README.md

Customers Who Never Order

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

直接使用exists语句查询

Code

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

exists语句

exists语句后面接一个select子查询，但实际并不返回任何数据，只返回是否有数据，存在返回true，否则false.