Here are some popular papers: https://github.com/rxin/db-readings

Once you finish the major papers, CMU has a ton of talks from more recent database research projects: https://db.cs.cmu.edu/seminar2020/

You could also try building a simple database from scratch, like this: https://www.awelm.com/posts/simple-db/

Also if you are specifically interested in SQL, make sure to learn relational algebra/calculus.