LeetCode（数据库）- 指定日期的产品价格

题目链接：​​点击打开链接​​

题目大意：略。

解题思路：注意用表链接来避免条件查询去掉了部分不符合的数据（但题目又有默认值的数据）。

AC 代码

-- 解决方案(1)
SELECT t1.product_id, IFNULL(price, 10) price
FROM 
(SELECT product_id FROM Products GROUP BY product_id) t1
LEFT JOIN 
(SELECT * FROM (SELECT product_id, ROW_NUMBER() OVER(PARTITION BY product_id ORDER BY change_date DESC) rw, new_price price
FROM Products
WHERE change_date <= '2019-08-16') t2
WHERE rw = 1) t3
ON t1.product_id = t3.product_id

-- 解决方案(2)
select p1.product_id, ifnull(p2.new_price, 10) as price
from (
    select distinct product_id
    from products
) as p1 -- 所有的产品
left join (
    select product_id, new_price 
    from products
    where (product_id, change_date) in (
        select product_id, max(change_date)
        from products
        where change_date <= '2019-08-16'
        group by product_id
    )
) as p2 -- 在 2019-08-16 之前有过修改的产品和最新的价格
on p1.product_id = p2.product_id