SQL实现LeetCode(183.从未下单订购的顾客)

[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 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：

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