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"
))

题目要求的是采用子查询，这里把各个子查询写到一行里面，方便大家理解，写的时候也是从第一行一路写下来的，这样写很顺畅