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You'll likely find the PostgreSQL docs[0] very useful. They're organized differently than the MySQL reference making it easier (in my experience) for reading stand-alone.† They're quite well-written and up-to-date.

[0]: https://www.postgresql.org/docs/

† The MySQL reference is very useful particularly when you're looking to compare feature differences between versions.

You may like PostgreSQL Exercises: https://www.pgexercises.com/

I'd start with the docs: https://www.postgresql.org/docs/

They are pretty good.