三层子查询,先从最里层写，查询到prod_id ="BR01"的订单编号

select order_num from OrderItems where prod_id ="BR01"

查询到prod_id ="BR01"的客户id

select cust_id from Orders where order_num in (select order_num from OrderItems where prod_id ="BR01")

查询到prod_id ="BR01"的客户的邮箱

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"))

当嵌套过多时容易被绕进去