Referencing outer query's tables in a subquery

Question

Is it possible to reference an outer query in a subquery with MySQL? I know there are some cases where this is possible:

SELECT *
FROM table t1
WHER         


        

Answer 1

Isn't this what you're after?

SELECT u.username, c._postCount
FROM User u
INNER JOIN (
    SELECT p.user, COUNT(*) AS _postCount
    FROM Posting p
    GROUP BY p.user    
) c ON c.user = u.id
WHERE u.joinDate < '2009-10-10';

The reason this will work is that the nature of the join itself will filter on user. You don't need to have a WHERE clause explictly filtering on user.

Answer 2

This would work fine

SELECT u.id as userid,u.username, c._postCount
FROM User u
INNER JOIN (
    SELECT p.user, COUNT(*) AS _postCount
    FROM Posting p
    --# This is the reference I would need:
    WHERE p.user = userid
    GROUP BY p.user
) c ON c.user = u.id
WHERE u.joinDate < '2009-10-10';

Answer 3

This solution is for postgresql. You could use LATERAL JOIN which is available in postgresql. Here is how you could use it in your query.

SELECT u.username, c._postCount
FROM User u
INNER JOIN LATERAL (
    SELECT p.user, COUNT(*) AS _postCount
    FROM Posting p
    WHERE p.user = u.id
    GROUP BY p.user
) c ON c.user = u.id
WHERE u.joinDate < '2009-10-10';

Here is a reference you could use. https://medium.com/kkempin/postgresqls-lateral-join-bfd6bd0199df

Answer 4

This is probably better:

SELECT u.username,
(SELECT COUNT(*) FROM Posting WHERE user = u.id) as _postCount
FROM User u WHERE u.joinDate < '2009-10-10';

Answer 5

i think that won't work, because you're referencing your derived table 'c' as part of a join.

however, you could just take out the WHERE p.user = u.id though and replace with a GROUP BY p.user in the derived table, because the ON c.user = u.id will have the same effect.