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深入讲解如何删除仅为第一列相同的记录

来源：赛迪网作者：若水 2008-03-25/14:29

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编程学院

Sql Server

技巧运用

正文

问题：关系数据库中手工输入表InputData中有如下记录:

TagName TimeStamp Value 
MM0008 2007-08-12 0:00:00 1000 
MM0008 2007-08-13 0:00:00 1001 
MM0008 2007-08-14 0:00:00 1002 
MM0008 2007-08-15 0:00:00 1003 
MM0008 2007-08-16 0:00:00 1004 
MM0008 2007-08-17 0:00:00 1005 
MM0008 2007-09-07 9:50:22 4444 
MM0008 2007-09-07 15:10:56 100 
MM0008 2007-09-07 15:11:56 200 
MM0008 2007-09-11 15:53:13 500 
MM0008 2007-09-11 15:54:51 500 
MM0008 2007-09-11 15:55:03 123 
MM0008 2007-09-11 15:58:08 500 
MM0008 2007-09-11 16:01:58 0 
MM0009 2007-09-11 15:58:08 501 
MM0009 2007-09-11 16:01:58 0 
MM0010 2007-09-11 15:58:08 502 
MM0010 2007-09-11 16:01:58 0 
MM0011 2007-09-11 15:58:08 503 
MM0011 2007-09-11 16:01:58 0 
MM0018 2007-09-11 15:58:08 504 
MM0018 2007-09-11 16:01:58 0 
MM0019 2007-09-11 15:58:08 505 
MM0019 2007-09-11 16:01:58 0 
MM0020 2007-09-11 15:58:08 506 
MM0020 2007-09-11 16:01:58 0 
MM0021 2007-09-11 15:58:08 507 
MM0021 2007-09-11 16:01:58 0 
MM0022 2007-09-11 15:58:08 508 
MM0022 2007-09-11 16:01:58 0 
MM0023 2007-09-11 15:58:08 509 
MM0023 2007-09-11 16:01:58 0 
MM0024 2007-09-11 15:58:08 510 
MM0024 2007-09-11 16:01:58 0 
MM0025 2007-09-11 15:58:08 511 
MM0025 2007-09-11 16:01:58 0

如何获得每个TagName离当前时间最近的一条记录，把它的值及其时间插入到实时数据库中？最后得到的记录集如下所示：

TagName TimeStamp Value 


MM0008 2007-09-11 16:01:58 0
MM0009 2007-09-11 16:01:58 0
MM0010 2007-09-11 16:01:58 0
MM0011 2007-09-11 16:01:58 0
MM0018 2007-09-11 16:01:58 0
MM0019 2007-09-11 16:01:58 0
MM0020 2007-09-11 16:01:58 0
MM0021 2007-09-11 16:01:58 0
MM0022 2007-09-11 16:01:58 0
MM0023 2007-09-11 16:01:58 0
MM0024 2007-09-11 16:01:58 0
MM0025 2007-09-11 16:01:58 0

解决方法：

1. 使用distinct：可以删除完全相同的记录，但是我现在只需要删除TagName列相同的记录，不是很好实现。

2. 使用group by

SELECT     Tagname, TimeStamp, Value
FROM         InputData
WHERE     (TimeStamp IN
(SELECT     MAX(TimeStamp) AS TimeStamp
FROM          InputData  
GROUP BY Tagname))

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