select product,
sales_date,
current_price - prev_price as diff
from (
select product,
sales_date,
price as current_price,
lag(price) over (partition by product order by sales_date) as prev_price,
row_number() over (partition by product order by sales_date desc) as rn
from the_unknown_table
) t
where rn = 1;
SQLFiddle示例：http://sqlfiddle.com/#!15/9f7d6/1
