mysql - 仅当两个相关表中都存在时才在具有多对多关系的中间表上选择计数（*）

Question

我有 3 个表users：courses和courseusers。Courseusers 是连接 courses.idCourse 和 users.idUser 的中间表。但是，中间表没有外键约束 and ON DELETE CASCADEor ON UPDATE CASCADE。

Users:
idUser|name

Courses:
idCourse|name

Courseusers:
id|idUser|idCourse

我的问题是，我如何获得订阅最多的前 3 门课程（ 中的大多数条目），同时忽略和表courseuser中手动删除的用户（它们仍将作为条目存在）。userscoursescourseuser

我现在拥有的：

SELECT c.idCourse, c.name, count(*) as count 
FROM courseusers as cu 
     JOIN course as c 
         ON cu.idCourse=c.idCourse 
     JOIN users as usr 
         ON (usr.idUser=u.idUser) 
GROUP BY u.idCourse
ORDER BY count DESC 
LIMIT 3

Answer 1

尝试使用以下查询

SELECT c.idCourse, c.name, count(*) as count  
FROM courseusers as cu  
   LEFT JOIN course as c  
       ON cu.idCourse=c.idCourse  
   LEFT JOIN users as usr  
       ON (usr.idUser=u.idUser)  
GROUP BY u.idCourse 
ORDER BY count DESC  
LIMIT 3 

Answer 2

http://sqlfiddle.com/#!2/0567a/1

SELECT
  c.name,
  COUNT(1) AS total
FROM Courceusers cu
JOIN Cources c USING(idCource)
JOIN Users u USING(idUser)
GROUP BY 1
ORDER BY 2 DESC
LIMIT 3;

Answer 3

select 
    CourseUsers.idCourse, 
    Courses.name, 
    COUNT(distinct CourseUsers.idUser) as Subscribers
from CourseUsers
    inner join Courses on CourseUsers.idCourse = Courses.idCourse
    inner join Users on CourseUsers.idUser = Users.idUser
group by CourseUsers.idCourse, Courses.name
order by Subscribers desc
limit 3

Answer 4

基于Users不在中间表上的表连接所有表

SELECT  a.idUser, a.Name, COUNT(c.idCourse) totalCount
FROM    Users a
            INNER JOIN  CourseUsers b
                ON a.idUser = b.idUser
            INNER JOIN Courses c
                ON b.idCourse = c.idCourse
GROUP BY a.idUser, a.Name
ORDER BY totalCount DESC
LIMIT 3