Ø
Extended
feature of ER diagram –
1. Generalization
2. Specialization
3. Aggregtion
1) Generalization –
·
The process of generalizing entities,
where the generalized entities contain the properties of all the generalized
entities, is called generalization.
·
Generalization is opposite of specialization
(reverse of specialization).
·
In generalization there exists a
bottom up approach.
·
In generalization, a number of entities are
brought together into one generalized entity based on their similar
characteristics.
·
Employee and customer can be a person
so we are removing some attributes.
·
Lower level entities are producing
higher level entity.
v
Inherit
– inherit is an important feature of generalization, in
generalization it allow upper level entity to inherit lower level entity
attribute.
2) Specialization –
·
The process of classifying an entity
into more specialized entity.
·
Specialization is the just opposite of
generalization.
·
In specialization a group of entity is divided
into sub group on their characteristic.
·
It is a top-down approach in which one
higher level entity can be broken down into two lower level entities.
·
In specialization, a higher level entity may
not have any lower-level entity sets, it's possible.
v
Inherit
– inherit is an important feature of specialization, in specialization
it allow lower level entity to inherit upper level entity attribute.
3) Aggregation – A
feature of entity relationship model that allows a relationship set to
participate in another relationship set. This is indicated on an ER diagram by drawing
a dashed box around the aggregation.
·
The meaning of aggregation is way of
abstraction.
·
The process through which one can treat the
relationship as higher level entities is known as aggregation.
·
It shows relationship among
relationship
