Getting to Know Relations

I don’t really KNOW relational databases. I’ve hung around with them quite a bit but I have no real experience. I know there are things I don’t understand about them. As with anything I’m genuinely uncertain about, I don’t even know what it is I don’t understand. I am in unknown unknowns territory.

While getting my  Umbrello UML modelling tool working again, I tripped over this:
https://docs.kde.org/stable4/en/kdesdk/umbrello/uml-elements.html#entity-relationship-diagram

It was a bit of a surprise because:
a) entity-relationship diagrams aren’t part of UML and
b) the explanation of the ‘overlapping specialisation’ example didn’t make sense to me.

I now knew this was one of the things I didn’t understand, but the KDE document wasn’t helping me. I searched for a better explanation and it took me here:

http://www.tomjewett.com/dbdesign/dbdesign.php?page=subclass.php

It uses UML class diagrams to model relations. I think this might sort me out.

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