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深入讲解如何删除仅为第一列相同的记录

来源:互联网  宽屏版  评论
2008-06-01 02:22:42

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))

 
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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