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 c.cust_email
FROM Customers AS c
WHERE c.cust_id in (
                  SELECT o.cust_id
                  FROM Orders AS o
                  WHERE o.order_num in (
                                        SELECT oi.order_num
                                        FROM OrderItems AS oi
                                        WHERE oi.prod_id = 'BR01'));