分组后取每组内排名的Top N的SQL语句

给个MySQL例子参考

-----查询每门课程的前2名成绩

CREATE TABLE StudentGrade(
stuId CHAR(4), --学号
subId INT, --课程号
grade INT, --成绩
PRIMARY KEY (stuId,subId)
)
GO
--表中数据如下
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '001 ',1,97);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '001 ',2,50);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '001 ',3,70);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '002 ',1,92);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '002 ',2,80);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '002 ',3,30);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '003 ',1,93);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '003 ',2,95);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '003 ',3,85);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '004 ',1,73);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '004 ',2,78);
INSERT INTO StudentGrade(stuId,subId,grade) VALUES( '004 ',3,87);
GO

--查看数据
select * from StudentGrade

--假如出现并列时，也只取两个同学的话。
--方法一：
select distinct *
from studentgrade as t1
where stuid in
(select top 2 stuid
from studentgrade as t2
where t1.subid=t2.subid
order by t2.grade desc)
order by subid, grade desc

--方法二：
select * from StudentGrade a where (select count(1) from studentGrade where subId=a.subId and grade> =a.grade) <=2

--方法三：
select * from StudentGrade t
where (select count(1) from StudentGrade where subid=t.subid and grade> t.grade) <=1
order by subId,grade desc

--结果

drop table StudentGrade