sql JOIN 和 COUNT

【问题标题】：sql JOINs and COUNTsql JOIN 和 COUNT
【发布时间】：2012-01-06 18:09:57
【问题描述】：

假设我有这个查询

 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

我还想从表forum_replies 中获取行数，其中id = ft.id。我怎么能这样做？

【问题讨论】：

标签： mysql sql join count

【解决方案1】：

根据您的数据库实现，这可能有效：

 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

HTH

【讨论】：

【解决方案2】：

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

【讨论】：