题解 | #返回购买 prod_id 为 BR01 产品#

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'))

SELECT c.cust_email
FROM Customers c
JOIN Orders o ON c.cust_id = o.cust_id
JOIN OrderItems oi ON o.order_num = oi.order_num
WHERE oi.prod_id = 'BR01'

跟着行文逻辑就直接写出来了，但是一直子查询好像会重复扫描表格，因此用join会比较好