SQL计数/查询援助

Question

所以我学习MySQL和我想做到以下几点：

对于每个教练名单他/她的名字和他/她辅导者的学生人数。

架构的相关部分是：

Person(ID, Name) 
Student(StudentID, MentorID) 
Instructor(InstructorID) 

两个InstructorID和StudentID地图Person.ID，并MentorID映射到InstructorID（每个学生都有导师的导师，和教员和学生人数）。

我已经尝试了以下无济于事：

select InstructorDetails.Name, count(Mentees) 
from Instructor i 
inner join Person as InstructorDetails 
    on InstructorDetails.ID = i.InstructorID 
inner join Student as Mentees 
    on Mentees.MentorID = i.InstructorID; 

任何建议：

select p.Name, count(select s.StudentID 
        from Student s 
        where s.MentorID = i.InstructorID) 
from Person p, Instructor i 
where p.ID = i.InstructorID; 

而且这个阅读StackOverflow的一些事情之后？

Answer 1

你缺乏GROUP BY您的查询，

SELECT InstructorDetails.Name, count(*) totalCount 
FROM Instructor i 
     INNER JOIN Person as InstructorDetails 
      ON InstructorDetails.ID = i.InstructorID 
     INNER JOIN Student as Mentees 
      ON Mentees.MentorID = i.InstructorID 
GROUP BY InstructorDetails.Name