Make sure you know about database transactions and the various ways they can be configured and what effects it will have on data consistency. That way you will

Make sure you know about database transactions and the various ways they can be configured and what effects it will have on data consistency. That way you will not make code that seems to work but which randomly corrupts data. That is the most important thing of all. Beyond that I would go with learning what you need on whatever job you get, just be aware that choice of DB is just as fundamental as what programming language is used to a backend project.