Customers Who Never Order

Problem Description

Given two tables, Customers and Orders, write a query to find all customers who never ordered anything.

Table: Customers

Column Name	Type
id	int
name	varchar
id is the primary key for this table.

Table: Orders

Column Name	Type
id	int
customerId	int
id is the primary key for this table. customerId is a foreign key referencing the id column in the Customers table.

Examples

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

Approach

1. Perform a LEFT JOIN between the Customers table and the Orders table on the id and customerId columns.
2. Use the WHERE clause to filter out the customers who have NULL in the Orders table, which indicates that they never placed an order.

Solution

SQL

-- SQL solution to find customers who never order anything

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

Explanation

• LEFT JOIN: Joins the Customers table with the Orders table. This join includes all customers, even those who have not placed any orders.
• WHERE Orders.customerId IS NULL: Filters the result to include only those customers who do not have a corresponding entry in the Orders table.

Complexity Analysis

• Time Complexity: O(n), where n is the number of rows in the Customers table.
• Space Complexity: O(n), where n is the number of rows in the Customers table.

References

• LeetCode Problem: Customers Who Never Order