Problem:
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       |
+-----------+

Solution: 根据题意,先选出Order表里出现的所有CustomerId,然后用not in排除即可,SQL语句如下:

1
2
select c.Name Customers
from Customers c where c.Id not in ( select CustomerId from Orders);