MySQL的：相同的左內多計數JOIN表不工作

Question

我有這3個表：

1. posts (num, title, createdDate) 
2. comments (num, post_num, parent_comment) 
3. likes (comment_num) 

我想查詢得到以下結果：

1. all posts 
2. comments count including replies 
3. replies count only (comments.parent_comment != 0) 
4. total likes count 
5. total participants in a post 

到目前爲止我是用好除了＃3的一切，即回覆數只（comments.parent_comment！= 0）

這是查詢：

SELECT 
posts.num, 
posts.title, 
DATEDIFF(NOW(),posts.createdDate) as NumOfDays, 
COUNT(comments.num) AS totalComments, 
COUNT(CASE WHEN comments.parent_comment=0 THEN 0 ELSE comments.parent_comment END) AS totalReplies, 


COUNT(likes.comment_num ) AS totalLikes, 
COUNT(DISTINCT comments.member_num) AS participants, 
cms_uploads.urlPath 

FROM posts 
LEFT JOIN comments ON comments.post_num = posts.num 
LEFT JOIN likes ON likes.comment_num = comments.num 


GROUP BY posts.num 
ORDER BY totalComments DESC 

我現在的結果是「totalReplies」 count類似於「totalComments」 count。

任何想法如何通過獲得正確的totalReply數來得到這個工作？

謝謝！

Answer 1

COUNT（）會計數爲零。使用SUM（），a：

SUM(CASE WHEN comments.parent_comment = 0 THEN 1 ELSE 0 END) AS totalReplies, 

Answer 2

我想這裏有一些事情正在進行。如上所述，您的案例陳述的結構可以使用修改

SELECT 
posts.num, posts.title, urlpath(? dont see this in any table), DATEDIFF(NOW(),posts.createdDate) as NumOfDays, 
SUM(comments.num) AS totalComments, 
SUM(CASE WHEN comments.parent_comment=0 THEN 1 ELSE 0 END) AS totalReplies, 
SUM(likes.comment_num ) AS totalLikes, 
COUNT(DISTINCT comments.member_num) AS participants, 
FROM posts 
LEFT JOIN comments ON comments.post_num = posts.num 
LEFT JOIN likes ON likes.comment_num = comments.num 
GROUP BY posts.num, posts.title, NumOfDays 
ORDER BY totalComments DESC 

試試這個。它會照顧你的選擇性組合，並且你的COUNT會在CASE結構上誤解。