[Year 12 IT Apps] ERD - curiouser and curiouser

[Judy Zuccon]

Hi everyone,
If I am reading it correctly, the ERD in the Potts textbook (Pg 107) has the same issues.
While researching this topic, I found that Chen's notation is predominantly used in the USA and the Crow's Foot in the UK.
Many thanks
Judy Zuccon

[cid:image001.jpg at 01CBF2E4.7C9F21A0]

From: itapps-bounces at edulists.com.au [mailto:itapps-bounces at edulists.com.au] On Behalf Of Mark KELLY
Sent: Monday, 4 April 2011 3:45 PM
To: Year 12 IT Applications Teachers' Mailing List
Subject: [Year 12 IT Apps] ERD - curiouser and curiouser

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<mailto: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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