Question

这里的情景是：

我有4张桌子

Course
Student
Department
EnrollCourse

当学生要参加课程时，课程列表应该从课程表中下拉。但是已经在enroll_courses表中的course_id不应该加载到该课程下拉列表中。因为学生不能注册单个课程2次。在这里，我使用not in操作，它将检索不在enroll_courses表中的course_id列表。

但是我收到了以下错误信息：

您的SQL语法有错误;检查手册对应于您的MariaDB服务器版本，以获得正确的语法 near＆＃39; enroll_courses.course_id LIMIT 0,25＆＃39;在第6行

这是查询：

SELECT students.name, students.email,departments.name as d_name,
        courses.name as c_name
        FROM students JOIN departments on students.department_id=departments.id 
        JOIN courses on departments.id = courses.department_id
        LEFT JOIN enroll_courses on enroll_courses.course_id=courses.id 
        WHERE courses.id NOT IN (enroll_courses.course_id) AND students.id=8

有谁能帮我找到解决方案？

Answer 1

试试这个，

SELECT  students.name ,
        students.email ,
        departments.name AS d_name ,
        courses.name AS c_name
FROM    students
        JOIN departments ON students.department_id = departments.id
        JOIN courses ON departments.id = courses.department_id
        JOIN enroll_courses ON enroll_courses.course_id = courses.id
WHERE   students.id = 8
        AND courses.id NOT IN ( enroll_courses.course_id );

Answer 2

有多种方法可以获得所需的结果，这与原始查询非常接近：

SELECT  students.name ,
        students.email ,
        departments.name AS d_name ,
        courses.name AS c_name
FROM    students
JOIN departments
  ON students.department_id = departments.id
JOIN courses
  ON departments.id = courses.department_id
WHERE students.id = 8   
-- up to here it's all possible courses

-- now remove already enrolled courses
  AND courses.id NOT IN  
   (
     SELECT course_id
     FROM enroll_courses
     WHERE student_id = 8
   )

或相关版本：

  AND courses.id NOT IN  
   (
     SELECT course_id
     FROM enroll_courses
     WHERE student_id = students.student_id
   )

或不是EXISTS：

  AND NOT EXISTS 
   (
     SELECT *
     FROM enroll_courses
     WHERE student_id = students.student_id
       AND course_id = courses.course_id
   )

正确使用Not In子句

2 个答案: