LeetCode: 183. Customers Who Never Order

题目描述

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       |
+-----------+

解题思路

将 Customers 和 Orders 表进行 左外连接，其中 CustomerId 为 null 的行对应的客户就是没有下过订单的用户。

数据库在通过连接两张或多张表来返回记录时，都会生成一张中间的临时表，然后再将这张临时表返回给用户。
在使用 LEFT OUTER JOIN 时，ON 和 WHERE 条件的区别如下：
1、ON 条件是在生成临时表时使用的条件，它不管 ON 中的条件是否为真，都会返回左边表中的记录。
2、WHERE 条件是在临时表生成好后，再对临时表进行过滤的条件。这时已经没有 LEFT OUTER JOIN 的含义（必须返回左边表的记录）了，条件不为真的就全部过滤掉。

AC 代码

SELECT Name AS Customers
FROM Customers LEFT OUTER JOIN Orders
ON Customers.Id = Orders.CustomerId WHERE Orders.CustomerId is null

LeetCode: 183. Customers Who Never Order相关推荐

leetcode 183. Customers Who Never Order
题意:查找没有点过单的客人题目链接:https://leetcode.com/problems/customers-who-never-order/ select Name from Custome ...
Leetcode#183. Customers Who Never Order
题目描述:查找没有预定(orders)的顾客(customers) 普通方法: select Name as Customers from Customers c where c.Id not in ...
LeetCode：183.Customers Who Never Order
题目: Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQ ...
【Leetcode】Customers Who Never Order
题目链接:https://leetcode.com/problems/customers-who-never-order/ 题目: Suppose that a website contains tw ...
183. Customers Who Never Order
原题链接:https://leetcode.com/problems/customers-who-never-order/description/ 做完这道题目我才发现,我跟不就不懂 in, exis ...
183. Customers Who Never Order没有下过订单的顾客MySQL
假设一个网站包含两个表,Customers表和Orders表.编写SQL查询以查找所有没订购任何商品的客户. customers orders Output 用到left outer join: s ...
[SQL]LeetCode183. 从不订购的客户 | Customers Who Never Order
★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★★ ➤微信公众号:山青咏芝(shanqingyongzhi) ➤博客园地址:山青咏芝(https://www.cnblog ...
LeetCode: 107. Binary Tree Level Order Traversal II
题目 Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from l ...
LeetCode 107. Binary Tree Level Order Traversal II
LeetCode 107. Binary Tree Level Order Traversal II Solution1:我的答案比102那道题多了一行代码... /*** Definition f ...

LeetCode: 183. Customers Who Never Order

LeetCode: 183. Customers Who Never Order

题目描述

解题思路

AC 代码

LeetCode: 183. Customers Who Never Order相关推荐

最新文章

热门文章