题解 | 返回购买 prod_id 为 BR01 的产品的所有顾客的电子邮件（一）
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'
)
)
一定注意在最里面那层where语句后面的括号的层次是不是正确的
不过你这绩点不考研浪费了吧