转自：http://www.cnblogs.com/wangpei/p/6600584.html
--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
/*
要查询每门课程的前2名成绩
001 1 97
003 1 93
003 2 95
002 2 80
004 3 87
003 3 85
如何实现？
*/
--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
/*
stuId subId grade
----- ----------- -----------
001 1 97
003 1 93
003 2 95
002 2 80
004 3 87
003 3 85
(6 row(s) affected)
*/
共有三种方案，从难易程度上讲我倾向于后两种，从查询逻辑思想上来讲后两种是一样的
select
*
from
StudentGrade t
where
(
select
count
(1)
from
StudentGrade
where
subid=t.subid
and
grade>t.grade)<=1
order
by
subId,grade
desc
我是这样理解的，看成两张表A和B，条件为A表的学科=B表的学科，
select
count
(1)
from
StudentGrade
where
subid=t.subid
and
grade>t.grade，返回A表的学科=B表的学科并且A表的成绩小于B表的成绩的影响行数，如果所影响的行数为零说明它的成绩是最高的，如果等于1的话就是最高的两个成绩。这就是查询条件，再按 subId,grade 排序。这种查询思想很值得我学习
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