一条语句简单解决“每个Y的最新X”的经典sql语句- MSSQL

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MSSQL 中的一条语句简单解决“每个Y的最新X”的经典sql语句

出处:互联网 整理: 软晨网（RuanChen.com） 发布: 2009-09-13 浏览: 116 ::

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在查询结果中添加一列表示记录的行数的sql语句 LINQ to SQL:处理char(1)字段的方式会引起全表扫描问题

“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：

复制代码代码如下:

/****** 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

复制代码代码如下:

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。

在查询结果中添加一列表示记录的行数的sql语句 LINQ to SQL:处理char(1)字段的方式会引起全表扫描问题

MSSQL 中的 一条语句简单解决“每个Y的最新X”的经典sql语句

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MSSQL 中的一条语句简单解决“每个Y的最新X”的经典sql语句