How Does a WHERE Clause Affect a LEFT JOIN's Behavior in SQL?

Left Join's Behavior in WHERE Clause

In SQL, a left join is intended to include all rows from the left table, even if there are no matching rows in the right table. However, certain conditions can alter this default behavior.

Consider the following 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'

In this query, the left join between tableTwo and tableThree is effectively transformed into an inner join due to the condition in the WHERE clause, c.foobar = 'somethingelse'. This is because specifying a non-NULL value from the right side of a left join (here, 'somethingelse') in the WHERE clause eliminates any rows where the corresponding column is NULL, resulting in an inner join behavior.

Therefore, the query returns only rows from tableOne and tableTwo that have matching rows in tableThree with a non-NULL value for c.foobar. To maintain the desired left join behavior, consider the following solutions:

1. Include NULL in WHERE Clause:

Replace the condition c.foobar = 'somethingelse' with AND (c.foobar = 'somethingelse' OR c.foobar IS NULL) to include rows where foobar is either 'somethingelse' or NULL.

2. Move Condition to Join Predicate:

Move the c.foobar = 'somethingelse' condition from the WHERE clause to the join predicate:

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

By doing this, the query retains the left join behavior while still filtering out rows based on the value of c.foobar.

The above is the detailed content of How Does a WHERE Clause Affect a LEFT JOIN's Behavior in SQL?. For more information, please follow other related articles on the PHP Chinese website!