16 - 平均售價

# 注意：between 小值 and 大值 select u.product_id, round(sum(u.units*p.price)/sum(u.units),2) average_price 
from Prices p left join UnitsSold u 
on p.product_id=u.product_id 
-- and
where u.purchase_date between p.start_date and p.end_date 
group by u.product_id;