I recently installed the Sololearn Application on my phone and have been working my way through the SQL and Python courses. It has been a great learning resource to use whilst on the go.
The SQL course was a good starting point in learning some of the fundamentals for writing SQL queries and seeing them in action. Whilst there was plenty of information to read through, there is also ample opportunity within the Application to put it into Practice with a sandboxed coding area. There is also a brilliant community to test yourself against in coding challenges.