保留最后的一条数据为Y

update prem_info_tax pi
   set status = 'N'
where status = 'Y'
   and exists (select 1
          from (select pk_serial, max(created_date) mcd
                  from prem_info_tax
                 where created_date > date '2016-05-01'
                   and status = 'Y' having count(1) > 1
                 group by pk_serial) a
         where a.pk_serial = pi.pk_serial
           and a.mcd <> pi.created_date);