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Or:

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Or:

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It can become:

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How to remove a generalization from an ER Model

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How to remove a generalization from an ER Model

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We start from a model like this:

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Represents existing logical links between two or more entities.
Among the entities involved we distinguish:

• one and only one parent entity (father entity)
• one or more entities daughter

Daughter entities constitute "special cases" of the entity father.
Every parent entity attribute is also an attribute of all the entities daughter, but daughter entities may have attributes that are different from them and from the father.

Generalizations are divided into total and partial.
A generalization is total when: "the union of the subsets of the children is the set of his father". A generalization is partial when "the union of subsets of children not globally identifies the set of father".

A generalization can also be exclusive or overlapping.
A generalization is exclusive when the intersection of subsets of children is empty; is instead overlapped when the intersection of subsets of children is not empty.
In conclusion, a generalization can be:
* total exclusive (t,e)
* total overlapped (t, s)
* partial exclusive (p,e)
* partial overlapped (p, s)

Examples:

the person generalization in man and woman is total and exclusive;

the generalization of the vehicle in the car and motorbike is partial and exclusive;

the person generalization in student and employee is partial and overlapping.

How to design a generalization in the ER Model:

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ACTIVITIES:
1) Listen to and watch the video very carefully, taking notes about everything you see and hear

2) Then answer the following questions:
a) What shapes are used to represent:
◦ Entities
◦ attributes
◦ relationships

b) Write the name of the entities and the attributes of each entity
c) What kind of key is the attribute Address, and what can it have?
d) Write the name of each relationship between all of the entities
e) Then reproduce the same ER diagram displayed in the video
f) Find the way of expressing the relationship between entities

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ER- Diagram Explanation Google student club activity 3 subject DBMS CSE Btech 5th SEM

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Uploaded by prabhdeep kaur on 2013-08-27.