leetcode183-Customers Who Never Order

问题描述：

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

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。

问题求解：

1、NOT IN

select Name from Customers c
where c.Id not in
(select CustomerId from Orders o);

2、not exists

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

3、left join

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