Left Join vs. Inner Join: How Does a WHERE Clause Affect a Left Join's Results?

Left Join vs. Inner Join

In SQL, left joins and inner joins are two fundamental join types that yield distinct result sets. A left join returns all rows from the left table and matches rows from the right table based on a join condition. If no match is found, the missing values in the right table are represented as NULLs. In contrast, an inner join only returns rows that fulfill the join condition, excluding rows from both tables that do not satisfy that condition.

Left Join Behavior Modification

In the provided SQL query:

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'

The presence of the WHERE c.foobar = 'somethingelse' clause after the LEFT JOIN condition has a surprising effect. It appears to convert the left join into an inner join, resulting in no rows being returned if 'somethingelse' is not present in tableThree.

Explanation

This seemingly odd behavior arises from the evaluation order of SQL statements. The WHERE clause is processed after the joins have been performed. Therefore, when c.foobar = 'somethingelse' is specified in the WHERE clause, it effectively filters out all rows from the left join where c.foobar is NULL. As a result, only rows where c.foobar matches the specified value ('somethingelse') are retained in the final result set.

Solution

To maintain the desired left join behavior, there are two options:

1. Modify the WHERE clause to include OR c.foobar IS NULL, as shown below:

   WHERE a.foo = 'something'
   AND (c.foobar = 'somethingelse' OR c.foobar IS NULL)

   This allows rows where c.foobar is NULL to be included in the result set.

2. Move the c.foobar = 'somethingelse' comparison into the ON clause of the left join, ensuring that only rows satisfying that condition are included in the result set:

   LEFT JOIN tableThree AS c ON b.pk = c.fk AND c.foobar = 'somethingelse'

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