Course 03 — SQL, by doing

Ask your dataand watch it answer.

A real SQLite database runs in this page. Every query you write is executed for real, and the rows that answer it visibly travel from the tables into your result — so joins, grouping and indexes stop being abstract.

Start lesson one Open the sandbox

15 lessons · 5 chapters · free

SQL — a join, then a GROUP BY
  1. 01 · Read

    Each lesson is one idea, explained in a few short paragraphs — no walls of theory.

  2. 02 · Do

    Type into a live environment and watch it react. Tab completes, ↑ recalls, undo takes back any step.

  3. 03 · Match

    Reach the goal any way you like — you're judged on the result, not the keystrokes. Par is the shortest route.

01

Asking questions

SELECT, WHERE, ORDER BY and LIMIT — reading exactly the rows you need.

  1. 01 Your first query SELECT and FROM — reading a whole table.
  2. 02 Filter with WHERE Only the rows you need, only the columns you need.
  3. 03 Sort and limit ORDER BY and LIMIT — the top three of anything.
  4. 04 Combine conditions AND, OR, IN and parentheses.
  5. 05 DISTINCT, and the trouble with NULL Each value once, and the value that isn't one.
02

Joining tables

Combine rows from related tables, and keep the ones that don't match.

  1. 06 Join two tables Orders only store a customer's id. JOIN looks up the name.
  2. 07 Join three tables Customer, product and order in one result.
  3. 08 Find what's missing LEFT JOIN keeps rows that have no match.
03

Summarising

COUNT, SUM and AVG, GROUP BY and HAVING — turning rows into answers.

  1. 09 Count with GROUP BY Turn many rows into one per group.
  2. 10 Filter groups with HAVING WHERE filters rows, HAVING filters groups.
  3. 11 Averages per category AVG, ROUND, and sorting by a computed column.
04

Changing data

INSERT, UPDATE and DELETE, and transactions that keep them safe.

  1. 12 Add a row INSERT INTO … VALUES.
  2. 13 Change rows UPDATE … SET … WHERE — and the WHERE you must not forget.
  3. 14 Remove rows DELETE FROM … WHERE.
05

Speed

Indexes, and reading the query plan to prove they're used.

  1. 15 Make a lookup fast EXPLAIN QUERY PLAN, and the index that skips the scan.

This is the genuine SQLite engine compiled to WebAssembly, so every query behaves exactly as it would in a SQLite file. Other databases (PostgreSQL, MySQL) differ in small details, which the lessons point out.