The language every database understands. Not a general-purpose language like Python — SQL is for asking questions of data: filtering, joining, grouping, and summarizing. Copy-paste examples plus live queries, running in a real database in your own browser.

Create a table

Every practice box on this page runs against its own fresh, empty database — so each example starts by creating and filling its own table.

CREATE TABLE students (
    id INTEGER PRIMARY KEY,
    name TEXT,
    grade INTEGER
);

INSERT INTO students (id, name, grade) VALUES
    (1, 'Ada', 92),
    (2, 'Linus', 88),
    (3, 'Grace', 95);

SELECT & WHERE

SELECT * FROM students;              -- every column, every row
SELECT name, grade FROM students;   -- just these columns
SELECT * FROM students WHERE grade >= 90;  -- filter rows
SELECT * FROM students WHERE name LIKE 'A%'; -- starts with "A"

ORDER BY & LIMIT

SELECT * FROM students ORDER BY grade DESC;   -- highest grade first
SELECT * FROM students ORDER BY grade DESC LIMIT 1;  -- just the top student

Aggregate functions

SELECT COUNT(*) FROM students;        -- how many rows
SELECT AVG(grade) FROM students;      -- average grade
SELECT MAX(grade), MIN(grade) FROM students;

GROUP BY

Splits rows into buckets, then runs an aggregate function on each bucket separately.

CREATE TABLE orders (id INTEGER, customer TEXT, amount REAL);
INSERT INTO orders VALUES (1,'Sam',20), (2,'Sam',35), (3,'Ana',50);

SELECT customer, SUM(amount) FROM orders GROUP BY customer;
-- Sam: 55, Ana: 50 — one total PER customer, not one grand total

JOIN

Combines rows from two tables based on a matching column — the whole point of having separate tables instead of one giant one.

CREATE TABLE customers (id INTEGER, name TEXT);
INSERT INTO customers VALUES (1, 'Sam'), (2, 'Ana');

CREATE TABLE orders2 (id INTEGER, customer_id INTEGER, amount REAL);
INSERT INTO orders2 VALUES (1, 1, 20), (2, 2, 50);

SELECT customers.name, orders2.amount
FROM orders2
JOIN customers ON orders2.customer_id = customers.id;

UPDATE & DELETE

UPDATE students SET grade = 100 WHERE name = 'Ada';
DELETE FROM students WHERE grade < 60;

NULL handling

SELECT * FROM students WHERE grade IS NULL;      -- NOT "= NULL" — that never matches
SELECT name, COALESCE(grade, 0) FROM students;  -- replace NULL with a default value

🚀 Practice problems

Runs against a real in-browser SQLite database (via sql.js/WebAssembly) — no server round-trip, same as Python and JavaScript.

Filter and sort EASY

CREATE TABLE students (id INTEGER, name TEXT, grade INTEGER);
INSERT INTO students VALUES
    (1,'Ada',92), (2,'Linus',78), (3,'Grace',95), (4,'Sam',65);

SELECT name, grade FROM students
WHERE grade >= 80
ORDER BY grade DESC;

Aggregate a table EASY

CREATE TABLE sales (id INTEGER, amount REAL);
INSERT INTO sales VALUES (1,20.5), (2,35.0), (3,12.25);

SELECT COUNT(*) AS num_sales, SUM(amount) AS total FROM sales;

GROUP BY with HAVING MEDIUM

CREATE TABLE orders (id INTEGER, customer TEXT, amount REAL);
INSERT INTO orders VALUES
    (1,'Sam',20), (2,'Sam',35), (3,'Ana',10), (4,'Ana',15);

-- only show customers whose total spend is over 30
SELECT customer, SUM(amount) AS total
FROM orders
GROUP BY customer
HAVING SUM(amount) > 30;

Join two tables HARD

CREATE TABLE authors (id INTEGER, name TEXT);
INSERT INTO authors VALUES (1,'Orwell'), (2,'Austen');

CREATE TABLE books (id INTEGER, title TEXT, author_id INTEGER);
INSERT INTO books VALUES (1,'1984',1), (2,'Animal Farm',1), (3,'Emma',2);

SELECT authors.name, books.title
FROM books
JOIN authors ON books.author_id = authors.id
ORDER BY authors.name, books.title;