Question

我有两个表，用户和帖子。我正在尝试编写一个查找用户查找最新帖子的查询，但我遇到了麻烦。这是我到目前为止所拥有的。

select a.username, b.last_post from logins as a join (select login_id, entry as last_post from posts) as b where a.id = b.login_id

+-----------+---------------------+
| username  | last_post           |
+-----------+---------------------+
| something | 2013-10-08 22:12:00 |
| other     | 2013-10-08 22:13:00 |
| test      | 2013-10-08 22:13:03 |
| test      | 2013-10-08 22:14:20 |
| hello     | 2013-10-08 22:12:53 |
| hello     | 2013-10-08 22:12:56 |
+-----------+----------+----------+

所以现在last_post只是它拉动的帖子的时间戳。如何获取显示 ONLY 这些用户的最后一篇文章的表格？

Answer 1

如果您只需要两列，则可以直接使用MAX()

SELECT a.username, 
       MAX(b.entry) last_post 
FROM   logins a 
       INNER JOIN posts b 
         ON a.id = b.login_id
GROUP  BY a.username

否则，如果要显示所有表中的所有列，则可以使用子查询，为每个entry分别获取最新login_id

SELECT  a.*, b.*
FROM    logins a 
        INNER JOIN posts b 
            ON a.id = b.login_id
        INNER JOIN
        (
            SELECT  login_id, MAX(entry) entry
            FROM    posts
            GROUP   BY login_id
        ) c ON  b.login_id = c.login_id AND
                b.entry = c.entry

只收到最新的帖子

1 个答案: