sql - 获取最后一个不同的行？

Question

我有下表(表称为帖子):

id  datecreated                 threadid
1   2013-03-17 17:36:12.9281661 1
2   2013-03-17 17:56:18.3472836 1
3   2013-03-18 07:09:08.2733381 2
4   2013-03-18 07:35:58.1251661 3
5   2013-03-18 22:04:41.6053307 3
6   2013-03-19 03:30:56.0803165 3
7   2013-03-19 16:26:59.1518276 4
8   2013-03-19 16:27:47.4339124 4
9   2013-03-19 16:28:13.3455579 4
10  2013-03-19 16:55:16.3930089 5

我希望查询只返回 threadid 被列出三次的行，但我只想要每个 threadid 的一行，并且它必须是最后一行(按日期排序)。

我该怎么做？

Answer 1

select p.*
from posts p
inner join
(
  select threadid, max(datecreated) maxdate 
  from posts
  group by threadid
  having count(*) = 3
) x on x.threadid = p.threadid and x.maxdate = p.datecreated