从不订购的客户

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。

Customers 表：

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+----+-------+
| Id | Name  |
+----+-------+
| 1  | Joe   |
| 2  | Henry |
| 3  | Sam   |
| 4  | Max   |
+----+-------+

Orders 表：

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+----+------------+
| Id | CustomerId |
+----+------------+
| 1  | 3          |
| 2  | 1          |
+----+------------+

例如给定上述表格，你的查询应返回：

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+-----------+
| Customers |
+-----------+
| Henry     |
| Max       |
+-----------+

解题思路：

  1. 第一种NOT IN

    • 找出所有订购过东西的客户Id，作为临时表

    • 找出不在临时表内的Id的客户姓名

  2. 第二种LEFT JOIN左外连接

代码：

NOT IN

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SELECT 
NAME AS Customers 
FROM Customers 
WHERE 
id NOT IN (
SELECT DISTINCT 
CustomerId 
FROM Orders)

LEFT JOIN

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SELECT c.Name AS Customers
FROM Customers c 
LEFT JOIN Orders o 
ON c.Id = o.CustomerId
WHERE o.customerId IS NULL