Several years ago, I spent a lot of time developing a relational database management system - ThinkSQL.
In a future post I'll write down the reasons why, and probably release the development diaries. But for now, the source code for the server is available here: https://github.com/ggaughan/ThinkSQL
As I say in the README:
The source code is linear per se, but while writing it, it was an organic thing, generating great interwoven trees in the computer memory and in my head that were many-layered and that were modified and traversed, and impacted on a dynamic multi-versioned data store alongside many other threads, causing and needing deep psychological flow. The code comments are released as-is, and are often streams-of-consciousness.