题解 | 返回购买 prod_id 为 BR01 的产品的所有顾客的电子邮件（一）

/* 方式1：用连表来实现 
SELECT
    cust_email
FROM
    Customers
INNER JOIN
    Orders ON Orders.cust_id = Customers.cust_id
INNER JOIN
    OrderItems ON OrderItems.order_num = Orders.order_num
WHERE
    OrderItems.prod_id = 'BR01'
;
*/

/* 方式2：用多层嵌套的子查询来实现 */
SELECT
    cust_email
FROM
    Customers
WHERE
    cust_id IN( -- 次内层返回prod_id = 'BR01'的cust_id
        SELECT
            cust_id
        FROM
            Orders
        WHERE
            order_num IN( -- 最内层返回prod_id = 'BR01'的order_num
                SELECT
                    order_num
                FROM
                    OrderItems
                WHERE
                    prod_id = 'BR01'
            )
    )