mysql如何通过group by和左连接获得第二高的值(value)

Question

(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

对于第二个左连接语句，我能够获得最高的 mb 值。有人可以帮我添加第三个左连接语句，以便我可以获得第二高的 mb 值吗？？

Answer 1

首先，您根本不需要第三次连接。您可以在一次连接中进行计算:

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;

获取 mb 的第二大值然后使用一个技巧，涉及 substring_index() 和 group_concat():

   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;

想法是将值连接在一起，按 mb 排序。然后取列表的第二个元素。一个缺点是该值会转换为字符串，即使它以数字开头也是如此。