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

[Garth, Lucas A]

Hi Mark

Surely this is doable in terms of the drawing – to link customer and shoe with M:N and use the verb PURCHASES or BUYS:

Cardinality – The number of entities to which another entity can be associated through a relationship

The diagrams on the right show, in order:

one-to-one

one-to-many

many-to-one

many-to-many






[cid:image002.png at 01D245A3.161DE750]



I agree that the answer should be many to many (M:N) because of the use of the word customers rather than customer.  Many customer can buy Nike Air Jordans.  One customer can buy Reebok Pumps and Nike Air Jordans (but not in the same purchase).

This situation is where an ERD is going to differ to how the database is actually built. Whenever there’s a M:N, generally there’s a linking table created in the middle with the primary keys of the entities.  In this case the middle table could conceivably be the sales table that can then link the item and shoe and contain the sales date and amount values.  (But this would potentially make it an entity…shh)

I also agree there should be a sales table.  It’s part of a list of the types of databases (sales & customer, payments, music, bookings) that I teach my students to be ready for the exam (and real life).

Lucas

From: informatics-bounces at edulists.com.au [mailto:informatics-bounces at edulists.com.au] On Behalf Of Mark
Sent: Wednesday, 23 November 2016 3:14 PM
To: Year 12 VCE Informatics Teachers' Mailing List
Subject: [Informatics] Info exam - B1b - The Curse of Chen !

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<mailto:mark at vceit.com>
http://vceit.com

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