Mysql sql连接和计数

假设我有这个疑问

 SELECT ft.*, m.*
   FROM forum_topics ft
     INNER JOIN members m ON ft.author = m.id
     WHERE ft.forum =  '$forum'
     ORDER BY ft.lastpost DESC 

我还想从

forum\u repress

表中获取其中的行数，其中

id=ft.id

。

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我如何才能做到这一点？

根据您的DB实现，这可能会起作用：

 SELECT ft.*, 
        m.*,
        ( SELECT count(1)  
            FROM forum_replies fr 
           WHERE fr.id = ft.id) AS nr_of_replies 
   FROM forum_topics ft
     INNER JOIN members m ON ft.author = m.id
     WHERE ft.forum =  '$forum'
     ORDER BY ft.lastpost DESC

嗯

SELECT ft.*, m.*, fr.ReplyCount
FROM forum_topics ft
INNER JOIN members m ON ft.author = m.id
inner join (
   Select Id, Count(*) as ReplyCount
   from forum_replys 
   group by id
 ) as fr 
 on ft.Id = fr.Id
 WHERE ft.forum =  '$forum'
 ORDER BY ft.lastpost DESC