#返回购买 prod_id 为 BR01所有顾客邮件
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'
)
)
最笨的办法？层层提取
