https://www.codecademy.com/learn/learn-sql begineer
https://www.w3schools.com/sql/ begineer
https://sqlzoo.net/ begineer - intermediate
http://sqlfiddle.com/ intermediate
https://sqlbolt.com/lesson/select_queries_introduction intermediate
https://sqlpad.io/playground/mysql/ intermediate - advanced
https://www.hackerrank.com/challenges/15-days-of-learning-sql/problem intermediate - advanced