Problem

Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.

Table: Customers.

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

Table: Orders.

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

Using the above tables as example, return the following:

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

即找到没有订单的客户

MySQL

解法一

SELECT Name AS Customers FROM Customers c WHERE c.Id NOT IN (SELECT CustomerId FROM Orders);

解法二

SELECT Customers FROM (SELECT c.Name AS Customers, o.Id AS oid FROM Customers c LEFT JOIN Orders o ON c.Id = o.CustomerId) AS r WHERE r.oid IS NULL;

分析

无