DB-Fiddle
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A browser-based playground where you paste schema and query SQL, choose a MySQL, MariaDB, PostgreSQL or SQLite version, run it, and share the result by link. Useful for reproducing query behaviour and comparing engines without local installs.
More resources on Relational Databases
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MySQL 8.0 Reference Manual
The official and most comprehensive documentation for MySQL. It's an invaluable resource for understanding all aspects of MySQL, from installation and configuration to advanced features, SQL syntax, and administration. While not a traditional 'book' in print, it functions as a highly detailed and authoritative reference available online for free.
use-the-index-luke.com
A practical guide to database indexing and query optimization, offering clear explanations of how indexes work, how to design them, and tutorials with SQL examples across major relational databases.
sqlzoo.net
SQLZoo.net is an interactive SQL learning resource offering free tutorials and hands-on practice with live databases. It provides step-by-step lessons and coding exercises on SQL topics (SELECT, JOIN, aggregation, etc.) with immediate feedback.
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Learn SQL fundamentals with Khan Academy! Master querying and managing data in this free introductory course.