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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">Hi everyone,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">If I am reading it correctly, the ERD in the Potts textbook (Pg 107) has the same issues.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">While researching this topic, I found that Chen’s notation is predominantly used in the USA and the Crow’s Foot in the UK.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">Many thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D">Judy Zuccon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D"><o:p> </o:p></span></p>
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color:#1F497D"><img width="624" height="151" id="Picture_x0020_1" src="cid:image001.jpg@01CBF2E4.7C9F21A0" alt="judy_zuccon_email_sig copy"><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:
"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;
font-family:"Tahoma","sans-serif""> itapps-bounces@edulists.com.au [mailto:itapps-bounces@edulists.com.au]
<b>On Behalf Of </b>Mark KELLY<br>
<b>Sent:</b> Monday, 4 April 2011 3:45 PM<br>
<b>To:</b> Year 12 IT Applications Teachers' Mailing List<br>
<b>Subject:</b> [Year 12 IT Apps] ERD - curiouser and curiouser<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">The more I look at the sample ERD, the odder it seems.<br>
<br>
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.<br>
<br>
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.<br>
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.<br>
<br>
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.<br>
<br>
Any thoughts from ERD-heads? Has anyone else seen an ERD relationship with more than 2 branches? I'd like to get this right.<br>
<br>
Cheers<br>
Mark<br clear="all">
<br>
-- <br>
Mark Kelly<br>
Manager of ICT, Reporting, IT Learning Area<br>
McKinnon Secondary College<br>
McKinnon Rd McKinnon 3204, Victoria, Australia<br>
Direct line / Voicemail: +613 8520 9085, Fax +613 9578 9253<br>
<a href="mailto:kel@mckinnonsc.vic.edu.au" target="_blank">kel@mckinnonsc.vic.edu.au</a><br>
VCE IT Lecture Notes: <a href="http://vceit.com" target="_blank">http://vceit.com</a><br>
Moderator: IT Applications Edulist<br>
<br>
All generalisations are false, except this one.<o:p></o:p></p>
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