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

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。

SQL语句：

使用NOT IN（Accepted 777ms）：

# Write your MySQL query statement below
SELECT Name FROM Customers c WHERE c.Id NOT IN (SELECT CustomerId FROM Orders o);

使用NOT EXISTS（Accepted 745ms）：

# Write your MySQL query statement below
SELECT Name FROM Customers c WHERE NOT EXISTS (SELECT CustomerId FROM Orders o WHERE o.CustomerId = c.id);

使用LEFT JOIN（Accepted 726ms）：

# Write your MySQL query statement below
SELECT Name FROM Customers c LEFT JOIN Orders o ON c.Id = o.CustomerId WHERE o.Id IS NULL;

 

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