I like it because as a beginner you can just follow the learning path. But if you just need a refresher about how to write a window function you can jump right to that section.
Oh, and no account sign-ups which I really appreciate.
You can also use it to validate what you've done using other tools. E.g. if you write some PHP or Python to interact with a database, you can open it in Sqlectron to double-check for yourself that it worked correctly.
not by itself a general resource but it's the best book on postgresql i have read recently. (i have read 2 or 3 but this stood out).
learning postgresql 11: https://www.amazon.com/Learning-PostgreSQL-beginners-high-pe...
i have also subscribed to the following newsletter to keep myself up to date with what is going on in the community. keep in mind that postgresql 12 has been out for a minute.
postgres weekly: https://postgresweekly.com/
in my mind postgresql is one of the most interesting projects out there in the open source community. and that newsletter sometimes includes articles that compares it to other big names like mysql/maria, oracle, etc. this helps expand your sql scape a little.