05. 取SQL分組中的某幾行數據

對表中數據分組，有時只需要某列的聚合值；有時卻需要返回整行數據，常用的方法有：子查詢、ROW_NUMBER、APPLY，總體感覺還是ROW_NUMBER比較直觀。
測試數據：

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 
select 1,10 union all
select 1,20 union all
select 1,30 union all
select 2,100 union all
select 2,200 union all
select 3,1000 union all
select 3,2000 union all
select 3,3000 union all
select 3,4000

 

一. 取分組中第1行(最大/最小值)
1. 取出分組中某列最大/最小值，不要求顯示其他列
最常見的分組聚合，用group by 分組時，只有參加分組/聚合的列才可以被顯示。

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

 2. 取出分組中某列最大/最小值，要求顯示其他列

要顯示表中其他列，用group by 不好實現，可以借助子查詢。

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 1 ID from testGroup b where a.UserID = b.UserID order by b.OrderID desc)
--或者
select * from testGroup a
where not exists(select 1 from testGroup b where a.UserID = b.UserID and a.OrderID < b.OrderID)
--或者
select * from testGroup a
where (select count(1) from testGroup b where a.UserID = b.UserID and a.id <= b.id) = 1

 

二. 取分組中前N行(排名前幾名)
前N行為正向排序(ASC)，后N行改為反向排序(DESC)即可，N=1時也就是取最大/最小值的行。下面以前2名(N=2)為例。
1. SQL Server 2000的寫法
(1)子查詢

select * from testGroup as a
where a.ID in (select top 2 ID from testGroup b where a.UserID = b.UserID order by b.OrderID)
--或者
select * from testGroup a
where not exists (select 1 from testGroup b where a.UserID = b.UserID and a.OrderID > b.OrderID 
having count(1) >= 2)
--或者
select * from testGroup a
where (select count(1) from testGroup b where a.UserID = b.UserID and a.ID >= b.ID) <= 2
--沒有唯一標識的表，可以用checksum來標識每行
select * from testGroup as a
where checksum(*) in (select top 2 checksum(*) from testGroup b where a.UserID = b.UserID order by b.OrderID)

 2. SQL Server 2005新語法

(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 1 and 2

 (3) APPLY(TOP)

select distinct t.* from testGroup a
cross apply (select top 2 ID, UserID, OrderID from testGroup b
where a.UserID = b.UserID order by b.OrderID) as t

 

三. 取分組中第N行(排名第N名)
把上面的查詢中，范圍值都改為固定值，就可以取具體某一行了，下面以第3名(N=3)為例。
(1) 子查詢

select * from testGroup a
where (select count(1) from testGroup b where a.UserID = b.UserID and a.OrderID >= b.OrderID) = 3
--或者
select * from testGroup a
where exists (select 1 from testGroup b where a.UserID = b.UserID and a.OrderID >= b.OrderID 
having count(1) = 3)

 (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 = 3