MySQL - 从双嵌套子查询引用外部表值

Question

The query I really need to execute is follows:

SELECT      u.points
        (SELECT COUNT(1) FROM (SELECT 1 FROM checkin c INNER JOIN wineries w ON w.id = c.winery_id WHERE c.user_id = u.id GROUP BY region_id) b) as states_visited
FROM        users u
GROUP BY u.id
ORDER BY points DESC

However, this causes the following error:

Unknown column 'u.id' in 'where clause'

I've tried with user-defined variables, no errors, but it's not actually referencing the user-defined variable value for some reason:

SELECT      @uid := u.id, u.points
        (SELECT COUNT(1) FROM (SELECT 1 FROM checkin c INNER JOIN wineries w ON w.id = c.winery_id WHERE c.user_id = @uid GROUP BY region_id) b) as states_visited
FROM        users u
GROUP BY u.id
ORDER BY points DESC

Any thoughts how I can make this work? Without the obvious resorting to doing two separate queries?

Answer 1

Why do you need the double nesting? And if the id is the primary key of table user , you don't need the GROUP BY either:

SELECT      u.points,
        (SELECT COUNT(1) FROM reviews WHERE user_id = u.id) AS review_count
FROM        users AS u
-- GROUP BY u.id
ORDER BY points DESC ;

You could also GROUP BY in a derived table - and then join:

SELECT      u.points,
            COALESCE(r.review_count,0) AS review_count
FROM        users AS u
    LEFT JOIN
        (SELECT user_id, COUNT(1) AS review_count
         FROM reviews
         GROUP BY user_id
        ) AS r
        ON  r.user_id = u.id
ORDER BY points DESC ;

or join and then GROUP BY :

SELECT      u.points,
            COUNT(r.user_id) AS review_count
FROM        users AS u
    LEFT JOIN
            reviews AS r
        ON  r.user_id = u.id
GROUP BY u.id, u.points
ORDER BY points DESC ;

---

The edited version is harder but can be done without double nesting, too:

SELECT      u.points,
        (SELECT COUNT(DISTINCT region_id) 
         FROM checkin c INNER JOIN wineries w ON w.id = c.winery_id 
         WHERE c.user_id = u.id 
        ) AS states_visited
FROM        users u
ORDER BY points DESC ;

Answer 2

It seems like you can do what you want with a simpe table join

SELECT u.id AS `id`, u.points AS `points`, COUNT(r.review_id) AS `review_count` /* or whatever the id record for reviews table is */
FROM users AS u
INNER JOIN reviews AS r
  ON u.id = r.user_id
GROUP BY `id`
ORDER BY `points` DESC

Answer 3

SELECT users.id, users.points, count(*)
FROM users, reviews
WHERE users.id = reviews.user_id
GROUP BY id,points
ORDER BY points DESC

sqlFiddle example