具有两个联接的SQL查询COUNT

Question

I'm trying to make a SQL Query from 3 tables (messages, user_comment_messages and user_like_messages), I need to get a list of each message with the total number of likes and comments, actually I'm trying with this:

SELECT m.id, COUNT(c.id) AS t_comment, COUNT(l.id) AS t_like
FROM messages m
JOIN user_comment_messages c ON c.message_id = m.id
JOIN user_like_messages l ON l.message_id = m.id
GROUP BY m.id;

When I execute the query I get the same value in the two columns and is incorrect. So I've tried with a subquery:

SELECT m.id,
(SELECT COUNT(*) FROM user_comment_messages c WHERE m.id = c.message_id) AS t_like,
(SELECT COUNT(*) FROM user_like_messages l WHERE m.id = l.message_id) AS t_comment
FROM messages m
ORDER BY m.id

This works well, but on my development environment I can't use subselects, only joins. Is there another way to accomplish that?

Thanks!

Answer 1

I think you can use distinct in count function to count distinct occurrences of commentid s and likeid s:

SELECT m.id, 
       COUNT(DISTINCT c.id) AS t_comment, 
       COUNT(DISTINCT l.id) AS t_like
FROM messages m
LEFT JOIN user_comment_messages c ON c.message_id = m.id
LEFT JOIN user_like_messages l ON l.message_id = m.id
GROUP BY m.id;

Answer 2

You use the LEFT JOIN, by this you can get all message either having like or comment or have no like or comment.

SELECT m.id, 
       IFNULL(COUNT(DISTINCT c.id),0) AS t_comment, 
       IFNULL(COUNT(DISTINCT l.id),0) AS t_like
FROM messages m
LEFT JOIN user_comment_messages c ON c.message_id = m.id
JOIN user_like_messages l ON l.message_id = m.id
GROUP BY m.id;