从具有连接和空值的 4 个表中计数

Question

表格：

 Posts           Comments           Likes            Users
ID | User_id     ID | Post_id      ID | Post_id     ID | Name
-------          --------          --------         --------
1  | 101          1 | 1             1 | 3          101 | 'Michael'
2  | 101          2 | 3             2 | 3          202 | 'Daniel'
3  | 303          3 | 3             3 | 3          303 | 'Scott'

我想查询的是：

ID | Total_comments | Total_likes | Name
1  |       1        |     0       | 'Michael'
2  |       0        |     0       | 'Michael'
3  |       2        |     3       | 'Scott'

然而这是我得到的：

ID | Total_comments | Total_likes | Name
1  |       1        |     0       | 'Michael'
2  |       0        |     0       | 'Michael'
3  |       6        |     6       | 'Scott'

使用此查询：

SELECT Posts.ID,
       COUNT(Comments.Post_id) as Total_comments, 
       COUNT(Likes.Post_id) as Total_likes,
       Users.Name
       FROM Posts INNER JOIN Users 
       ON Users.ID = Posts.User_id 
       LEFT JOIN Comments 
       ON Comments.Post_id = Posts.ID 
       LEFT JOIN Likes 
       ON Likes.Post_id = Posts.ID 
       GROUP BY Posts.ID

我想知道，上面的查询可以通过什么方式修改为 return 预期的结果？实现此目标的好方法是什么？

Answer 1

您正在沿两个维度连接，因此您将获得笛卡尔积。这就是发生的事情。

一个棘手的解决方案——如果对给定的 post 没有太多评论和点赞，它会很好地工作——是使用 COUNT(DISTINCT):

SELECT Posts.ID,
       COUNT(DISTINCT Comments.Post_id) as Total_comments, 
       COUNT(DISTINCT Likes.Post_id) as Total_likes,
       Users.Name

不过，最有效的解决方案可能是相关子查询：

SELECT p.ID,
       (SELECT COUNT(*) FROM Comments c WHERE c.Post_Id = p.ID) as Total_comments, 
       (SELECT COUNT(*) FROM Likes l WHERE l..Post_id = p.ID) as Total_likes,
       u.Name
FROM Posts p INNER JOIN
     Users u
     ON u.ID = p.User_id ;

这样更快，因为它避免了外部聚合。但是，它需要 comments(post_id) 和 likes(post_id).

上的索引

从具有连接和空值的 4 个表中计数

Count from 4 tables with join and null values

mysql

sql

database

count

left-join