查每个分组前N条记录

---------------------------------
--  Author: liangCK 小梁
--  Title : 查每个分组前N条记录
--  Date  : 2008-11-13 17:19:23
---------------------------------

--> 生成测试数据: #T
IF OBJECT_ID('tempdb.dbo.#T') IS NOT NULL DROP TABLE #T
CREATE TABLE #T (ID VARCHAR(3),GID INT,Author VARCHAR(29),Title VARCHAR(39),Date DATETIME)
INSERT INTO #T
SELECT '001',1,'邹建','深入浅出sqlServer2005开发管理与应用实例','2008-05-10' UNION ALL
SELECT '002','胡百敬','sqlServer2005性能调校','2008-03-22' UNION ALL
SELECT '003','格罗夫Groff.J.R.','sql完全手册','2009-07-01' UNION ALL
SELECT '004','KalenDelaney','sqlServer2005技术内幕存储引擎','2008-08-01' UNION ALL
SELECT '005',2,'Alex.Kriegel.Boris.M.Trukhnov','sql宝典','2007-10-05' UNION ALL
SELECT '006','飞思科技产品研发中心','sqlServer2000高级管理与开发','2007-09-10' UNION ALL
SELECT '007','sqlServer2005数据库开发详解','2008-06-15' UNION ALL
SELECT '008',3,'陈浩奎','sqlServer2000存储过程与XML编程','2005-09-01' UNION ALL
SELECT '009','赵松涛','sqlServer2005系统管理实录','2008-10-01' UNION ALL
SELECT '010','黄占涛','sql技术手册','2006-01-01'

--SQL查询如下:

--按GID分组,查每个分组中Date最新的前2条记录


--1.字段ID唯一时:
SELECT * FROM #T AS T WHERE ID IN(SELECT TOP 2 ID FROM #T WHERE GID=T.GID ORDER BY Date DESC)

--2.如果ID不唯一时:
SELECT * FROM #T AS T WHERE 2>(SELECT COUNT(*) FROM #T WHERE GID=T.GID AND Date>T.Date)

--sql Server 2005 使用新方法

--3.使用ROW_NUMBER()进行排位分组
SELECT ID,GID,Author,Title,Date
FROM
(
   SELECT rid=ROW_NUMBER() OVER(PARTITION BY GID ORDER BY Date DESC),*
   FROM #T
) AS T
WHERE rid<=2

--4.使用APPLY
SELECT disTINCT b.*
FROM #T AS a
CROSS APPLY
(
    SELECT TOP(2) * FROM #T WHERE a.GID=GID ORDER BY Date DESC
) AS b


--结果
/*

ID   GID         Author                        Title                                   Date
---- ----------- ----------------------------- --------------------------------------- -----------------------
003  1           格罗夫Groff.J.R.                 sql完全手册                                 2009-07-01 00:00:00.000
004  1           KalenDelaney                  sqlServer2005技术内幕存储引擎                   2008-08-01 00:00:00.000
005  2           Alex.Kriegel.Boris.M.Trukhnov sql宝典                                   2007-10-05 00:00:00.000
007  2           胡百敬                           sqlServer2005数据库开发详解                    2008-06-15 00:00:00.000
009  3           赵松涛                           sqlServer2005系统管理实录                     2008-10-01 00:00:00.000
010  3           黄占涛                           sql技术手册                                 2006-01-01 00:00:00.000

(6 行受影响)
*/