【Leetcode】Customers Who Never Order

题目链接：https://leetcode.com/problems/customers-who-never-order/

题目：
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 |
+———–+

思路：
left或right join，选择customerid为空的就是没有进行过交易的。

算法：

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