Question

i use a sql query like this to get some results i need:

SELECT
    *
FROM
    pictures p
WHERE
    p.id NOT IN 
        (
            SELECT
                picture_id
            FROM
                guesses g
            WHERE 
                g.user_id = XXX 
        )
    AND
        p.user_id != XXX
;

Relation is as follows: A user has many pictures and a picture belongs to one user. A user has many guesses and a guess belongs to one picture. The tricky part is that a user is only allowed one guess for the same picture.

XXX = $user_id

I guess that there is a way to rewrite this sub-select using a left join but i can’t get it working.

Can anyone help?

Anja

Answer 1

Because it is a NOT IN condition you should use a LEFT OUTER JOIN. This is the direct translation to left outer join of your query:

SELECT
    distinct p.*
FROM
    pictures p
LEFT OUTER JOIN
   guesses g      ON g.picture_id = p.id and g.user_id = XXX
WHERE
   p.user_id != XXX
   and g.user_id is null
;