I never learnt SQL "properly" until I took one of the first MOOC courses, Introduction to Databases by Jennifer Widom. Really good for getting a better understanding. Also, for a fantastic source on database transactions and isolation levels, check out "Designing Data-Intensive Applications" (chapter 7) by Martin Kleppmann. Really a great book!

I have written about both these here:

https://henrikwarne.com/2011/12/18/introduction-to-databases...

https://henrikwarne.com/2019/07/27/book-review-designing-dat...