For example, I've been building a new web app with Elixir and Phoenix for the last few weeks, learning as I go. Couple hours a day (when I can basically). Did nothing but skim the docs and look at open source examples.
I got pretty far and made real progress but the other day I decided to just buy the Programming Phoenix 1.4 book which is written by the people who made Elixir and Phoenix.
I'm now 80% done with the book and I'm very happy I bought it. It filled in SO many gaps and I have a ton of actionable "TODOs" in my app to make changes based on what I learned in the book. Some of these are really big wins and the insights learned help me understand how to write better code in the future (even outside of Elixir).
When it comes to courses, I can't speak for OP but I personally offer free life time updates and support (I happen to create courses too, but not on SQL). Having a get out of jail free card where you can ask someone a question and get an answer is worth the course's price alone. Especially considering if you were to hire someone for that level of support you would probably end up paying 3 or 4 times the cost of the entire course just for 1 hour's worth of support.
> ...instead of just learn it on your own on postgresql.org...
The important thing is to just do it, accept failures, apply some self-criticism and keep going. How you get started is far less important in terms of quality of output as time progresses in one's career.
Courses are a perfectly good way to jump-start the process as is learning from source materials. Different things work for different people.
I still contented the £2K (back in the day) that my employer spent on a week long intro to Oracle was a good investment for them and Me.
As for how my employer at the time pointed me toward SQL was something like; "Here is yahoo.com, learn SybaseSQL by Monday on your own, you start a new project on Monday that requires it". It was Friday afternoon, in 1998 and Google was still in it's infancy, hence all searches were done via yahoo or altavista. And I had to learn it by myself over the week-end. Not even a £0.02 investment from my boss.
Most people are not going to "learn databases" over the weekend. A boss or company that has employees capable of this type of independent learning and routinely expects it will soon lose the former. That costs more than 2 cents.