MySQL count, group by 和 join 查询

Question

我有两个表:users 和 comments，其中每个评论都属于一个用户。

用户

id  user  class
1   mike    A
2   joe     B
3   john    A

评论

id  user_id    comment
1     1      "some text"
2     2      "some text"
3     1      "some text"
4     2      "some text"
5     3      "some text"
6     2      "some text"
7     2      "some text"

我想计算每个用户类别中的评论数。 结果应该是:

A 3
B 4

我已经试过了:

SELECT users.class, count(*) AS count 
  FROM comments 
 GROUP BY comments.user_id.class
 ORDER BY count DESC;

但不起作用。

Answer 1

你需要像这样加入

Select u.class, count(*) 
From users u
Join comments c on c.user_id = u.id
Group by u.class