Question

我有4张桌子。

1. Course 2. Semester 3. AssignTeacher 4. Teacher.

 Course has id, name, code.
 Semester has id, name, course_id.
 AssignTeacher has id,course_id,teacher_id.
 Teacher has id, name.

现在我想找到courses.code，courses.name，semesters.name和teachers.name 从上表。其中teachers.name应该与AssignTeacher表中具有id的教师一起提供。现在使用课程表，当没有教师ID与该课程而不是显示空值时，我可以设置一些默认值，如＆＃39;无＆＃39;在查询？

这是我直到现在写的查询，虽然我没有得到我的预期：

SELECT courses.code, courses.name, semesters.name,teachers.name 
FROM semesters 
JOIN courses on courses.semester_id = semesters.id 
JOIN assign_teachers on assign_teachers.course_id=courses.id
JOIN teachers on assign_teachers.id=teachers.id

Answer 1

如果我理解正确，请将选择列表更改为

COALESCE(teachers.name, 'None')

Answer 2

当然，您应该使用COALESCE()将null替换为所需值：

SELECT courses.code, courses.name, semesters.name,COALESCE(teachers.name,'None') as teacher_name
from semesters join courses on  courses.semester_id = semesters.id 
  join assign_teachers on assign_teachers.course_id=courses.id
   join teachers on assign_teachers.id=teachers.id

但是当你没有使用teacher.id时，你得到空值似乎很奇怪。因为你使用普通连接，所以应该过滤这个记录。我认为你应该使用LEFT加入：

SELECT courses.code, courses.name, semesters.name,COALESCE(teachers.name,'None') as teacher_name
from semesters join courses on  courses.semester_id = semesters.id 
  left join assign_teachers on assign_teachers.course_id=courses.id
   left join teachers on assign_teachers.id=teachers.id

MysQL：如何设置空行的默认值？

2 个答案: