轉自：http://www.cnblogs.com/wangpei/p/6600584.html
--查詢每門課程的前2名成績
CREATE
TABLE
StudentGrade(
stuId
CHAR
(4),
--學號
subId
INT
,
--課程號
grade
INT
,
--成績
PRIMARY
KEY
(stuId,subId)
)
GO
--表中數據如下
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
/*
要查詢每門課程的前2名成績
001 1 97
003 1 93
003 2 95
002 2 80
004 3 87
003 3 85
如何實現？
*/
--查看數據
select
*
from
StudentGrade
--假如出現並列時，也只取兩個同學的話。
--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
--結果
/*
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)
*/
共有三種方案，從難易程度上講我傾向於后兩種，從查詢邏輯思想上來講后兩種是一樣的
select
*
from
StudentGrade t
where
(
select
count
(1)
from
StudentGrade
where
subid=t.subid
and
grade>t.grade)<=1
order
by
subId,grade
desc
我是這樣理解的，看成兩張表A和B，條件為A表的學科=B表的學科，
select
count
(1)
from
StudentGrade
where
subid=t.subid
and
grade>t.grade，返回A表的學科=B表的學科並且A表的成績小於B表的成績的影響行數，如果所影響的行數為零說明它的成績是最高的，如果等於1的話就是最高的兩個成績。這就是查詢條件，再按 subId,grade 排序。這種查詢思想很值得我學習
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