Nested Join in Subquery and failing correlation - mysql

Question

I have 3 tables sc_user, sc_cube, sc_cube_sent
I wand to join to a user query ( sc_user) one distinct random message/cube ( from sc_cube ), that has not been sent to that user before ( sc_cube_sent), so each row in the result set has a disctinct user id and a random cubeid from sc_cube that is not part of sc_cube_sent with that user id associated there.
I am facing the problem that I seem not to be able to use a correlation id for the case that I need the u.id of the outer query in the inner On clause. I would need the commented section to make it work.
# get one random idcube per user not already sent to that user
SELECT u.id, sub.idcube
FROM sc_user as u
LEFT JOIN (
SELECT c.idcube, sent.idreceiver FROM sc_cube c
LEFT JOIN sc_cube_sent sent ON ( c.idcube = sent.idcube /* AND sent.idreceiver = u.id <-- "unknown column u.id in on clause" */ )
WHERE sent.idcube IS NULL
ORDER BY RAND()
LIMIT 1
) as sub
ON 1
I added a fiddle with some data : http://sqlfiddle.com/#!9/7b0bc/1
new cubeids ( sc_cube ) that should show for user 1 are the following : 2150, 2151, 2152, 2153
Edit>>
I could do it with another subquery instead of a join, but that has a huge performance impact and is not feasible ( 30 secs+ on couple of thousand rows on each table with reasonably implemented keys ), so I am still looking for a way to use the solution with JOIN.
SELECT
u.id,
(SELECT sc_cube.idcube
FROM sc_cube
WHERE NOT EXISTS(
SELECT sc_cube.idcube FROM sc_cube_sent WHERE sc_cube_sent.idcube = sc_cube.idcube AND sc_cube_sent.idreceiver = u.id
)
ORDER BY RAND() LIMIT 0,1
) as idcube
FROM sc_user u

Answer 1

without being able to test this, I would say you need to include your sc_user in the subquery because you have lost the scope
LEFT JOIN
( SELECT c.idcube, sent.idreceiver
FROM sc_user u
JOIN sc_cube c ON c.whatever_your_join_column_is = u.whatever_your_join_column_is
LEFT JOIN sc_cube_sent sent ON ( c.idcube = sent.idcube AND sent.idreceiver = u.id )
WHERE sent.idcube IS NULL
ORDER BY RAND()
LIMIT 1
) sub

Answer 2

If you want to get messagges ids that has not been sent to the particular user, then why use a join or left join at all ?
Just do:
SELECT sent.idcube
FROM sc_cube_sent sent
WHERE sent.idreceiver <> u.id
Then the query may look like this:
SELECT u.id,
/* sub.idcube */
( SELECT sent.idcube
FROM sc_cube_sent sent
WHERE sent.idreceiver <> u.id
ORDER BY RAND()
LIMIT 1
) as idcube
FROM sc_user as u

Answer 3

Got it working with NOT IN subselect in the on clause. Whereas the correlation link u.id is not given within the LEFT JOIN scope, it is for the scope of the ON clause. Here is how it works:
SELECT u.id, sub.idcube
FROM sc_user as u
LEFT JOIN (
SELECT idcube FROM sc_cube c ORDER BY RAND()
) sub ON (
sub.idcube NOT IN (
SELECT s.idcube FROM sc_cube_sent s WHERE s.idreceiver = u.id
)
)
GROUP BY u.id
Fiddle : http://sqlfiddle.com/#!9/7b0bc/48