The [Database Management System (DMBS) course](https://youtube.com/playlist?list=PL9ysvtVnryGpnIj9rcIqNDxakUn6v72Hm&si=4fAC6xYpLUyZt2HK) taught by Jennifer Widom at Stanford University is a free introduction to databases offered on YouTube. This is a perfect introduction to database theory. You'll need to couple it learning a DBMS like [[Postgres]] as it doesn't teach any software. There is also a textbook [Database Systems: The Complete Book](http://infolab.stanford.edu/~ullman/dscb.html).