mySQL - 為每個帖子選擇2條最新評論

Question

我試圖限制每個帖子只有2條評論，
我在郵政表中選擇，我希望每個都有2條評論

架構：

帖子表

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

評論表

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

你能幫我朋友嗎，我很困惑！
在此之前，任何幫助將不勝感激。

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

在你的情況下

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

語法可能不完美，沒有時間創建小提琴。 但這有子查詢，應該得到與帖子相關的最新2條評論並將其加入帖子本身。 必須考慮這樣一個事實，即可能根本沒有任何評論，因此來自左連接的Is Null測試。

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

此查詢返回每個帖子的最后2條評論：

SQLFiddle演示

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)