Pulled together for a reply on Twitter, a collection of tutorial projects for databases (and a bit of dist-sys):

* https://howqueryengineswork.com
* https://cstack.github.io/db_tutorial/
* https://github.com/pingcap/talent-plan
* https://fly.io/dist-sys/
* https://eli.thegreenplace.net/2020/implementing-raft-part-0-introduction/
* https://skyzh.github.io/mini-lsm/
* https://github.com/cmu-db/bustub course projects