There are a few options. If you've done the basics through Codecademy, then you're ready to move on to the specifics of a platform (such as PostgreSQL as you've mentioned). Though this can equally be a minefield.

The reporting engines/formats differ from platform to platform. As does SQL believe it or not, in the sense each of the mainstream vendors adds their own specific bits to the standard. They are all compliant for the basics though. All of them will support the latest version of SQL (ANSI standard 2011), but PL/SQL is different to Transact-SQL in the way it uses cursors.

In addition, each has an even bigger variation when it comes to multidimensional queries such as MDX, since there is no standard for MDX and some vendors use their own expressions and languages to carry out such queries.

I can certainly help with tutoring, having been working in SQL since 1997 (and still do), although I think it's worth focussing on getting a little better at the specifics of the SQL vendor extensions you want to use, including their Cube, Pivot and Grouping functions as well as more complex joins, as they'll be the baseline skills for future reporting exercises, including analytical processing.

Hello, my name is Erik and I'm a Senior Analyst Programmer. To build more complex queries you must practice, practice and practice. Challenge yourself! Think of complex results you want in a report and do the query, however how long it will take. Over time, you'll become more efficient in building queries and you'll be able to analyse any data.

If you need help to get a step by step guideline to follow or want private SQL training, I'm available to help you.

Regards,

Erik

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.

Depending where you live there is a great organization I teach for that has locations all over the US. It's called General Assembly. Website: https://generalassemb.ly/
I teach a Data Viz 101 there but they have, at the very least access to, lots of professionals who are more than qualified to help you out. I may be able to help or, depending on what you need, can put you in touch with someone who can.