Upper table visibility in select with multiple subqueries - mysql

Question

I have a query:
SELECT b.user_id, b.active FROM users b
WHERE b.followers_count != (SELECT COUNT(*)
FROM (SELECT u.user_id
FROM user_follow uf,users u,user_follow_request ufr
WHERE
uf.following_id = b.user_id AND
uf.following_id = ufr.friend_id AND
ufr.status = 'approved' AND
ufr.user_id = u.user_id AND u.user_id != b.user_id AND u.active != 0
GROUP BY u.user_id) a)
AND b.active = -1 limit 5;
It has to select all the user_id's from users that have different followers_count in the column from the one being calculated by sql.
But the problem is that I'm receiving error message
Error Code: 1054. Unknown column 'b.user_id' in 'on clause'
What I'm doing wrong? Highly appreciate your help.

Answer 1

Did you mean uf.user_id = b.user_id instead of uf.following_id = b.user_id? Anyway, what user_follow table is for?
Agree to Strawberry, you'd better use explicit joins. They are much easier to read and understand. Here is what you may have mentioned (note that user_follow table is not used at all):
SELECT u.user_id, u.followers_count, IF(ISNULL(uafc.actual_fc),0,uafc.actual_fc) AS afc
FROM users AS u
LEFT JOIN (
SELECT u.user_id AS user_id, COUNT(*) AS actual_fc
FROM users AS u
JOIN user_follow_request AS ufr
ON ufr.friend_id = u.user_id
JOIN users AS fu
ON fu.user_id = ufr.user_id
WHERE ufr.user_id != u.user_id
AND ufr.status = 'approved'
AND fu.active != 0
GROUP BY u.user_id, u.followers_count
) AS uafc
ON uafc.user_id = u.user_id
WHERE u.followers_count != IF(ISNULL(uafc.actual_fc),0,uafc.actual_fc)
;
There may be some nicer solution for NULL workaround.
See it on SQLFiddle: http://sqlfiddle.com/#!2/71f6c/3