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_num
        from 
            Products
        where 
            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';