【MySQL刷题】从不订购的客户

思路

判断客户是否曾经下过订单的条件是：如果一个客户 ID 在 orders 表中不存在，这就意味着他们从未下过订单。

• 方法1：左连接

SELECT  c.name AS 'Customers'
FROM    Customers AS c
LEFT
    JOIN Orders AS o
ON      c.id = o.customerId
WHERE   o.customerId IS NULL

• 方法2：distinct去重

SELECT  `name` AS 'Customers'
FROM    Customers AS c
WHERE   c.id
NOT IN  (
            SELECT  customerId
            FROM    Orders AS o
            );