group-by - 关于 GROUP BY 和 ORDER BY

Question

我对 group by 的查询顺序有疑问。

我有一个表 download_category 并下载。下载表有一个名为“thumb”的列（它是一个图像 - varchar）。我想获取类别名称和插入的最后一个拇指。

我正在这样做：

    SELECT download_category.id
             , download_category.link
             , download.thumb
      FROM download  

RIGHT JOIN download_category ON (download_category.id = download.idCategoria )   
     WHERE download_category.link IN ('link-of-category') 

GROUP BY download_category.id
       , download_category.link
ORDER BY download.id DESC 

如果不存在下载，则需要 RIGHT JOIN。

问题是：拇指列返回第一个插入的记录，而不是最后一个“download.id DESC”。我究竟做错了什么？

Answer 1

试试这个：

SELECT download_category.id
         , download_category.link
         , download.thumb
from download_category 
left join (select max(id) maxId,
                  idCategoria 
           from download 
           group by idCategoria) t
      ON (download_category.id = t.idCategoria ) 
left join download 
      ON t.maxid=download.id

 WHERE download_category.link IN ('link-of-category') 

 GROUP BY download_category.id
        , download_category.link
 order by t.maxid DESC;