I also recently began learning SQL properly, and I also regret not having done so earlier. The PG docs/tutorial is really great to get started. After I worked

I also recently began learning SQL properly, and I also regret not having done so earlier. The PG docs/tutorial is really great to get started. After I worked through those, I feel it was essential to work my way through pgexercises.com. It's really easy to think you get it, but actually having to puzzle through stuff is obviously quite different. When I reached some of the more complicated subquery stuff, I decided to get 'Learn SQL' by Alan Beaulieu (recommended by pgexercises), and I just worked my way from the start all over again. Book's excellent too. I do find that I hit a 'barrier' when all the basic parts are introduced and it becomes all about translating a goal to 'thinking in SQL', especially when there seem to be multiple valid approaches. But the combination of reading a ton, then doing the exercises, then re-reading and trying to play around with the Learning SQL db (Sequel Pro is great for Mac, as is Postico (Postgres)) has been very effective so far.