php - Mysql查询在最近的行上连接表

Question

我有一个类别页面，在我加入之前它们工作正常。

类别显示如下:

category 1-----------------------0----------0
discussion 1 by someone

category 2-----------------------0----------0
discussion 2 by someoneelse

现在显示讨论的地方，我需要根据它的 discussion_id 显示发布到该类别的最后一个讨论。我试过 ORDER BY ... DESC 但它按类别名称而不是讨论名称和发布者排序。

 $sql = "SELECT *, COUNT(d.cat_id) as count 
         FROM discussions as d 
         LEFT JOIN categories c ON (c.cat_id = d.cat_id) 
         RIGHT JOIN soldiers s ON (s.uid = d.discussion_poster) 
         GROUP BY d.cat_id";
 $result = query($sql);  

    
while (($row = mysqli_fetch_assoc($result)) != false) {       
   $cat_id = $row['cat_id']; 
   $discussion_id = $row['discussion_id'];
   $cat_title = $row['cat_title'];
   $discussion_title = $row['discussion_title']; 
   $discussion_time = $row['discussion_time'];
   $count = $row['count'];
   $discussion_poster_id = $row['discussion_poster'];
   $discussion_poster = $row['soldier'];
}

Answer 1

$sql = "
SELECT *
FROM categories c
INNER JOIN
(
   SELECT MAX(discussion_id) discussion_id,
   COUNT(discussion_id) as count,
   cat_id
   FROM
   discussions
   GROUP BY
   cat_id
) as d1
ON (c.cat_id = d1.cat_id)
INNER JOIN discussions as d
ON (d1.discussion_id = d.discussion_id)
INNER JOIN soldiers s
ON (s.uid = d.discussion_poster)
GROUP BY d.cat_id";