sql16（Leetcode1251平均售价）

代码：

# Write your MySQL query statement below
 
select product_id,ifnull(round(sum(sum)/sum(units),2),0) as average_price
from(
    select p.product_id,p.price,u.units,price*units as sum
    from
        Prices p        
    left join 
        UnitsSold u
    on u.product_id=p.product_id and datediff(u.purchase_date,p.start_date)>=0 and datediff(u.purchase_date,p.end_date)<=0
) t
group by product_id