Problem: 1164. 指定日期的产品价格

• coalesce 的使用
  

简洁版

👨‍🏫 参考题解

select 
    distinct p1.product_id,
    coalesce((select p2.new_price
    from Products p2
    where p2.product_id = p1.product_id and p2.change_date <= '2019-08-16'
    order by p2.change_date DESC
    limit 1),10) as price 
from Products p1

高效版

👨‍🏫 参考题解

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