SQL实现LeetCode(183.从未下单订购的顾客) SQL实现LeetCode(183.从未下单订购

[LeetCode] 183.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 CustomersLEFT JOIN Orders ON Customers.Id = Orders.CustomerIdWHERE 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 cWHERE 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
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