My favorite has been https://pgexercises.com/. Instead of going through theory, you practice writing progressively more complex queries to achieve a specific result.

I've only used it as a reference, but the art of postgresql is pretty solid. In general any webdev who isn't skilled at SQL is missing out on an easy way to level up. SQL is crazy powerful, versatile, and scalable, and takes a small amount of time to gain proficiency, and stays with you. I methodically read one 350 page book in 2012 and still feel I understand SQL better than most I come across. That's kind of silly. I recommend learning enough to understand the gist of aggregates and window functions. Once you understand what they can do, you can easily reference the syntax when needed.

[1]: https://theartofpostgresql.com

The postgresql documentation is actually a pretty good start (and finish!).

I like learning by doing something so I took an old side project and I'm making it on PostgreSQL with PostgREST in front of it. It is crazy all the things that the DB can do! Triggers after an actions happens, virtual columns, JWT Tokens, Unit Testing...

Theory: Database Lessons by Dr. Daniel Soper (watch at x2 speed) https://www.youtube.com/playlist?list=PL1LIXLIF50uXWJ9alDSXC...

Learn by watching short videos and practicing: Mastery with SQL (paid course, $49) - https://www.masterywithsql.com/

I would recommend SQL and Relational Theory by CJ Date.

It completely changed how I use SQL. Before reading it, I had about a decade of experience with SQL. I didn't really understand the theory underneath. I also didn't understand how to write SQL in a way that adheres to the theory.

You can think of this book as being the closest we have to "SQL: The Good Parts".

Execute Program has a great interactive course:

https://www.executeprogram.com/courses/sql