“每个Y的最新X”是一个经典的SQL问题，工作中经常碰到。当然不是“按Y分组求最新的X值”那么简单，要求最新X的那条记录或主键ID。用一条SQL语句可以简单的解决此问题。 生成实例表和数据：
代码如下:
/****** 创建表 ******/
if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[Table]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [dbo].[Table]
GO
Create TABLE [dbo].[Table] (
[ID] [int] IDENTITY (1, 1) NOT NULL ,
[Y] [nvarchar] (50) COLLATE Chinese_PRC_CI_AS NOT NULL ,
[X] [smalldatetime] NOT NULL
) ON [PRIMARY]
GO
--插入数据
Insert INTO [Table](Y, X) values('CCC', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('AAA', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('AAA', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('BBB', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('BBB', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('BBB', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('CCC', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('AAA', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('CCC', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('DDD', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('DDD', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('DDD', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('EEE', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('EEE', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('EEE', '2007-03-03 03:03:03')
GO
/****** 创建表 ******/
if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[Table]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [dbo].[Table]
GO
Create TABLE [dbo].[Table] (
[ID] [int] IDENTITY (1, 1) NOT NULL ,
[Y] [nvarchar] (50) COLLATE Chinese_PRC_CI_AS NOT NULL ,
[X] [smalldatetime] NOT NULL
) ON [PRIMARY]
GO
--插入数据
Insert INTO [Table](Y, X) values('CCC', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('AAA', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('AAA', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('BBB', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('BBB', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('BBB', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('CCC', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('AAA', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('CCC', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('DDD', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('DDD', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('DDD', '2007-03-03 03:03:03')
Insert INTO [Table](Y, X) values('EEE', '2007-01-01 01:01:01')
Insert INTO [Table](Y, X) values('EEE', '2007-02-02 02:02:02')
Insert INTO [Table](Y, X) values('EEE', '2007-03-03 03:03:03')
GO解决“每个Y的最新X”经典SQL问题：以下几种方法真是八仙过海
代码如下:
Select ID, Y, X
FROM [Table] T1
Where (NOT EXISTS
(Select 1
FROM [Table] T2
Where (T2.Y = T1 .Y) AND (T2.X > T1 .X or
T2.X = T1 .X AND T2.ID > T1 .ID)))
/*****************************************************************************/
Select *
FROM [Table]
Where ID IN
(Select MAX(T1.ID)
FROM [Table] T1 JOIN
(Select y, MAX(x) x
FROM [Table]
GROUP BY y) T2 ON T1.Y = T2.Y AND T1.x = T2.x
GROUP BY T1.y)
/*****************************************************************************/
Select T .ID, T .Y, T .X
FROM [Table] T INNER JOIN
(Select MAX(T1.ID) AS ID
FROM [Table] T1 JOIN
(Select y, MAX(x) x
FROM [Table]
GROUP BY y) T2 ON T1.Y = T2.Y AND T1.x = T2.x
GROUP BY T1.y) T2 ON T .ID = T2.ID
/*****************************************************************************/
Select *
FROM [Table] T1
Where ID IN
(Select TOP 1 ID
FROM [Table]
Where Y = T1.Y
orDER BY X DESC)
/*****************************************************************************/
Select *
FROM [Table] T1
Where (ID =
(Select TOP 1 ID
FROM [Table]
Where Y = T1.Y
orDER BY X DESC, ID DESC))
/*****************************************************************************/
/*****************************************************************************/
Select ID, Y, X
FROM [Table] T1
Where (NOT EXISTS
(Select 1
FROM [Table] T2
Where (T2.Y = T1 .Y) AND (T2.X > T1 .X or
T2.X = T1 .X AND T2.ID > T1 .ID)))
/*****************************************************************************/
Select *
FROM [Table]
Where ID IN
(Select MAX(T1.ID)
FROM [Table] T1 JOIN
(Select y, MAX(x) x
FROM [Table]
GROUP BY y) T2 ON T1.Y = T2.Y AND T1.x = T2.x
GROUP BY T1.y)
/*****************************************************************************/
Select T .ID, T .Y, T .X
FROM [Table] T INNER JOIN
(Select MAX(T1.ID) AS ID
FROM [Table] T1 JOIN
(Select y, MAX(x) x
FROM [Table]
GROUP BY y) T2 ON T1.Y = T2.Y AND T1.x = T2.x
GROUP BY T1.y) T2 ON T .ID = T2.ID
/*****************************************************************************/
Select *
FROM [Table] T1
Where ID IN
(Select TOP 1 ID
FROM [Table]
Where Y = T1.Y
orDER BY X DESC)
/*****************************************************************************/
Select *
FROM [Table] T1
Where (ID =
(Select TOP 1 ID
FROM [Table]
Where Y = T1.Y
orDER BY X DESC, ID DESC))
/*****************************************************************************
/效率嘛,在不同的字段建立索引速度都不尽相同,使用者见仁见智了.
第一种方法速度在各方面都不错,而且在Y列在建立索引，可以大大优化查询速度。
