Chapter 01 · basic · 4 min
Start here: SQL & this tutorial
SQL is the language you use to ask a database questions: "which customers spent the most?", "how many orders shipped late?". It's the single most-tested skill in data-analyst, data-scientist, and data-engineer interviews across Southeast Asia, and it's what this free tutorial is built to get you fluent in, from your first SELECT to the window functions that show up in senior rounds.
Before the first real chapter, two quick things: how these lessons work, and what's actually running your queries.
The dataset
A tiny `greetings` table, just enough to run your very first query below.
Every example here is live: run it, then break it
This isn't a read-only tutorial. Every code block below is a real editor connected to a real database. Hit Run on the query below and you'll get actual rows back. Then change something: swap message for language, add a WHERE, delete a line, and run it again. Nothing you do here can break anything, and experimenting is the fastest way to learn. Each chapter also ends with links into the question bank, so the moment a concept clicks you can go practise it for real.
Your first query: press Run
This tutorial runs real PostgreSQL, in your browser
The database powering every example is PostgreSQL, one of the most widely used databases in the industry and a common choice for interview SQL rounds. It's running entirely inside your browser (no server, no signup, nothing to install), but it's the genuine engine, not a simulation: what runs here runs the same way in a production Postgres.
Why call this out? Because SQL has small dialect differences between databases (Postgres, MySQL, SQL Server, and others). The core (SELECT, WHERE, JOIN, GROUP BY, window functions) is identical everywhere and is 95% of what you'll learn here. A handful of convenience functions are Postgres-specific though; you'll meet DATE_TRUNC, SPLIT_PART, and FILTER later, and this tutorial flags them each time so you know what's portable and what isn't.
How the tutorial is structured
Chapters are grouped into three tiers you can see in the sidebar, meant to be read in order:
| Tier | What it covers |
|---|---|
| Basic | Single-table skills: reading, filtering, sorting, and reshaping data |
| Intermediate | Summarizing data and combining it across tables |
| Advanced | Multi-step reasoning: the techniques that separate interview candidates |
You can jump around with the sidebar any time, but if you're newer to SQL, top-to-bottom is the intended path, since each chapter builds on the ones before it.
That's the whole orientation. Next up is the statement every single SQL query begins with: SELECT.