SQL--leetcode183. 從不訂購的客戶

某網站包含兩個表，Customers 表和 Orders 表。編寫一個 SQL 查詢，找出所有從不訂購任何東西的客戶。

Customers 表：

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

Orders 表：

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

例如給定上述表格，你的查詢應返回：

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

---

這一題考察左連接，left join以左表爲基礎，右表裏若是沒有會顯示null值。

# Write your MySQL query statement below
select a.Name as Customers from Customers a left join Orders b on b.CustomerId=a.Id where b.CustomerId is null