返回购买 prod_id 为 BR01 的产品的顾客
返回购买 prod_id 为 BR01 的产品的所有顾客的电子邮件（一）
https://www.nowcoder.com/practice/962b16554fbf4b99a87f4d68020c5bfb
解1.
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' ))
解2.
FROM Customers
WHERE cust_id IN
(SELECT cust_id
FROM Orders
WHERE order_num IN
(SELECT order_num
FROM OrderItems
WHERE prod_id='BR01' ))
SELECT cust_email
FROM Customers C
,Orders O
,OrderItems OI
WHERE C.cust_id=O.cust_id
AND O.order_num=OI.order_num
AND OI.prod_id='BR01'
#sql#FROM Customers C
,Orders O
,OrderItems OI
WHERE C.cust_id=O.cust_id
AND O.order_num=OI.order_num
AND OI.prod_id='BR01'
