从mysql中的多个表中提取数据

Question

我的问题是写一个SQL查询来返回所有教过课程的老师的名字，其中至少有2名学生在课程中获得B-或更好。

我的表是：

Courses(id, name, teacher_id)
Grades(student_id, course_id, grade)
Students(id, name, email, password)
Teachers(id, name)

我使用了以下查询：

SELECT * FROM grades JOIN teachers WHERE grades.grade = 'B-'

但显示所有老师喜欢

Answer 1

您可以使用exists（）来检查每位教师是否存在超过2'b-'或更好的课程

所以你的查询应该是：

SELECT * FROM Teachers t
WHERE EXISTS(SELECT c.teacher_id from Courses c
             inner join grades g ON c.id= g.course_id
             where t.id = c.teacher_id and g.grade in('B-','B','A-','A','A+')
             group by c.teacher_id having count(*) > 1)

Answer 2

这应该有效：

Select distinct Teachers.name from Courses join Teachers on Courses.teacher_id = Teachers.id where Courses.id in (Select course_id from (Select course_id, count(student_id) as 'total' from Grades where grade = 'B-' or grade = 'B' or grade = 'B+' or grade = 'A-' or grade = 'A' group by course_id) a where a.total > 1)