返回购买 id 为 BR01 的产品的所有顾客的电子邮件
返回购买 prod_id 为 BR01 的产品的所有顾客的电子邮件（二）
https://www.nowcoder.com/practice/c7aa73afc41f4dfc925baebdd175c345
select Customers.cust_email from Customers inner join Orders on Customers.cust_id=Orders.cust_id inner join OrderItems on Orders.order_num=OrderItems.order_num where prod_id="BR01"
