题解 | #返回BR01 的产品的所有顾客的电子邮件（一）#

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'
    )
)

-- 解法2；合并表解法，表较大时不推荐；但代码可读性高，表之间的关系也很清晰
SELECT cust_email
FROM Customers
JOIN Orders USING(cust_id)
JOIN OrderItems USING(order_num)
WHERE prod_id = 'BR01'