Question

我有一个包含3个字段的表：

ProfessorID    StudentID    Mark
P1             S1              9
P1             S2              8
P1             S3             10
P2             S1              7
P2             S2              2
P2             S3              4
P3             S4              5
P4             S1              6

教授可以教许多学生，反之亦然，学生可以向许多教授学习。当一个学生从教授那里学到东西时，他就会得到他的印记。

我的问题是显示教授至少2名学生的教授名单，以及2名从这些教授那里获得最佳分数的学生。例如，该表的查询结果为：

ProfessorID    StudentID    Mark
P1             S1              9
P1             S3             10
P2             S1              7
P2             S3              4

我尝试了一些解决方案，但它们无效正确。

我该如何正确地做到这一点？

Answer 1

declare  @table table (ProfessorID nvarchar(2), StudentID nvarchar(2),Mark int)

insert into @table
select 'P1', 'S1', 9
union all
select 'P1', 'S2', 8
union all
select 'P1', 'S3', 10
union all
select 'P2', 'S1', 7
union all
select 'P2', 'S2', 2
union all
select 'P2', 'S3', 4
union all
select 'P3', 'S4', 5
union all
select 'P4', 'S1', 6

select *
from @table o
where o.StudentID IN (select top 2 s.StudentID from @table s where s.ProfessorId = o.ProfessorId order by Mark DESC)
and o.ProfessorID IN (select p.ProfessorID from @table p group by p.ProfessorID having count(*) >= 2)

Answer 2

假设你想在SQL 2005（或2008）中使用它，这也可以正常工作

select *
from
(
select *
    ,rank() over  (partition by professorid order by mark desc) as ranking
    ,(select count(distinct studentid) 
        from marks m2 
        where m2.professorid = m1.professorid 
        group by professorid
        ) as students
from marks m1
) subs
where ranking < 3
and students > 2

-Edoode

Answer 3

我刚刚醒来......但是这里有：

select *
FROM 
   (select professorID, count(distinct studentID) as studentsTaught
    FROM table
    ) s
WHERE s.studentsTaught = 2
UNION ALL    
SELECT top 2 *
FROM table
WHERE professorID IN (select professorID
                      FROM (
                            select professorID, count(distinct studentID) as studentsTaught
                            FROM table
                            ) s
                      )
WHERE s.studentsTaught = 2
ORDER BY mark

应该工作......基本上你是在询问教授并把它们算作子查询，然后从那个子查询中选择那些有2名学生的人。对于他们的学生，你必须联合他们并找到相同的教授，但是ORDER BY足以让两个最好的学生。

Answer 4

SELECT ProfessorID, StudentID, MAX(Mark)
FROM table
GROUP BY ProfessorID, StudentID

如何在SQL中获得每位教授的两位最佳学生？

4 个答案: