How does the SQL run without a database server?

It uses SQLite compiled to WebAssembly (sql.js). A full SQL database runs inside your browser tab, so your queries execute locally and instantly.

Which SQL features can I practice?

The challenges cover SELECT and WHERE, ORDER BY, INNER and LEFT JOIN, GROUP BY, HAVING, aggregate functions, self-joins, subqueries, LIMIT, date functions and window functions such as RANK().

SQL Practice Playground

Write real SQL against a sample database and run it instantly — right here in your browser, no signup and no setup. Work through 13 interview challenges from a simple SELECT up to window functions, and check your answers as you go.

query.sql

SQLite

Loading SQL engine…

Database schema

departments

id, name, location

employees

id, name, department_id → departments, salary, manager_id → employees, hire_date

customers

id, name, country, signup_date

products

id, name, category, price

orders

id, customer_id → customers, order_date, status

order_items

id, order_id → orders, product_id → products, quantity

Challenges

0 / 13

Easy#1

Return every column for all employees.

SELECT * FROM employees;

Easy#2

List the name and salary of employees earning more than 65000, highest salary first.

SELECT name, salary FROM employees
WHERE salary > 65000
ORDER BY salary DESC;

Easy#3

Show the number of customers from each country (column names: country, customers).

SELECT country, COUNT(*) AS customers
FROM customers
GROUP BY country;

Medium#4

For each department show its name and the number of employees in it. Departments with no employees should still appear with a count of 0.

SELECT d.name, COUNT(e.id) AS employees
FROM departments d
LEFT JOIN employees e ON e.department_id = d.id
GROUP BY d.id, d.name;

Medium#5

Show only the departments that have more than 3 employees, with their employee count.

SELECT d.name, COUNT(e.id) AS employees
FROM departments d
JOIN employees e ON e.department_id = d.id
GROUP BY d.id, d.name
HAVING COUNT(e.id) > 3;

Medium#6

Show each department's name and its average salary, rounded to the nearest whole number.

SELECT d.name, ROUND(AVG(e.salary)) AS avg_salary
FROM departments d
JOIN employees e ON e.department_id = d.id
GROUP BY d.id, d.name;

Medium#7

List each employee's name next to their manager's name (columns: employee, manager). Employees with no manager should still appear, with a NULL manager.

SELECT e.name AS employee, m.name AS manager
FROM employees e
LEFT JOIN employees m ON e.manager_id = m.id;

Medium#8

Find the names of customers who have never placed an order.

SELECT name FROM customers
WHERE id NOT IN (SELECT customer_id FROM orders);

Medium#9

List the names of products that have never been ordered.

SELECT name FROM products
WHERE id NOT IN (SELECT product_id FROM order_items);

Hard#10

Show the total revenue (quantity × price) for each product category, highest revenue first (columns: category, revenue).

SELECT p.category, SUM(oi.quantity * p.price) AS revenue
FROM order_items oi
JOIN products p ON p.id = oi.product_id
GROUP BY p.category
ORDER BY revenue DESC;

Hard#11

Show the top 3 customers by total amount spent (quantity × price), highest first (columns: name, total_spent).

SELECT c.name, SUM(oi.quantity * p.price) AS total_spent
FROM customers c
JOIN orders o ON o.customer_id = c.id
JOIN order_items oi ON oi.order_id = o.id
JOIN products p ON p.id = oi.product_id
GROUP BY c.id, c.name
ORDER BY total_spent DESC
LIMIT 3;

Hard#12

Find the second highest salary among all employees (single value, column: second_highest).

SELECT MAX(salary) AS second_highest
FROM employees
WHERE salary < (SELECT MAX(salary) FROM employees);

Hard#13

For each employee show name, department, salary, and their salary dept_rank within their department (1 = highest). Order by department name, then rank.

SELECT e.name, d.name AS department, e.salary,
       RANK() OVER (PARTITION BY e.department_id ORDER BY e.salary DESC) AS dept_rank
FROM employees e
JOIN departments d ON d.id = e.department_id
ORDER BY d.name, dept_rank;

Practice SQL for interviews, in your browser

Most SQL practice sites make you sign up, install a database, or watch your query disappear behind a paywall. This playground does none of that. A full SQLite database runs locally in your browser tab (via WebAssembly), so you can write a query, press Run, and see results instantly. Nothing you type leaves your machine.

The sample database models a small company and an online store, which is exactly the shape of data most SQL interview questions use: employees and departments for joins and aggregation, and customers, orders, products and order_items for multi-table revenue questions.

What you'll practice

Filtering & sorting — WHERE, ORDER BY, LIMIT
Joins — INNER JOIN, LEFT JOIN, and self-joins (an employee to their manager)
Grouping — GROUP BY, HAVING, and aggregate functions like COUNT, SUM, AVG
Subqueries — including the classic "second highest salary" and "never ordered" patterns
Window functions — RANK() OVER (PARTITION BY …)
Dates — grouping and filtering by month with SQLite's strftime

How to use it

Pick a challenge on the right, hit Try it to load the prompt into the editor, and write your query. When you run it, the playground checks your result against the expected answer and marks the challenge solved if it matches. Stuck? Every challenge has a Show solution button. Broke the data with an UPDATE or DELETE? Hit Reset data to restore the sample database.

When you're ready for the theory behind these queries, read the SQL interview questions guide, or take a break and try the Guess the Big O game.

Frequently asked questions

Is this SQL playground free?

Yes — completely free, no signup, and it runs entirely in your browser. Nothing you type is sent to a server.

How does SQL run without a database server?

It uses SQLite compiled to WebAssembly (sql.js). A real SQL engine runs inside your browser tab, so queries execute locally and instantly.

What SQL dialect is this?

SQLite. Almost all standard SQL you'd write in an interview works identically; a few functions such as strftime for dates are SQLite-specific.