Mysql JOIN/SUM 查询

Question

我有 2 张 table :

• 类(class):公开类(class)列表
• student_courses:已报名类(class)的学生

它们的描述如下:

courses:
id
name
approved_by

student_courses:
id
student_id
course_id
paid

我想列出所有类(class)，每门类(class)包含以下信息:类(class) ID 和报名该类(class)的学生人数。我尝试过这样的事情:

SELECT courses.id, SUM(student_courses.id)
FROM courses as courses
LEFT JOIN student_courses as student_courses on student_courses.course_id=courses.id
WHERE courses.approved_by != '0'

但这只返回 1 行。

Answer 1

SELECT courses.id
     , courses.name
     , COUNT(student_courses.id) AS number_of_students    ---COUNT(), not SUM()
FROM courses 
  LEFT JOIN student_courses  
    ON student_courses.course_id = courses.id
WHERE courses.approved_by <> '0'
GROUP BY courses.id

HAVING COUNT(student_courses.id) < courses.student_slots