LeetCode183從不訂閱的客戶

題目：

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

 思路一：進行左連接，然後取Id爲空的

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

注意：不能寫o.Id=null.

思路二：先在Orders中查詢CustomerId，再在Customers中查Id不在CustomerId的範圍的數

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