购买prod_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 c.cust_email from Customers c inner join Orders o1 on c.cust_id = o1.cust_id inner join OrderItems o2 on o1.order_num = o2.order_num where o2.prod_id ='BR01'