【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