Re: Database Design Diagram
Well - it is tricky indeed.
I'll say it depends on the task at hand and the focus of the project, and the development methods used for the project. Scope is also a part to consider.
Especially because it is a educational project, there is also the time-constraint whether it is worth to actually include such a diagram compared to simply argue the points.
So it will be pretty much impossible to give a straight answer :)
If optimizing performance in the database is a part, then I'd most certainly include a database diagram, so you can map out which querys will be performed and thus which indexes to build while mapping out the database.
If not, then I'd personally - in an educational situation - use a class diagram as foundation for the database. I never made a project without a class diagram, well - once but then we were following a design method which didn't include class diagrams in its method. (was not an OO method).
So if database performance/design is not a (significant) part of the project you are working on, I'd properly then argue that the database diagram would be very similar to a class diagram and instead mention when I would make it and why. But because of the focus of the project, combined with the time-constraints I would not make it.
But as said - the "right" answer, the text-book answer if you will, is yes - it should be included.
But the pragmatic answer (and more often then not real life answer) is - depends on the scope and properly not. As long as you argue the why and why nots. :D