Question

我有三张桌子：

用户：包含注册用户。
课程：包含可用课程。
userscourses：有两列：user_id和course_id。每当用户加入课程时，都会创建一个与该用户相关的新记录。

我想创建一个mysql查询;给定user_id，将获取用户加入的所有课程。此外，对于每个课程行，我想添加一个列，表示该课程中有多少用户。

我做了一个查询，但它没有给我想要的结果：

SELECT courses.id AS CourseID, courses.name AS Name, COUNT(usersCourses.userId) AS JoinedUsersNumber 
FROM courses, users, usersCourses 
WHERE users.id = courses.userId 
AND usersCourses.courseId = courses.id 
AND usersCourses.courseId=courses.id 
AND usersCourses.userId='31'

有人能告诉我如何实现这个目标或者告诉我我的查询有什么问题吗？

修改这是一个例子：

用户表

   id       |     username
--------------------------
   u1       |      user01
   u2       |      user02
   u3       |      user03
   u4       |      user04

课程表：

   id       |     name
--------------------------
   c1       |    course01
   c2       |    course02
   c3       |    course03
   c4       |    course04

userscourses表：

 userId     |    courseId
--------------------------
   u1       |      c1
   u1       |      c2
   u1       |      c3
   u2       |      c1
   u3       |      c1
   u4       |      c1
   u2       |      c2
   u3       |      c2

预期结果（user_id = u1）：

  name      |  Joined Users
--------------------------
 course01   |       4
 course02   |       3
 course03   |       1

Answer 1

加入userscourses表以获得该课程的所有用户： -

SELECT courses.id AS CourseID, courses.name AS Name, COUNT(usersCourses2.userId) AS JoinedUsersNumber 
FROM users
INNER JOIN usersCourses 
ON users.id = usersCourses.userId 
INNER JOIN courses
ON usersCourses.courseId = courses.id 
INNER JOIN usersCourses usersCourses2
ON usersCourses.courseId = usersCourses2.courseId
WHERE usersCourses.userId='31'
GROUP BY courses.id AS CourseID, courses.name AS Name

或者如果您希望该课程的用户数排除所选用户（即您想要其他用户数）： -

SELECT courses.id AS CourseID, courses.name AS Name, COUNT(usersCourses2.userId) AS JoinedUsersNumber 
FROM users
INNER JOIN usersCourses 
ON users.id = usersCourses.userId 
INNER JOIN courses
ON usersCourses.courseId = courses.id 
LEFT OUTER JOIN usersCourses usersCourses2
ON usersCourses.courseId = usersCourses2.courseId
AND users.id != usersCourses2.userId 
WHERE usersCourses.userId='31'
GROUP BY courses.id AS CourseID, courses.name AS Name

Answer 2

首先进行一个返回课程用户计数的查询：

 SELECT course.id, course.name, count(*) as JoinedUsersNumber 
 FROM course 
 JOIN usersCourses
 ON course.id=usersCourses.courseId
 GROUP BY course.id, course.name

然后将其用作子查询，如

SELECT 
  c.id AS CourseID, 
  c.name AS Name, 
  c.JoinedUsersNumber 
FROM 
     SELECT course.id, course.name, count(*) as JoinedUsersNumber 
     FROM course 
     JOIN usersCourses
     ON course.id=usersCourses.courseId
     GROUP BY course.id, course.name
) c
JOIN 
  usersCourses ON c.id = usersCourses.courseId = 
JOIN
  users ON users.id = usersCourses.userId   
WHERE usersCourses.userId=31

Answer 3

您的查询是

SELECT courses.id AS CourseID, courses.name AS Name, COUNT(usersCourses.userId) AS JoinedUsersNumber 
FROM courses, users, usersCourses 
WHERE users.id = courses.userId 
AND usersCourses.courseId = courses.id 
AND usersCourses.courseId=courses.id 
AND usersCourses.userId='31'

根据您的架构

您已加入users.id = courses.userId，但您的课程表中没有userId
您尚未使用其桥表创建用户的连接条件。你已经两次加入同一列

AND usersCourses.courseId = courses.id AND usersCourses.courseId = courses.id

希望这会对你有所帮助。

 SELECT courses.id AS CourseID, courses.name AS Name, COUNT(usersCourses.userId) AS JoinedUsersNumber 
        FROM courses, users, usersCourses 
        WHERE 
        usersCourses.courseId = courses.id 
        AND usersCourses.userId=users.userId 
        AND usersCourses.userId='31'
        Group By courses.name

Answer 4

这是我的SQL小提琴：http://sqlfiddle.com/#!9/6a134/4/0

SELECT *
FROM
  (
    SELECT
      userscourses.user_id   AS user_id,
      users.user_name        AS user_name,
      userscourses.course_id AS course_id,
      courses.course_name    AS course_name
    FROM
      users, courses, userscourses
    WHERE
      users.user_id = userscourses.user_id
      AND courses.course_id = userscourses.user_id
      AND userscourses.user_id = 1
  ) userTable
  LEFT JOIN
  (
    SELECT
      course_id,
      COUNT(user_id) AS sum_participant
    FROM
      userscourses
    GROUP BY
      course_id
  ) courseTable
    ON userTable.course_id = courseTable.course_id

Answer 5

这样可行：

Can't open perl script "script.pl" : No such file or directory

在多个表上执行mysql查询

5 个答案: