深入讲解如何删除仅为第一列相同的记录
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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