Could you give me some examples of a weak-entity type.

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Request for Question Clarification by livioflores-ga on 24 Jun 2003 00:41 PDT

Hi 888!!

I am not an expert on this subject, so before to post an answer I want
to confirm if the data that I found is useful to you.

"Reflexive Relationships", this powerpoint presentation has good
explanation and examples about this topic, just download it and see
with Ms PowerPoint:
http://www.comp.dit.ie/cliu/courses/ft2282id/coursenotes/dbi42.ppt

"The Entity-Relationship (ER) Model":
http://www.cis.ohio-state.edu/~gurari/course/cis670/cis670Ch2.html

"The E/R Model":
http://www.cs.jcu.edu.au/teaching/Subjects/cp1500/1998/foils/introER.html#section28
http://www.cs.jcu.edu.au/teaching/Subjects/cp1500/1998/foils/introER.html

"Entity-Relationship Modeling":
http://www.siue.edu/~dbock/cmis450/3-ermodel.htm

"E/R and Object Modeling": Another Powerpoint presentation to
download.
http://cs.gmu.edu/~aobaidi/spring-02/eerandobjectmodeling.ppt

"Mapping from ER or EER to Relational Model":
http://nas.cl.uh.edu/yang/teaching/csci5333fall02/csci5333ch9.htm

"Example sheet":
http://www.cs.man.ac.uk/~horrocks/Teaching/cs2312/Examples/example-sheet4-answers.pdf

Please see this sources and let me know if they satisfy your request,
and I will post them as the answer.

I will wait for your response.
Best regards.
livioflores-ga

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Clarification of Question by 888-ga on 24 Jun 2003 01:45 PDT

You could put it into answer, but could you explain "weak-entity" more
specificly, Thank you !

Hi 888!! 

Weak entity type:
A weak entity type is an entity that needs the key attributes from
another entity to uniquely identify tuples. Weak entities lack keys.

"What is a weak entity?
It is one where to determine the existence of an instance of the
entity, you need to know the existence of at least an instance of one
parent. The corollary states that if a parent instance does not exist,
the child instance can not exist. For example in a relationship
between MOTHER and CHILD, a child instance (Chelsea Clinton) cannot
exist without a mother instance (Hillary Clinton)."
From "College of Business, Iowa State University" website, "Sree
Nilakanta, Ph.D." page:
http://www.bus.iastate.edu/nilakant/MIS%20433/Sp2003/FAQ.htm


You can find a good definition in the following set of pages of the
"School of Computing Science at SFU" website:
"The Entity-Relationship Model":
http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node1.html

At this page visit these links following the order (you can do that
just visiting the first link and after reading the page pressing the
button NEXT):
1 - Entities and Entity Sets:
http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node2.html

2- Relationships & Relationship Sets:
http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node3.html

3- Attributes:
http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node4.html

4- Mapping Constraints:
http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node5.html

5- Keys:
http://www.cs.sfu.ca/CC/354/zaiane/material/notes/Chapter2/node6.html


You can find several examples and explanations in the following
documents:

"Reflexive Relationships", this powerpoint presentation has good
explanation and examples about this topic, just download it and see
with Ms PowerPoint:
http://www.comp.dit.ie/cliu/courses/ft2282id/coursenotes/dbi42.ppt 
 
"The Entity-Relationship (ER) Model": 
http://www.cis.ohio-state.edu/~gurari/course/cis670/cis670Ch2.html 
 
"The E/R Model": 
http://www.cs.jcu.edu.au/teaching/Subjects/cp1500/1998/foils/introER.html#section28
http://www.cs.jcu.edu.au/teaching/Subjects/cp1500/1998/foils/introER.html
 
"Entity-Relationship Modeling": 
http://www.siue.edu/~dbock/cmis450/3-ermodel.htm 
 
"E/R and Object Modeling": Another Powerpoint presentation to
download.
http://cs.gmu.edu/~aobaidi/spring-02/eerandobjectmodeling.ppt 
 
"Mapping from ER or EER to Relational Model": 
http://nas.cl.uh.edu/yang/teaching/csci5333fall02/csci5333ch9.htm 
 
"Example sheet": 
http://www.cs.man.ac.uk/~horrocks/Teaching/cs2312/Examples/example-sheet4-answers.pdf

Search strategy:
"weak entity type"
"weak entity" "what is"


I hope this helps you. Please feel free to post a request for an
answer clarification if you need it, I will gladly reponse to your
request.

Best regards.
livioflores-ga