Since you are already good on SQL basics, it will be easy for you to get into advanced concepts. There are plenty of SQL tutorials available online, I like this one on w3schools -

https://www.w3schools.com/SQl/sql_join.asp

You can start with the SQL JOIN chapter, (or recap the basic concepts again if you wish). They have good examples and are detailed but simple enough for beginner like you.

If you are doing PostgreSQL, you can follow their online documentation here for reference on any topic. Its very detailed.
https://www.postgresql.org/docs/9.4/static/index.html

Also to get even further, buy any advanced SQL programming book, and read it offline. Books are the best way to improve your SQL skills, knowledge and concepts shared in books are usually not found online or at-least not in once place.