inner join
返回购买 prod_id 为 BR01 的产品的所有顾客的电子邮件（一）
https://www.nowcoder.com/practice/962b16554fbf4b99a87f4d68020c5bfb
select c.cust_email from OrderItems a inner join Orders b on a.order_num=b.order_num inner join Customers c on b.cust_id=c.cust_id where a.prod_id='BR01'
