深入講解如何刪除僅為第一列相同的記錄

2024-07-21 02:42:11

字體：大中小

來源：轉載

供稿：網友

問題：關系數據庫中手工輸入表InputData中有如下記錄:

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

如何獲得每個TagName離當前時間最近的一條記錄，把它的值及其時間插入到實時數據庫中？最后得到的記錄集如下所示：

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

解決方法：

1. 使用distinct：可以刪除完全相同的記錄，但是我現在只需要刪除TagName列相同的記錄，不是很好實現。

2. 使用group by

SELECT     Tagname, TimeStamp, ValueFROM         InputDataWHERE     (TimeStamp IN(SELECT     MAX(TimeStamp) AS TimeStampFROM          InputData  GROUP BY Tagname))

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