Question

我有五张桌子：1。课程2.departmetns 3.students 4. enroll_courses 5.results

Departments has id
Courses has id, department_id
Students has id,department_id
Enroll_courses has id, courses_id,students_id
Results has id, courses_id,students_id

我必须选择在enroll_courses表中但不在结果表中的courses.name。

我写了查询，我无法得到确切的答案。如果有人可以帮我找到解决方案。

查询：

SELECT courses.name 
FROM courses 
    JOIN departments ON courses.department_id = departments.id 
    JOIN students ON departments.id = students.department_id 
WHERE students.id = 9
  AND courses.id IN (SELECT course_id FROM enroll_courses)

Answer 1

由于你没有提供任何DDL's，它有点盲目拍摄，但我认为results表有一个course_id列，如果没有，则将其更改为关系。您可以使用EXISTS()和NOT EXISTS()：

执行此操作

SELECT courses.name 
FROM courses 
    JOIN departments ON courses.department_id = departments.id 
    JOIN students ON departments.id = students.department_id 
WHERE EXISTS(SELECT 1 FROM enroll_courses ec WHERE ec.course_id = courses.id)
  AND NOT EXISTS(SELECT 1 FROM results r WHERE r.course_id = courses.id)
  AND students.id =9

MySql查询：如何编写内部选择连接查询

1 个答案: