题解 | SQL93
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 cust_email from Customers where cust_id in (select cust_id from Orders where order_num in (select order_num from OrderItems where prod_id = 'BR01'))
