地址:https://leetcode-cn.com/problems/customers-who-never-order/

## 某网站包含两个表，Customers 表和 Orders 表。编写一个 SQL 查询，找出所有从不订购任何东西的客户。    
     示例：
    Customers 表：
    
    +----+-------+
    | Id | Name  |
    +----+-------+
    | 1  | Joe   |
    | 2  | Henry |
    | 3  | Sam   |
    | 4  | Max   |
    +----+-------+
    Orders 表：
    
    +----+------------+
    | Id | CustomerId |
    +----+------------+
    | 1  | 3          |
    | 2  | 1          |
    +----+------------+
    例如给定上述表格，你的查询应返回：
    
    +-----------+
    | Customers |
    +-----------+
    | Henry     |
    | Max       |
    +-----------+
 
`解题思路`

1 not in

    `select Name as Customers from Customers where id not in (
         select CustomerId from Orders
     )`
     
2left join on

        `select c.Name as Customers from Customers c left join Orders  o on 
         c.Id = o.CustomerId where CustomerId is null`