ON vs. WHERE Clause in JOINs: When Should I Use Which?

ON Clause vs. WHERE Clause in JOIN Operations

When using INNER JOIN operations, both the ON clause and WHERE clause can be employed to filter data. While they may seem interchangeable in some cases, there are subtle differences that warrant attention.

ON Clause

The ON clause is used to specify specific conditions that must be met for rows to be included in the join result. It filters data at the join level, ensuring that only rows from both tables that satisfy the specified conditions are combined.

WHERE Clause

The WHERE clause is used to filter rows after the join operation has taken place. It applies conditions to the combined dataset, further refining the results.

Performance Considerations

In general, there is no significant performance difference between using the ON clause or WHERE clause for filtering data in INNER JOIN operations. However, there are exceptions.

Outer Joins

When using outer joins, such as LEFT OUTER JOIN, the placement of the filter condition can affect the results. Applying the filter in the ON clause will exclude rows from the right table (for LEFT OUTER JOIN) that do not meet the condition. On the other hand, applying the filter in the WHERE clause will filter rows from the combined dataset after the join, potentially returning more rows.

Example:

Consider the following example:

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

In this case, using the ON clause with the filter (b.IsApproved = 1) will exclude all rows from Bar that do not have IsApproved set to 1.

Alternatively,

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

This will first perform the join operation and then filter the resultset based on the IsApproved condition, potentially returning rows from Bar where IsApproved is NULL.

Other Considerations

• For simple inner joins, using the ON clause is generally preferred as it keeps the WHERE clause from becoming cluttered.
• Complex conditions that involve multiple joins or subqueries may be easier to manage in the WHERE clause.
• Index usage should be considered when selecting the best approach. A filter in the ON clause may utilize an index on the join column, while a filter in the WHERE clause may require a table scan.

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