SQL-Customers Who Never Order_sql里的customers 什么意思-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_23908539/article/details/51454495

本文介绍如何使用SQL查询从两个表——客户表和订单表中找出从未下单的客户。提供了两种方法：一是使用子查询排除已存在的订单客户；二是利用左外连接筛选未匹配到订单记录的客户。

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

思路：
way 1：
用嵌套子查询，适合多表格比较的情况。在Customers表中选择Id不在Orders表的CustomerId集合里面的元组
way 2:
用左外连接实现。首先要搞明白普通的连接功能是找到两个表格的公共集合然后形成新的表。
左外连接的关键词是left join或者left outer join，然后再加一个on，其实质就是一种构造基本表的方式
通过左外连接得到新的表后，再选出相应的元组即可

way 1:

select Name 
from Customers
where Id not in 
    (
        select CustomerId
        from Orders
    )

way 2:

select Name 
from Customers as c
left outer join Orders as d
on c.Id = d.CustomerId
where CustomerId is null