mysql - 寻找只教一个类的老师

Question

我想问我要在查询中添加什么来选择只教一门课的类(class)。我的 table 是:

Teacher                         Course
ID    Name                      Course_ID   Teacher_ID
1     George                    1           1
2     Sam                       2           2
3     Julie                     3           1

我想获取只教过一个类的老师ID。我有:

select teacher.id
from teacher, course
where teacher.id = course.teacher_id;

我正在考虑添加

having (count(course.teacher_id)) = 1

或

where count(t) = (select count(*) from course) and t = 0

但我收到“无效使用组错误”。如何更改它来修复我的查询？

Answer 1

你已经尝试过的差不多了，只需添加group by，并使用join而不是,来合并表格:

select teacher.id
from teacher
join course
on teacher.id = course.teacher_id
group by teacher.id
having (count(course.Course_ID)) = 1 -- note count(Course_ID), not count(teacher_id)