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

這道題讓我們給了我們一個Customers表和一個Orders表，讓我們找到從來沒有下單的顧客，那麼我們最直接的方法就是找沒有在Orders表中出現的顧客Id就行了，用Not in關鍵字，如下所示：

解法一：

SELECT Name AS Customers FROM Customers 
WHERE Id NOT IN (SELECT CustomerId FROM Orders);

或者我們也可以用左交來聯合兩個表，只要找出右邊的CustomerId為Null的顧客就是木有下單的：

解法二：

SELECT Name AS Customers FROM Customers
LEFT JOIN Orders ON Customers.Id = Orders.CustomerId
WHERE Orders.CustomerId IS NULL;

我們還可以用Not exists關鍵字來做，原理和Not in很像，參見程式碼如下：

解法三：

SELECT Name AS Customers FROM Customers c
WHERE NOT EXISTS (SELECT * FROM Orders o WHERE o.CustomerId = c.Id);

參考資料：