Source

• leetcode: 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       |
+-----------+

MySQL

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