Leetcode 183. Customers Who Never Order (database)

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：Orders里面存在的CustomerId包含了所有至少买了一件产品的顾客。那么在Customers里面选Name时保证Id不在这里面有行了。

select Name as Customers from Customers where Customers.Id not in 
(select CustomerId from Orders)