MySQL-Customers Who Never Order

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

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题目大意：

假设一个网站包含两个表， 顾客表Customers和订单表Orders。编写一个SQL查询找出所有从未下过订单的顾客。

解题思路：

使用NOT IN，NOT EXISTS，或者LEFT JOIN均可。

使用NOT IN

# Write your MySQL query statement belowselect c1.Name as Customers from Customers c1 where c1.Id not in   (select o.CustomerId from Orders o) order by c1.Id;

使用NOT EXISTS

 SELECT Name as Customers from Customers c1 where not exists    (select CustomerId from Orders o where c1.Id = o.CustomerId );

使用LEFT JOIN

select Name as Customers from Customers c1 left join Orders o on c1.Id = o.CustomerId where o.Id is NULL;