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);
参考资料：
https://leetcode.com/discuss/22624/three-accepted-solutions
https://leetcode.com/discuss/53213/a-solution-using-not-in-and-another-one-using-left-join