Does SQL Guarantee Short-Circuit Evaluation in WHERE Clauses?

SQL WHERE Clause Evaluation: Short-Circuiting Explained

A frequent question regarding SQL's WHERE clauses concerns boolean expression evaluation: does SQL employ short-circuit evaluation? This means the evaluation stops as soon as the outcome is certain, potentially bypassing later parts of the expression.

ANSI SQL Standard and Evaluation Order

The ANSI SQL standard doesn't explicitly define short-circuit evaluation for WHERE clauses. The ANSI SQL Draft 2003 specifies that expression evaluation order is implementation-dependent. This leaves the decision of whether or not to implement short-circuiting to individual database vendors.

Database-Specific Implementation

Therefore, short-circuit behavior varies significantly across database systems:

• SQL Server: Supports short-circuit evaluation for AND and OR operators.
• Oracle: Doesn't support short-circuit evaluation by default, though it can be enabled using the OPTIMIZER_MODE hint.
• MySQL: Supports short-circuit evaluation for the AND operator only.

Illustrative Example

Consider this query:

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. In SQL Server, short-circuit evaluation would skip the second condition (@key IS NOT NULL AND @key = t.Key), as the overall result is already known.

Key Takeaway

The ANSI SQL standard doesn't guarantee short-circuit evaluation in WHERE clauses. Its presence and behavior are database-specific. Always consult your database system's documentation to understand its evaluation strategy.

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