I just finished learning Postgres! I used codecademy's SQL session for the intro, but then just decided to play around with queries. This diagram was my most useful resource, and one that I still look at on a daily basis when trying to create queries - I hope it helps! https://www.sqlite.org/images/syntax/select-stmt.gif
I also look at the postgres docs very frequently for syntax and format and those help me a lot. Stack Overflow is also a great resource!
I found that course excellent as well - although these days if you don't have the "pro" tier of Codecademy you miss out on the quizzes and other things. Still, it was good and pretty advanced. Learned a lot.
That's very true, I actually ended up subscribing to pro because I was also learning JavaScript and HTML/CSS at the time (new programmer), so I found the pro stuff useful. However, if you're not wanting to drop the $20 or so for the month, I'd say that you can find plenty of fantastic online resources.
I also look at the postgres docs very frequently for syntax and format and those help me a lot. Stack Overflow is also a great resource!