[Informatics] Info exam - B1b - The Curse of Chen !

[Mark]

Hi all. Apologies again if this question has already been discussed to
death, but I have restrained from reading exam discussion until I finish
the first draft of my post mortem.

So, B1b and Chen !

(I added the exclamation mark to make dull ERD discussion more exciting.
Did it work?)

*Point 1.*

I'm guessing the markers are expecting a diamond labelled "BUYS" with a 1
(customer) to M (shoe) relationship since the question said that *one*
customer can buy *more than one* pair of shoes.

But in the real world many types of shoes can be bought by many different
customers, so it should be many-to-many (M:N).

Will markers accept a common-sense answer?

*Point 2.*

"Date Sold" is shown as an attribute of the SHOE entity. This is absurd.
This would mean that each style of shoe could only ever be sold *once*.

Obviously, 'Date Sold' is an attribute of a *transaction *entity, not a *shoe
*entity, so the ERD violates 3NF.

How can a Chen ERD make the correct relationship clear? I've been trying to
sketch it out using VCAA conventions, without success.

Should the "Date Sold" actually be an attribute of the "Buys" *relationship*
?

Can Chen relationships even *have* attributes?

Not according to VCAA's 'ER Conventions' document
<http://www.vcaa.vic.edu.au/Documents/vce/computing/Informatics_examination_ER_Conventions.docx>
.

But it's the only way I can do it using VCAA's Chen conventions.


*Newsflash*. Research in The Real World (TM) reveals Chen relationships CAN
have attributes !

See http://www.conceptdraw.com/How-To-Guide/erd-how-to-draw-er-diagrams

Specifically -
http://www.conceptdraw.com/How-To-Guide/picture/Chen-ERD--sample.png

It shows how *relationships can have attributes attached*, for example the
"Has" relationship has attributes, "LastPlayed" and "CreatedOn".

But then again, the Conceptdraw site also tells us there are scary beasties
like : multivalue attributes; derived attributes; weak (identifying)
relationships; and associative entities - so I suppose we should be
grateful that students have been firewalled from *some* of The Real World.


Warning - pointless rant begins...
(don't say you weren't warned)

Maybe I just am prejudiced against nice Mr Chen (who probably did many
wonderful things in his career) but I can't accept an ERD that makes no
logical sense and makes me feel all icky inside*.

Maybe it's just VCAA's interpretation of his work that fails to satisfy.
If VCAA made a movie of 'The Sound of Music', I'd bet they would leave out
all of the songs in order to make the film shorter **.

So, long live *crows feet* ERDs !

OK. I've come out and said it proudly. I've been quiet on the issue until
now, but crows feet rock my world***.

Regards,
Mark

P.S. This entire post *may* be a cunning plan to get you to look deeply
into a single exam question and find the deliberate glaring error in my
analysis and formulate an equally-cunning substantiated repudiation of my
thesis.

FOOTNOTES

* Adam Sandler movies do much the same thing to me.
** This reputedly happened in South Korea
<http://unrealfacts.com/one-south-korean-theatre-removed-all-music-scenes-from-the-sound-of-music/>
.
*** It has been established before now that my life is not at all rocked.
It is sad and wretched. I eat *tinned* dolmades (sorry, Litsa), dogs are
*not* my best friend, and I *still* want Pluto to be a planet. Sob.

-- 

Mark Kelly

mark at vceit.com
http://vceit.com
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