跨越多個表的分層子查詢可以引用父查詢嗎？

Question

我有三個表格設置：帖子，討論和評論。佈局如下：

POSTS 
id | user_id | title | body | created 

DISCUSSIONS 
id | post_id | user_id | title | body | created 

COMMENTS 
id | post_id | discussion_id | user_id | title | body | created 

一個帖子有許多討論，然後有許多評論。

我需要工作的是一個查詢，它會給我所有的帖子，但是每個帖子中的用戶數涉及。現在，通過「參與」，我的意思是我需要對所有發佈過帖子討論的用戶進行計數，或者對帖子的討論發表評論。很顯然，我的查詢需要確保它不包含同樣post_id的DISCUSSIONS表中的COMMENTS表中的重複項。

我能得到這個由子查詢指定POST_ID這樣的「手動」工作：

SELECT posts.id AS posts_id, posts.user_id AS posts_user_id, posts.title AS posts_title, posts.created AS posts_created, users_profile.name AS users_profile_name,(anon_1.discussions_users_count + anon_1.comments_users_count) AS anon_1_users_count 
FROM users_profile, posts LEFT OUTER JOIN (
    SELECT discussions.post_id AS post_id, count(*) AS discussions_users_count, (
     SELECT count(discussion_comments.user_id) FROM discussion_comments WHERE discussion_comments.user_id NOT IN (
      SELECT discussions.user_id FROM discussions WHERE discussions.post_id = 1 GROUP BY discussions.user_id 
     ) 
     AND discussion_comments.post_id = 1 GROUP BY discussion_comments.user_id 
    ) AS comments_users_count FROM discussions GROUP BY discussions.post_id 
) AS anon_1 ON posts.id = anon_1.post_id WHERE posts.user_id = users_profile.user_id ORDER BY posts.created DESC LIMIT 10 OFFSET 0 

不過，這顯然沒有做任何我好，因爲我需要能夠有子查詢引用父查詢的post.id，以便每行返回每個帖子的適當計數。

這甚至可能嗎？需要做些什麼？

感謝， 賽斯

Answer 1

我會嘗試這一點。

SELECT post_id, count(DISTINCT user_id) 
FROM 
(
    SELECT id as post_id, user_id FROM posts 
    UNION 
    SELECT post_id, user_id FROM discussions 
    UNION 
    SELECT post_id, user_id FROM comments 
) a 
GROUP BY post_id