题解 | #返回购买价格为 10 美元或以上产品的顾客列表#
返回购买 prod_id 为 BR01 的产品的所有顾客的电子邮件（一）
http://www.nowcoder.com/practice/962b16554fbf4b99a87f4d68020c5bfb
OrderItems表和Orders表通过键order_num连接，Orders表和Customers表通过键cust_id连接，可以利用这两个键进行子查询。
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'
)
)
