-- learn by running, not reading

Practice SQL where
the queries actually run.

A notebook for MySQL, PostgreSQL, MariaDB & Oracle — your own live sandbox, pre-loaded with real data. No installs, no config.

PostgreSQL
MySQL
MariaDB
Oracle

query.sql postgres

SELECT   c.name,
         count(o.id) AS orders
FROM     customers c
JOIN     orders o ON o.customer_id = c.id
GROUP BY c.name
ORDER BY orders DESC
LIMIT    5;

name	orders
Amara Okafor	42
Liang Wei	39
Sofia Bianchi	31
Diego Herrera	27
Priya Nair	24

5 rows·11 ms

-- what you get

Everything the query needs to run

Instant sandbox

Start querying in seconds — no downloads, no setup. A persistent database that saves your work as you type.

Visual ER diagrams

Auto-generated entity diagrams show how tables relate, so you can see the schema before you write the JOIN.

Pre-loaded datasets

Real sample data across MySQL, PostgreSQL, MariaDB and Oracle — ready the moment you open a book.

-- curated collections

Featured Collections

Curated SQL courses and learning paths

📚

Building a Budget Tracker with Double-Entry Bookkeeping

Build a fully functional budget tracking system in SQL, from scratch. Each article adds to a working schema based on double-entry bookkeeping, the same system that has powered financial record-keeping since the 1400s. You'll design accounts, record balanced transactions, enforce integrity with triggers, and generate real financial reports. No toy examples. Every table, constraint, and query is something you could use in production.

4Books

📚

PostgreSQL Indexes: From First Index to Production Tuning

A series on PostgreSQL indexing, how indexes work, when to use them, and how to prove it with EXPLAIN ANALYZE. Starts with single-column basics and builds through composite indexes, covering indexes, and beyond.

1Book

📚

SQL Interview Questions Series

Practice SQL interview questions commonly asked at tech companies. Each book presents a real-world scenario, explains the underlying SQL pattern being tested, and provides a dataset to write your solution against. Topics covered include self-joins, window functions, gaps and islands, time-series comparisons, and aggregate filtering. Suitable for anyone preparing for data engineering, analytics, or backend engineering interviews where SQL proficiency is evaluated.

6Books

📚

Mastering CTEs (Common Table Expressions)

Learn to write clearer, more maintainable SQL queries with Common Table Expressions (CTEs). This collection shows practical examples of using CTEs to break down complex queries into logical steps, from simple filtering and aggregation to advanced patterns like recursive queries and gaps-and-islands problems. Each example includes working code you can run immediately in PostgreSQL, MySQL, Oracle, or MariaDB—no signup required. Whether you're analyzing time-series data, building hierarchical reports, or optimizing query performance, CTEs are an essential SQL skill.

6Books

📚

SQL Challenge series

Explore this SQL collection

2Books

📚

Movies datasets

Weekly collection of some interesting datasets and analysis

1Book

-- recent tutorials