left join turns into inner join

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.foo         


        

Answer 1

It doesn't turn a LEFT JOIN into an INNER JOIN, thought the effect may appear to be the same. When you set the WHERE condition

AND c.foobar = 'somethingelse'

you're getting rid of all the cases that allow a LEFT JOIN TO act as it does. In this case, some of the values for c.foobar will be NULL. Setting this on the JOIN condition still allows non-matching LEFT JOIN results, only restricting what is returned for C results.