Question

假设我们有两个实体：老师和学生。

每位老师都有多名学生。

现在我想：

最多可查询5名教师，每位教师最多不超过10名学生。

到目前为止，这可以通过以下方式轻松完成：

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

但这还不是整个故事。

和

如果任何一位教师的学生人数超过10人，则该教师应返回true，否则false

如何在SQL中执行此操作？

Answer 1

LINQ makes this super-easy

Answer 2

为每位所选教师使用子查询。

Answer 3

伪SQL：为5位教师中的每位教师分别为10名学生提供一行结果

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

SQL问题，挑战

3 个答案: