Passionate, business-oriented software architect and technologist with 2 decades of hands-on experience in software development, solution architecture, technology management and team building. Successfully implemented and delivered a broad range of software solutions for finance, banking, telecom, legal tech, e-commerce and retail domains. Team-player/builder/leader with a collaborative, can-do attitude. Extensive experience designing, developing, managing software projects. Implemented critical business solutions in Microsoft, Java and open source technologies for financial strategic planning, capital markets, securitization and syndication, e-billing, accounting, wholesale distribution, e-commerce, data analytics and integration projects. Build, managed and trained high functioning development teams.
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 -
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.
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.