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'
    )
)