Learn SQL from scratch: queries, tables and real examples

Almost every app stores its information in a database. SQL is one of the most requested skills: developers, data analysts and testers all use it.

What is a relational database?

Picture spreadsheets linked to each other. Each table stores one kind of thing (customers, orders, products) and each row is a record. Tables are related through keys: an order stores the id of the customer who placed it.

The queries you'll use most

-- Read: the 10 most recent orders
SELECT id, customer_id, total
FROM orders
ORDER BY created_at DESC
LIMIT 10;

-- Filter and group: how much each customer spent this year
SELECT customer_id, SUM(total) AS spent
FROM orders
WHERE created_at >= '2026-01-01'
GROUP BY customer_id;

-- Join tables: the customer name for each order
SELECT o.id, c.name, o.total
FROM orders o
JOIN customers c ON c.id = o.customer_id;

With SELECT, WHERE, ORDER BY, GROUP BY and JOIN you can answer most everyday questions.

Recommended learning order

1. Basic queries: SELECT, WHERE, ORDER BY, LIMIT.
2. Aggregations: COUNT, SUM, AVG, GROUP BY, HAVING.
3. JOINs: combining tables without duplicating data.
4. Design: tables, data types, primary and foreign keys, normalization.
5. Indexes: why a query is slow and how to speed it up.
6. Transactions: making sure a transfer never ends up half done.

How to practice

Install PostgreSQL (free) or use an online playground, load a sample database and ask real questions: which product sells the most? which customers haven't bought in 3 months?

Next step

Codea's free Databases Jr route teaches SQL with PostgreSQL plus an introduction to MongoDB, with challenges on real data. Route content is in Spanish.