子查询+where
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 o.cust_id
from Orders o, OrderItems oi
WHERE oi.prod_id = 'BR01' and o.order_num = oi.order_num
)
