I would pick one RDBMS and try to dissect it, there is a lot to chose from nowadays, you can check out db-engines to get a general sense of what's out there:

https://db-engines.com/en/ranking

From what I have seen most enterprises today will be using Oracle or Microsoft, however PostgreSQL seems to have gained popularity with the web developer and small business crowd (as well as with the HN community). I have been an Oracle database developer since 2015 and would definitely recommend going that route if it interests you, at the very least it might be a good starting point because of the fantastic documentation, here's a great guide I recommend to get you started with all the basic concepts:

https://docs.oracle.com/en/database/oracle/oracle-database/1...