34 - 指定日期的产品价格

-- row_number(行号) 生成连续的序号，不考虑分数相同
-- 在'2019-08-16'之前改的价格，使用最近一期的日期，没有在'2019-08-16'之前改的价格，默认价格为10
select t.product_id, t.new_price as price
from (select *,row_number() over (PARTITION BY product_id order by change_date desc) as row_numfrom Productswhere change_date<='2019-08-16') as t
where t.row_num=1
union
-- 没有在'2019-08-16'之前改的价格，默认价格为10
select product_id, 10 as price 
from Products 
group by product_id
having min(change_date)>'2019-08-16';