Left Joins and WHERE Clauses: How Do They Interact, and How Can I Avoid Unexpected Results?

The Quirks of Left Joins and WHERE Clauses

When performing data retrieval operations, developers often employ left joins to combine information from multiple tables. However, a common pitfall arises when using a WHERE clause in conjunction with a left join, potentially transforming the left join's behavior.

Consider the following SQL statement:

SELECT
a.foo
b.bar
c.foobar
FROM tableOne AS a
INNER JOIN tableTwo AS b ON a.pk = b.fk
LEFT JOIN tableThree AS c ON b.pk = c.fk
WHERE a.foo = 'something'
AND c.foobar = 'somethingelse'

Intuitively, one would expect this statement to perform a left join between tableThree and tableTwo, specifying a filter condition for c.foobar. However, the presence of the WHERE clause containing c.foobar = 'somethingelse' inadvertently changes the left join's characteristics.

When a value from the right side of a left join (in this case, c.foobar) is included in the WHERE clause, only rows where the value is NOT NULL are included in the result set. Thus, the left join effectively behaves like an inner join, eliminating all rows with NULL values for c.foobar.

To preserve the intended behavior of the left join in the above example, one can employ either of the following approaches:

• Rewrite the WHERE clause: Replace c.foobar = 'somethingelse' with a condition that allows for NULL values, such as AND (c.foobar = 'somethingelse' OR c.foobar IS NULL).
• Move the filter into the join predicate: Modify the LEFT JOIN clause as follows: LEFT JOIN tableThree AS c ON b.pk = c.fk AND c.foobar = 'somethingelse'. By placing the filter within the join itself, the left join's behavior remains unchanged, as NULL values in c.foobar are still preserved.

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