题解 | #返回电子邮件

直接用三个子查询

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"
            )
    );

直接使用WHER子句

SELECT
    cust_email
FROM
    Customers,
    OrderItems,
    Orders
WHERE
    Customers.cust_id = Orders.cust_id
    AND Orders.order_num = OrderItems.order_num
    AND OrderItems.prod_id = "BR01";

使用左连接

首先将Customers和Orders以cust_id为依据连接在一起，

之后以order_num为依据将连接后的表和OrderItems连接在一起。

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