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,它将在表中保留n个记录并删除其余的

我尝试了以下内容

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

但似乎我不能order by在嵌套选择中.

任何帮助赞赏

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
)