prod_id 为 BR01所有顾客的电子邮件（一）#
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
答案1：
select cust_email from Customers where cust_id in (select a.cust_id from Orders as a inner join OrderItems as b where a.order_num = b.order_num and b.prod_id = 'BR01')
答案2：
select cust_email from Customers a left join Orders as b ON a.cust_id = b.cust_id left join OrderItems c ON b.order_num = c.order_num where prod_id = 'BR01';
思路就是 你先把所有表管理起来 再弄条件去判断
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