SQL問題，挑戰

Question

假設我們有兩個實體：教師和學生。

每位老師有多個學生。

現在我想：

查詢最多5名教師，併爲每一位教師，不超過他的學生10。

到目前爲止，這可以很容易地通過做：

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，這並不是故事的全部呢。

和

如果教師的人已超過10名學生，應該返回true爲老師，否則false

如何做到這一點的SQL？

Answer 1

僞SQL：結果每一行對應一個10名學生的每個5名教師的

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

使用子查詢選擇的每一位老師。