O'Reilly has an excellent pocket book on generic SQL titled "SQL Pocket Guide: A Guide to SQL Usage" (it has several editions). That book worked really well for me. It is surprisingly thorough for a pocket book. I highly recommend it.

Besides that, I highly recommend https://sqlite.org/ as a reference. It's syntax pages for SQL are fantastic. But to understand the language you need a better resource, and for my money that's the book mentioned above.

w3schools and the online documentation for any database are good places to start (Postgres and SQL Server have pretty good documentation).

There are also easy-to-google SQL Puzzles, if you're looking for something more advanced.

I would also recommend learning about query plans and how to read them, they're invaluable for query optimization.

Basic SQL has lots of tutorials. Graduate from them to Markus Winand's use-the-index-luke.com and modern-sql.com