JOIN vs. WHERE Clause for Filtering: Which SQL Query is Faster?

Filter Criteria in SQL Queries: Join vs. Where Clause

When querying data in SQL, there are two common approaches for applying filter criteria: the Join Criteria and the Where Clause. Developers often grapple with the question of which approach is more efficient.

The Question:

Which SQL query performs faster: one that applies the filter on the Join Criteria or on the Where Clause?

Analysis:

Consider two queries:

Query 1 (Filter on Join Criteria):

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

Query 2 (Filter on Where Clause):

SELECT *
FROM TableA a
INNER JOIN TableXRef x ON a.ID = x.TableAID AND a.ID = 1
INNER JOIN TableB b ON x.TableBID = b.ID;

Performance Comparison:

Empirically, tests reveal that the Where Clause is slightly faster than the Join Criteria. This is counterintuitive as one might expect the Join Criteria to reduce the result set sooner.

Logical Consistency:

Apart from performance, consider the logical consistency of each approach. Using the Where Clause ensures that the operation remains valid even if the Join is replaced with a Left Join.

Recommended Approach:

Based on performance and logical consistency, it is generally recommended to apply filter criteria on the Where Clause.

Example:

Consider the following query:

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

This query is logically sound and performs as well as the equivalent query with the filter on the Join Criteria.

Conclusion:

While the Join Criteria and Where Clause perform similarly from a performance standpoint, the Where Clause is logically more consistent and thus the preferred approach for applying filter criteria in SQL queries.

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