题解 | 双重子查询
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'
)
)
