I used a number of online resources - don’t recall them all but notable ones were:

https://use-the-index-luke.com/

http://www.postgresqltutorial.com/

Postgres documentation

Postgres conference videos and slides (can find on Postgres website and on YouTube)

...you can guess I don’t rely on books too much!

I recommend everything by Joe Celko and CJ Date. Start with Celko.

The most advanced book on SQL I have read is https://www.goodreads.com/book/show/1255518.SQL_Design_Patte...