Question

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'

Having the and clause after the where clause seems to turn the left join into an inner join. The behavior i am seeing is if there isnt ‘somethingelse’ in tableThree there will be 0 rows returned.

If i move c.foobar = ‘somethingelse’ into the join clause the stored join will act like a left join.

    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
    AND c.foobar = 'somethingelse'
    WHERE a.foo = 'something'

Can someone point me at some documentation describing why this happens? THank you very much

Answer 1

It’s because of your WHERE clause.

Whenever you specify a value from the right side of a left join in a WHERE clause (which is NOT NULL), you necessarily eliminate all of the NULL values and it essentially becomes an INNER JOIN.

If you write, AND (c.foobar = 'somethingelse' OR c.foobar IS NULL) that will solve the problem.

You can also move the c.foobar portion into your join predicate, and that too will solve the issue.