1251. 平均售价
题目链接：1251. 平均售价
代码如下：
# Write your MySQL query statement below
select p.product_id,round(ifnull(sum(units*price)/sum(units),0),2) as average_price
from Prices as p
left join UnitsSold as u
on p.product_id=u.product_id
where u.purchase_date between p.start_date and p.end_date or u.product_id is null
group by product_id
)
