There are many free resources online for learning SQL. Here are some of them:
Khan Academy has a SQL course:
https://www.khanacademy.org/computing/computer-programming/s...
The one in the first link below is interesting in that it then sets you up for the SQL Murder Mystery in the second link:
https://mystery.knightlab.com/walkthrough.html
https://mystery.knightlab.com/
The "walkthrough" link above mentions that if you want to learn SQL more deeply to try selectstarsql.com, which looks good:
https://selectstarsql.com/
This looks interesting too:
https://sqlzoo.net/wiki/SQL_Tutorial
Most of the above have databases available for you to immediately try out what you're learning, which should be helpful (sqlzoo.net also seems to have some more advanced topics covered too).
There are also a bunch of (multi-hour) courses on YouTube:
https://www.google.com/search?q=site%3Ayoutube.com+sql+cours...
If you go the video route, there are sites like SqlFiddle.com and freedatabase.com, etc. that let you set up databases so you can practice what you learn from the videos without having to install anything (though you can do that too, of course):
https://www.google.com/search?online+sql+database
I hope you find at least some of the above to be useful. Good luck with it.