CustomersWhoNeverOrder.sql

-- Source : https://leetcode.com/problems/customers-who-never-order/
-- Author : Hamza Mogni
-- Date   : 2022-06-14

/***************************************************************************************************** 
 *
 * Table: Customers
 * 
 * +-------------+---------+
 * | Column Name | Type    |
 * +-------------+---------+
 * | id          | int     |
 * | name        | varchar |
 * +-------------+---------+
 * id is the primary key column for this table.
 * Each row of this table indicates the ID and name of a customer.
 * 
 * Table: Orders
 * 
 * +-------------+------+
 * | Column Name | Type |
 * +-------------+------+
 * | id          | int  |
 * | customerId  | int  |
 * +-------------+------+
 * id is the primary key column for this table.
 * customerId is a foreign key of the ID from the Customers table.
 * Each row of this table indicates the ID of an order and the ID of the customer who ordered it.
 * 
 * Write an SQL query to report all customers who never order anything.
 * 
 * Return the result table in any order.
 * 
 * The query result format is in the following example.
 * 
 * Example 1:
 * 
 * Input: 
 * Customers table:
 * +----+-------+
 * | id | name  |
 * +----+-------+
 * | 1  | Joe   |
 * | 2  | Henry |
 * | 3  | Sam   |
 * | 4  | Max   |
 * +----+-------+
 * Orders table:
 * +----+------------+
 * | id | customerId |
 * +----+------------+
 * | 1  | 3          |
 * | 2  | 1          |
 * +----+------------+
 * Output: 
 * +-----------+
 * | Customers |
 * +-----------+
 * | Henry     |
 * | Max       |
 * +-----------+
 ******************************************************************************************************/


SELECT
    c.name AS "Customers"
FROM 
    Customers c
LEFT JOIN
    Orders o
ON
    o.customerId = c.id
WHERE o.id IS NULL