Customers Who Never Order - Leetcode Solution

Customers Who Never Order problem on LeetCode is a SQL based question that requires you to write a query to find all the customers who have never made an order.

To solve this problem, we need to join two tables - Customers and Orders, and then use the NOT EXISTS operator to find all the customers who have not made any orders. Below is the detailed solution to this problem.

SELECT c.Name AS 'Customers Who Never Order' FROM Customers c
WHERE NOT EXISTS 
(SELECT o.CustomerId FROM Orders o WHERE o.CustomerId = c.Id);

In the above query, we first select the name of the customers from the Customers table and alias it as 'Customers Who Never Order'. Next, we use the NOT EXISTS operator to find all the customers who have not made any orders.

The subquery SELECT o.CustomerId FROM Orders o WHERE o.CustomerId = c.Id finds all the customer IDs from the Orders table where the customer has made an order.

The NOT EXISTS operator then returns all the customers whose IDs are not present in the above subquery result. These are the customers who have never made an order.

Therefore, the output of this query will be a list of customers who have never made an order.

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