SQL语句求优化,该怎么处理

select changeVal.val ,val.results,changeVal.changeTime,val.changeNum from (select sum(pur.purchase_change_cost) val,count(pur.sp_code) changeTime  from ba_netmessage_purchase pur  left join ba_sp_info sp on pur.sp_code = sp.sp_code  left join ba_area area on pur.area_code = area.AREA_CODE    where pur.sp_code in       (select t12.sp_code          from (select ba.purchase_lowest_cost, ba.sp_code                  from ba_netmessage_purchase ba,                       (select max(oprate_date) oprate_date, sp_code                          from ba_netmessage_purchase                         group by sp_code)  t1                 where ba.sp_code = t1.sp_code                   and ba.oprate_date = t1.oprate_date) t12,               (select ba.purchase_lowest_cost, ba.sp_code                  from ba_netmessage_purchase ba,                       (select min(oprate_date) oprate_date, sp_code                          from ba_netmessage_purchase                         group by sp_code)  t1                 where ba.sp_code = t1.sp_code                   and ba.oprate_date = t1.oprate_date) t2         where t12.sp_code = t2.sp_code           and t12.purchase_lowest_cost - t2.purchase_lowest_cost > 0)     and pur.oprate_date between       to_date('2012-1-19 00:00:00', 'yyyy-MM-dd HH24:MI:SS') AND       to_date('2012-12-19 23:59:59', 'yyyy-MM-dd HH24:MI:SS')   AND pur.area_code = '100'  ) changeVal,--and sp.sp_name like '%%'       ( select sum(t12.purchase_lowest_cost)results,count(t12.sp_code) changeNum          from  (select ba.purchase_lowest_cost, ba.sp_code,ba.purchase_change_cost                  from ba_netmessage_purchase ba,                       (select max(ba.oprate_date) oprate_date, ba.sp_code                          from ba_netmessage_purchase ba left join ba_sp_info sp on ba.sp_code=sp.sp_code                           where  ba.oprate_date between                    to_date('2012-1-1 00:00:00', 'yyyy-MM-dd HH24:MI:SS') AND                      to_date('2012-12-18 23:59:59', 'yyyy-MM-dd HH24:MI:SS')                            AND ba.area_code = '100'  -- and sp.sp_name like '%%'                         group by ba.sp_code)  t1                 where ba.sp_code = t1.sp_code                   and ba.oprate_date = t1.oprate_date                      ) t12,               (select ba.purchase_lowest_cost, ba.sp_code                  from ba_netmessage_purchase ba,                       (select min(oprate_date) oprate_date, sp_code                          from ba_netmessage_purchase                         group by sp_code)  t1                 where ba.sp_code = t1.sp_code                   and ba.oprate_date = t1.oprate_date) t2         where t12.sp_code = t2.sp_code           and t12.purchase_lowest_cost - t2.purchase_lowest_cost > 0 ) val;