SQL isn’t just about writing queries—it’s about solving real-world problems efficiently. Whether it's finding missing data, preventing bad entries, or making queries smarter, knowing how to handle these challenges can set you apart as a backend engineer.

Here are three fun challenges to sharpen your SQL skills.

🛒 Challenge #1: The case of missing orders

The problem

You run an e-commerce site, and your team wants to find users who signed up but never placed an order.

The solution

Use a LEFT JOIN to find customers who don’t have matching orders.

SELECT c.id, c.name
FROM customers c
LEFT JOIN orders o ON c.id = o.customer_id
WHERE o.id IS NULL;

Why this matters

  • Helps the marketing team target inactive users
  • Shows how JOINs can find missing data

💡 Bonus challenge: Modify the query to find customers with at least 3 orders instead.

💰 Challenge #2: The impostor employees

The problem

Your HR database has a big issue—some employees have negative salaries! Find and fix these invalid entries.

The solution

A simple WHERE condition will catch them:

SELECT id, name, salary
FROM employees
WHERE salary <= 0;

Why this matters

  • Ensures data consistency
  • Helps detect fraud or data entry mistakes

💡 Bonus challenge: Prevent this from happening again with a database constraint:

ALTER TABLE employees ADD CONSTRAINT check_salary CHECK (salary > 0);

🎵 Challenge #3: The playlist shuffle hack

The problem

You’re building a music app, and users want a random shuffle feature. But newly added songs should have a higher chance of appearing.

The solution

A weighted randomization query:

SELECT * FROM songs
ORDER BY RANDOM() * (1 + 0.2 * (CURRENT_DATE - created_at))
LIMIT 10;

Why this matters

  • Makes song shuffling smarter
  • Introduces probability-based ranking

💡 Bonus challenge: Ensure no song appears twice in the same session.

Final thoughts

SQL is more than just queries—it’s about problem-solving.

By practicing real-world challenges, you’ll build skills that go beyond syntax.

  • Missing data? Use JOINs smartly
  • Data validation? Enforce constraints
  • Smart sorting? Use weighted ranking

🚀 Want to learn from real-world challenges? Check out Backend Challenges and start building today!