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       |+-----------+
題目是想將Customers中某一類name顯示出來，這個條件通過Orders判斷。
可以通過left join將Customers與Orders連接起來，然后篩選出符合條件的。
以例題為例：
Customers left join Orders的結(jié)果：
條件是：沒有訂單的顧客的名字，表示為CustomerId is NULL;
select Customers.Name as Customersfrom Customers left join Orderson Customers.Id=Orders.CustomerIdwhere Orders.CustomerId is NULL;