LeetCode 183: Customers Who Never Order
LeetCode 183: Customers Who Never Order.
I'm Varchasv, a Data Engineer working on enterprise data integration.
Currently on a 90-problem challenge to level up my technical skills and switch to a more development-focused role.
What I'm doing:
- Solving 2 Leetcode problems daily (SQL + DSA).
- Blogging about each problem.
- Building in public.
My Goal - Land a better data engineering role by mid-2026.
Follow my journey !!
Date: January 19, 2026
Category: SQL
Time Taken: 10 minutes
Difficulty: Easy
Problem Statement
Write a solution to find all customers who never order anything.
Link: Customers Who Never Order
My Approach
Initial thought: Since we only need the names of customers who have never ordered, we can perform a left join on the customers and orders tables using customers.id = orders.customerId. Then, we can use the WHERE clause to find customers who have no order.id.
Final solution: A more efficient solution is to use NOT EXISTS with a subquery instead of a join. With this method, as soon as we find a customer with any orders, we can skip to the next customer. This saves time by not having to check all the data for each customer.
Why this works:
This works because as soon as we find any order.id for a customer, we stop looking for customers with no orders and move on to the next customer. This saves time.
Solution Code (Left Join)
SELECT c.name AS Customers
FROM Customers AS c
LEFT JOIN Orders AS o
ON c.id = o.customerId
WHERE o.id IS NULL
Solution Code (Optimized)
SELECT c.name AS Customers
FROM Customers AS c
WHERE NOT EXISTS (
SELECT 1
FROM Orders AS o
WHERE o.customerId = c.id
)
Key Takeaway: Better optimization with NOT EXISTS.
Pattern: Anti Join / Anti-Semi Join
Mistakes I Made: There were no mistakes to mention. I simply wasn't aware of the optimized solution for this.
Series: 90 Days of Data Engineering Progress: 9/90 problems completed
Tags: #DEQuest #LeetCode #SQL #DataEngineering #BuildInPublic