mySQL - 为每个帖子选择2条最新评论

Question

我试图限制每个帖子只有2条评论，我在帖子表中选择，我希望每个评论都有2条评论

架构：

帖子表

------------------------------------
  id   |   content   |    date     |
  25    |   hello     |  20/10/2013 |

评论表

------------------------------------------------
  id   |   content   |    post   |    date     |
  1    |   hello     |    25     |  20/10/2013 |

你可以帮我朋友吗，我很困惑！ 在此之前，感谢任何帮助。

Answer 1

MySQL supports the LIMIT keyword, which allows you to control how many rows are returned; ideal when displaying data over many pages. You can use LIMIT in your sql query like this

在你的情况下

select * from posts p join comments c on p.id=c.post and
      c.id> (select id from comments  where post=p.id order by id DESC LIMIT 2,1)

Answer 2

语法可能不完美，没有时间创建小提琴。但这有子查询，应该得到与帖子相关的最新2条评论并将其加入帖子本身。必须考虑这样一个事实，即可能根本没有任何评论，因此从左连接开始测试Is Null。

Select *
From Posts p
Left Outer Join Comments c
On c.post = p.id
Where 
    (        c.id Is Null
        Or    c.id In
        (
            Select c2.id
            From Comments c2
            Where c2.post = p.id
            Order by c2.id Desc
            Limit 2
        )
    )

Answer 3

此查询返回每篇帖子的最后2条评论：

SQLFiddle demo

select p.content post_content,
       c.content comment_content

from posts p
left join comments c on 
    (p.id=c.post)
    and 
    c.id>
    (select id from comments  
        where post=p.id
        order by id DESC LIMIT 2,1)