SQL问题，挑战

Question

假设我们有两个实体：教师和学生。

每位老师有多个学生。

现在我想：

查询最多5名教师，并为每一位教师，不超过他的学生10。

到目前为止，这可以很容易地通过做：

select *, 
(
select GROUP_CONCAT('<sid>',students.name,'</sid>') from students on 
teachers.id=students.teacher limit 10 
) as students 
from teachers limit 5 

但是，这并不是故事的全部呢。

和

如果教师的人已超过10名学生，应该返回true为老师，否则false

如何做到这一点的SQL？

Answer 1

伪SQL：结果每一行对应一个10名学生的每个5名教师的

select t.teacher_id, s.student_id, 
    case when t2.count > 10 then 'true' else 'false' end 
from 
    (select top 5 * 
    from teachers 
    order by teacher_id) t 
join 
    (select top 10 * 
    from students s1 join teachers t1 on s1.teacher_id = t1.teacher_id 
    order by student_id) s 
on t.teacher_id = s.teacher_id 
join 
    (select teacher_id, count(*) as count 
    from teachers t join students s on t.teacher_id = s.teacher_id 
    group by teacher_id) t2 
on t2.teacher_id = t.teacher_id 

Answer 2

LINQ makes this super-easy

Answer 3

使用子查询选择的每一位老师。