JOIN Criteria vs. WHERE Clause Filtering: Which SQL Query Performs Better?

Which SQL Query Is Faster: Join Criteria or Where Clause Filter?

Original Question:

Which approach is more efficient: filtering on the join criteria or in the WHERE clause?

Performance Analysis:

Contrary to popular belief, testing has revealed that the WHERE clause filter is marginally faster than the join criteria filter. The difference in performance, however, is insignificant.

Query Execution Plans:

Both query structures result in identical execution plans, as they produce the same filtered result set.

Logical Consistency:

From a logical standpoint, it is preferable to apply the filter to the WHERE clause, as it still makes sense when the INNER JOIN is replaced with a LEFT JOIN:

SELECT  *
FROM    TableA a
LEFT JOIN
        TableXRef x
ON      x.TableAID = a.ID
LEFT JOIN
        TableB b
ON      x.TableBID = b.ID
WHERE   a.id = 1

This query will only return records where a.id is 1, regardless of the JOIN type. Therefore, the WHERE clause filter is more logically consistent in scenarios where lazy evaluation is required.

Conclusion:

While there is a slight performance advantage to using the WHERE clause filter, the choice between the two approaches should primarily consider logical consistency and code readability.

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