有时候会用到查询倒数第二条记录 
last=HolderChangeHistory.find_by_sql
(["               SELECT * FROM holder_change_histories 
                  where treasure_id = ? 
                  order by id desc   
                  limit   1,1  ",
                  @hch.treasure_id])
select   *     
              from   tablename   a     
              where   (  
                  select   count(*)     
                       from   tablename   b     
                       where   a.id<b.id)=1  
select   *     
                from   tb1     
                order   by   tb1.id   desc     
                limit   1,1;