【leetcode Database】183. Customers Who Never Order

題目：

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       |
+-----------+

解析：本題可以用多重查詢來做。先把Orders表中的CustomerId字段取出來，然後把Customers表中所有Id不在CustomerId字段中的名字取出來即可。代碼如下：

# Write your MySQL query statement below
SELECT Name AS Customers FROM Customers WHERE Id NOT IN (SELECT CustomerId FROM Orders);