mysql - 关于分组 2 列的 SQL 选择查询

Question

我正在努力解决这个问题 - 看起来很简单

下表记录了哪个用户观看了哪个视频

topic_user_id | topic_id | user_id
     1        |   10     |   3
     2        |   10     |   4
     3        |   11     |   5
     4        |   11     |   3
     5        |   12     |   6
     6        |   13     |   6
     7        |   14     |   7
     8        |   11     |   8
     9        |   11     |   9
     10       |   14     |   10
     11       |   15     |   11
     12       |   16     |   11
     13       |   17     |   11

现在要了解有多少用户观看了特定视频 - 我有以下查询。

有多少用户观看了特定视频

select count(distinct(user_id)) as 'Number of Users',topic_id from topic_user
where user_id is not null
group by topic_id

输出

Number of Users | topic_id 
    2           |   10     
    4           |   11     
    1           |   12     
    1           |   13     
    2           |   14     
    1           |   15     
    1           |   16     
    1           |   17     

Read as: 2 users watched topic 10 , 4 watched topic 11 and so on

这工作正常 - 但我正在寻找的是找到:

有多少用户观看了 1 个视频
有多少用户观看了 2 个视频
有多少用户观看了 3 个视频

输出应该是这样的

Number of Users  | Number of Videos watched
      6          |       1
      2          |       2
      1          |       3

Read as - 6 people watched only 1 video, 2 people watched 2 videos and so on.

需要一些帮助。

提前致谢

Answer 1

可能有更简单的方法，但子查询会起作用

select
    videos as 'Number of videos',
    count(user_id) as 'Num of Users'
from (
   select
        count(distinct(topic_id)) as videos,
        user_id from topic_user
   group by
        user_id
 ) sub
 group by
    videos