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 | +-----------+
这道题让我们给了我们一个Customers表和一个Orders表，让我们找到从来没有下单的顾客，那么我们最直接的方法就是找没有在Orders表中出现的顾客Id就行了，用Not in关键字，如下所示：
解法一：
SELECT Name AS Customers FROM Customers WHERE Id NOT IN (SELECT CustomerId FROM Orders);
或者我们也可以用左交来联合两个表，只要找出右边的CustomerId为Null的顾客就是木有下单的：
解法二：
SELECT Name AS Customers FROM Customers LEFT JOIN Orders ON Customers.Id = Orders.CustomerId WHERE Orders.CustomerId IS NULL;
我们还可以用Not exists关键字来做，原理和Not in很像，参见代码如下：
解法三：
SELECT Name AS Customers FROM Customers c WHERE NOT EXISTS (SELECT * FROM Orders o WHERE o.CustomerId = c.Id);
本文转自博客园Grandyang的博客，原文链接：从未下单订购的顾客[LeetCode] Customers Who Never Order ，如需转载请自行联系原博主。
