mysql如何通过group by和在左连接中获得第二高的值

对于第二个左join语句，我能够获得最高的mb值。有人能帮我添加第三个左join语句，这样我就可以获得第二高的mb值吗？

首先，你根本不需要第三个join语句。您可以在一个联接中进行计算：

(select id from owner where date_format(auction_date,'%Y-%m-%d %H:%i:00') = date_format(NOW(),'%Y-%m-%d %H:%i:00')) as a
    left join (select owner_id,max(nb) as maxbid from auction group by owner_id) as b on a.id=b.owner_id
    left join (select owner_id,max(mb) as maxautobid from auction group by owner_id) as c on a.id=c.owner_id

为

mb

获取第二大值，然后使用一个技巧，包括

子字符串_index（）

和

组_concat（）

：

---

其思想是将这些值连接在一起，按

mb

排序。然后取列表中的第二个元素。其中一个缺点是，即使当它作为一个数字开始时，它也被转换为字符串。

当你询问SQL查询问题时，请考虑提供基于它的样本数据和期望输出。它可以帮助你更快更准确地得到答案。

from (select id
      from owner
      where date_format(auction_date,'%Y-%m-%d %H:%i:00') = date_format(NOW(),'%Y-%m-%d %H:%i:00')
     ) as a left join
     (select owner_id, max(nb) as maxbid, max(mb) as maxautobi
      from auction
      group by owner_id
     ) b
     on a.id=b.owner_id;

   from (select id
          from owner
          where date_format(auction_date,'%Y-%m-%d %H:%i:00') = date_format(NOW(),'%Y-%m-%d %H:%i:00')
         ) as a left join
         (select owner_id, max(nb) as maxbid, max(mb) as maxautobi,
                 substring_index(substring_index(group_concat(mb order by mb desc), ',', 2), ',', -1
                                ) as second_mb
          from auction
          group by owner_id
         ) b
         on a.id=b.owner_id;