取SQL分组中某几行数据

常用的方法有：子查询、ROW_NUMBER、APPLY，总体感觉还是ROW_NUMBER比较直观

if OBJECT_ID('testGroup') is not null
drop table testGroup
GO
create table testGroup
(
ID int identity primary key,
UserID int,
OrderID int
)
GO
insert testGroup
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一. 取分组中第1行(最大/最小值)
1. 取出分组中某列最大/最小值，不要求显示其他列
最常见的分组聚合，用group by 分组时，只有参加分组/聚合的列才可以被显示。

select UserID, MAX(OrderID) as MaxOrderID
from testGroup
group by UserID

2. 取出分组中某列最大/最小值，要求显示其他列

要显示表中其他列，用group by 不好实现，可以借助子查询。

select * from testGroup a
where ID = (select MAX(ID) from testGroup b where a.UserID = b.UserID)
order by ID
--或者
select * from testGroup
where ID in (select MAX(ID) from testGroup group by UserID)
--或者
select * from testGroup as a
where a.ID in (select top  ID from testGroup b where a.UserID = b.UserID order by b.OrderID desc)
--或者
select * from testGroup a
where not exists(select  from testGroup b where a.UserID = b.UserID and a.OrderID < b.OrderID)
--或者
select * from testGroup a
where (select count() from testGroup b where a.UserID = b.UserID and a.id <= b.id) = 

二. 取分组中前N行(排名前几名)
前N行为正向排序(ASC)，后N行改为反向排序(DESC)即可，N=1时也就是取最大/最小值的行。下面以前2名(N=2)为例。
1. SQL Server 2000的写法
(1)子查询

select * from testGroup as a
where a.ID in (select top  ID from testGroup b where a.UserID = b.UserID order by b.OrderID)
--或者
select * from testGroup a
where not exists (select  from testGroup b where a.UserID = b.UserID and a.OrderID > b.OrderID
having count() >= )
--或者
select * from testGroup a
where (select count() from testGroup b where a.UserID = b.UserID and a.ID >= b.ID) <= 

2. SQL Server 2005新语法

(2) ROW_NUMBER()

select ID, UserID, OrderID
from
(select *, ROW_NUMBER() over(partition by UserID order by OrderID) num
from testGroup ) t
where t.num between  and 

(3) APPLY(TOP)

select distinct t.* from testGroup a
cross apply (select top  ID, UserID, OrderID from testGroup b
where a.UserID = b.UserID order by b.OrderID) as t

三. 取分组中第N行(排名第N名)
把上面的查询中，范围值都改为固定值，就可以取具体某一行了，下面以第3名(N=3)为例。
(1) 子查询

select * from testGroup a
where (select count() from testGroup b where a.UserID = b.UserID and a.OrderID >= b.OrderID) =
--或者
select * from testGroup a
where exists (select  from testGroup b where a.UserID = b.UserID and a.OrderID >= b.OrderID
having count() = )

(2) ROW_NUMBER()

select ID, UserID, OrderID
from
(select *, ROW_NUMBER() over(partition by UserID order by OrderID) num
from testGroup ) t
where t.num =