key question regarding these Boolean expressions is whether database systems employ short-circuit evaluation.

Short-circuit evaluation optimizes Boolean expression processing by halting evaluation as soon as the final result is known. This avoids unnecessary computations, potentially boosting performance.

Consider this example:

SELECT * 
FROM Table t 
WHERE @key IS NULL OR (@key IS NOT NULL AND @key = t.Key) 

If @key is NULL, the first condition (@key IS NULL) is true. The ANSI SQL Draft 2003 standard leaves the evaluation of the second condition as implementation-defined. However, database systems often optimize by skipping subsequent evaluations when the outcome is already determined. Therefore, the second part might not be evaluated, but this isn't guaranteed across all systems.

Short-circuit evaluation's behavior in SQL WHERE clauses varies by database system:

• SQL Server: Generally supports short-circuit evaluation.
• Oracle: Typically supports short-circuit evaluation.
• MySQL: Supports short-circuiting for certain operators (AND and OR), but not all (&& and ||). The behavior can be database version-specific.

The potential for short-circuiting should be considered when writing efficient SQL queries, but relying on it for correctness is generally discouraged due to the lack of consistent behavior across different database systems.

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