I really suggest against building a database from scratch. It's just too annoying, and there's so much code to write (parser, storage, indexing, query planner, connections). If you're interested in internals, I'd say look at the sqlite codebase instead: https://sqlite.org/src/doc/trunk/README.md . If anything, reading code that works is probably more useful than writing code that almost certainly won't without months and possibly years of effort.
A lot of the more complex database things are only really learned by having a large database system. Performance, distributed databases, and complex schemas come to mind here. Most of the times with simple examples, you'll do something wrong performance wise, but you'll never know because of the scale (such as forgetting an index, or doing a bad join).
Many times, you don't need to know that much about database other than some basic SQL.
Comments
I really suggest against building a database from scratch. It's just too annoying, and there's so much code to write (parser, storage, indexing, query planner, connections). If you're interested in internals, I'd say look at the sqlite codebase instead: https://sqlite.org/src/doc/trunk/README.md . If anything, reading code that works is probably more useful than writing code that almost certainly won't without months and possibly years of effort.
A lot of the more complex database things are only really learned by having a large database system. Performance, distributed databases, and complex schemas come to mind here. Most of the times with simple examples, you'll do something wrong performance wise, but you'll never know because of the scale (such as forgetting an index, or doing a bad join).
Many times, you don't need to know that much about database other than some basic SQL.