[LeetCode] Customers Who Never Order 从未下单订购的顾客
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
