SQL删除表中最旧的记录

Question

我正在寻找一个在oracle表上运行的SQL查询，该表将在表中保留n条记录并删除其余的

我尝试了以下

delete from myTable where pk not in 
(SELECT pk FROM myTable where rownum <5 order by created DESC)

但似乎我在嵌套选择中不能有order by。

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Answer 1

当您使用ORDER BY和ROWNUM时，首先应用ROWNUM，因此您无法获得预期的结果。您可以将SQL修改为：

delete from myTable where pk not in 
( SELECT pk FROM 
   ( SELECT pk FROM myTable  order by created DESC)
  where rownum <5
)

还有很多其他方法可以写这个。如果表很大并且大多数行将被删除，那么这可能会更快：

delete from myTable where created < 
( SELECT MIN(created) FROM 
   ( SELECT created FROM myTable order by created DESC)
  where rownum <5
)