update product pa set price = (
select max(price) from product pb where pa.id = pb.id
) where price is null;

------解决方案--------------------

SQL code
update product pa set price = (
select max(price) from product pb where pa.id = pb.id
) where price is null and year=2012 and month=8;

------解决方案--------------------

如果要更新成最近一个价格不是空的月份的值，
SQL code
update product p set price = ( select price from
  (select id, mth, price, row_number() over (partition by id order by year desc, month desc) rn from product where price is not null) prc_list
where prc_list.id = p.id and prc_list.rn = 1)
where price is null and year=2012 and month=8;