MySQL: Multiple Counts within the same LEFT JOIN table not working

Question

I have these 3 tables:

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

I am trying a query to get the following results:

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

So far i am good with everything except the #3, ie replies count only (comments.parent_comment != 0)

This is the query:

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

The result i am having is "totalReplies" count is similar to "totalComments" count.

Any ideas how can i get this to work by getting correct totalReplies count?

Thanks!

Answer 1

COUNT() will count zeros. Use SUM(), a la:

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

Answer 2

I think there's a couple things going on here. The structure of your case statement as mentioned above could use a reworking

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

Try this. It takes care of your selective group by, and your COUNT misinterpretation on CASE struct.