WHERE vs. ON Clause in JOIN Queries: When Should I Use Which for Optimal Performance?

WHERE Clause vs ON in JOIN Query: Performance Implications

When using JOIN queries in T-SQL, it's common to encounter the choice between placing filtering conditions in the WHERE clause or the ON clause. While both approaches can yield the same result set, there are subtle performance differences to consider.

Specifically, let's examine the following query:

SELECT *
FROM Foo f
INNER JOIN Bar b ON (b.BarId = f.BarId);
WHERE b.IsApproved = 1;

Compared to:

SELECT *
FROM Foo f
INNER JOIN Bar b ON (b.IsApproved = 1) AND (b.BarId = f.BarId);

Both queries return identical sets of rows, but the performance characteristics differ.

In general, placing filters in the ON clause is preferred for the following reasons:

• Optimizes join operations: The ON clause is evaluated before any rows are joined, allowing the database to selectively join only rows that meet the specified criteria. This results in a more efficient join process.
• Minimizes result set size: By filtering in the ON clause, you reduce the number of rows that are subjected to the WHERE clause evaluation, leading to a potentially smaller result set.

However, there are cases where placing filters in the WHERE clause is necessary:

• Outer joins: Outer joins include rows from the left table even if there are no matching rows in the right table. Placing filters in the WHERE clause allows you to filter the result set after the join has occurred.
• OPTIONAL filtering: OPTIONAL filters allow rows to be returned even if the join condition is not met. For example, the following query requires b.IsApproved to be NULL or 1:

SELECT *
FROM Foo f
LEFT OUTER JOIN Bar b ON (b.BarId = f.BarId)
WHERE (b.IsApproved IS NULL OR b.IsApproved = 1);

Conclusion:

When performance is a concern, consider placing filtering conditions in the ON clause to optimize join operations and reduce result set size. For outer joins or OPTIONAL filtering, the WHERE clause must be used to achieve the desired filtering.

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