一个数据库查询问题的解决方案

/*

Author:Roboth

Subject:select one item from months[一月取一条的问题]

*/

/*

CSDN的一个问题

现在有表A和表B

表A

userid username

1 张三

2 李四

表B

userid userlevel

1 1

1 2

1 3

2 1

2 2

现在要用一条sql查询 或者用视图搞定表搞定表A中userid 在表B中userlevel最高的一条 然后输出userid username和usertoplevel.

userid username usertoplevel

1 张三 3

2 李四 2

*/

我的解决方式

--Prepare data

DECLARE @A TABLE(userid int,username varchar(5))

INSERT INTO @A

select 1,'张三 '

union all

select 2,'李四 '

DECLARE @B TABLE(userid int,userlevel int)

INSERT INTO @B

select 1,1

union all

select 1,2

union all

select 1,3

union all

select 2,1

union all

select 2,2

--Method1

select

MAX(A.username),

B.userid,MAX(B.userlevel) as TopLevel

from @B B

INNER JOIN @A A

ON A.userid=B.userid

group by B.userid

--Method2

select A.username,B.*

from @B B

INNER JOIN @A A

ON A.userid=B.userid

where

not exists(select 1 from @B B1 where B1.userlevel>B.userlevel and B1.userid=B.userid)

--Method3

select A.username,B.*

from @B B

INNER JOIN @A A

ON A.userid=B.userid

where

1 > (select COUNT(1) from @B B1 where B1.userlevel>B.userlevel and B1.userid=B.userid)

--Method4

SELECT A.*,(select MAX(B.userlevel) from @B B where B.userid=A.userid)

from @A A

--Method5

SELECT A.*,(select top 1 (B.userlevel) from @B B where B.userid=A.userid order by B.userlevel DESC)

from @A A

--Method6

select A.*,B.userlevel

from @A A

inner join

(

SELECT userid,MAX(userlevel) as userlevel

from @B

group by userid

) B

on A.userid=B.userid