[Year 12 IT Apps] ERD - curiouser and curiouser

[Mark KELLY]

The more I look at the sample ERD, the odder it seems.

The sample VCAA ERD has not one, but two relationships that have more than
one input and output ("supervises" has 3 branches, "gives" has 3).  I have
now looked at about 40 ERDs in The Real World (TM) and so far not one of
them has any relationship with more than two branches.

It seems to me that our sample ERD should not have just one "supervises"
relationship: it should have two.  One aims at Subject and the other at
Class.
Otherwise, if one were to assign cardinalities, you'd have to have the same
values for each relationship, which might not in fact be what is right.  For
example, the relationship between the teacher supervising subject and the
teacher supervising class might not both be 1:many.  With the single line
between teacher and supervises, I can't see how this difference could be
indicated.

The "gives" relationship is even more weird. It seems to be a shorthand
construct that doesn't actually work as a relationship. I've redrawn a
version (attached) that seems to be a bit more workable, using only 2
branches from each relationship.

Any thoughts from ERD-heads?  Has anyone else seen an ERD relationship with
more than 2 branches?  I'd like to get this right.

Cheers
Mark

-- 
Mark Kelly
Manager of ICT, Reporting, IT Learning Area
McKinnon Secondary College
McKinnon Rd McKinnon 3204, Victoria, Australia
Direct line / Voicemail: +613 8520 9085, Fax +613 9578 9253
kel at mckinnonsc.vic.edu.au
VCE IT Lecture Notes: http://vceit.com
Moderator: IT Applications Edulist

All generalisations are false, except this one.
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