SQL is one of the most widely used database management systems in the world. It is an essential skill for any data scientist, business analyst, or software developer. If you want to learn SQL or improve your SQL skills, there are several websites that offer free SQL courses and tutorials to help you get started. Here are the top 5 websites to learn SQL for free:

1. SQLBolt : http://sqlbolt.com

SQLBolt is a free interactive SQL tutorial that teaches you SQL basics with simple, interactive exercises. The website is user-friendly and provides a step-by-step guide to learning SQL. SQLBolt is suitable for beginners who want to learn SQL basics, as well as for experienced users who want to refresh their SQL knowledge.

2. SQLZoo : https://sqlzoo.net/wiki/SQL_Tutorial

SQLZoo is another free SQL tutorial that offers interactive SQL exercises. The website provides a wide range of SQL exercises, from basic to advanced levels. SQLZoo is suitable for both beginners and advanced users who want to learn SQL or improve their SQL skills.

3. SQLTest : http://sqltest.net

SQLTest is a free SQL course that provides a comprehensive set of SQL exercises. The website is designed for both beginners and advanced SQL users. SQLTest is suitable for users who want to learn SQL from scratch, as well as for users who want to improve their SQL skills.

4. W3Schools : http://w3schools.com/sql/

W3Schools is a popular website for learning web development. It also offers a free SQL tutorial that covers SQL basics and advanced concepts. The website provides clear examples and explanations. W3Schools is suitable for users who want to learn SQL as part of their web development skills.

5. Codecademy: http://codecademy.com/learn/learn-sql

Codecademy is a well-known website for learning programming. It offers a free interactive SQL course that teaches SQL fundamentals. The website is user-friendly and provides instant feedback on your progress. Codecademy is suitable for users who want to learn SQL as part of their programming skills.

These websites offer a great opportunity to learn SQL for free. They are user-friendly and offer various SQL exercises and examples. Start your SQL learning journey today with any of these websites.