One of the model for conceptual schema is ER(entity relational) model.
Note : weak entity does not mean total participation or having identifying relationship
but identifying relationship implies total participation.
ENHANCED ER NOTATIONS/ ENTENDED ER NOTATIONS:
An association between the subclass entity B with another entity A is called as
inheritance. In other words, an entity class B is said to be a subclass of other class A if
it share the “is a” relationship with A. example, A car “is a”Vehicle. Here a vehicle is a
more general class. A car is a vehicle but vehicle cannot necessarily car, vehicle may
be a bus, scooter or cycle etc.
An entity of class B is said to be specialization of entities of class A. conversely,
entities of A is said to be generalizations of class B entities. So we called car as
specialization of vehicle because it is specific entity and vehicle represents the general
class of these specific entities, so we called them generalizations of theses specialized
class. A specialized class may have more attributes than the general class but the
general class represent those attributes which are common among the specialized
● PROPERTIES OF GENERALIZATION AND SPECIALIZATION:
1.a subclass undergo type inheritance of the super class. That is, each member of
the subclass has the same attributes as the super class entities and participates in
the same relationship types.
2. if a general class participates in relationship types then the specialized class
should also participates in the same relationship types. That is if a vehicle is said
to go from point M to point N, then car or truck or cycle should also go from
point M to point N.
● Example :
● Specialization : the process of creating subclasses from the given entity type is
called specialization. Suppose in the UOD( universe of discourse) we found found
that there is a vehicle class and in the vehicle class , we found that there are cars,
trucks, scooter and cycle and I found are theses are vehicle. So we create them as
subclass of vehicle class.
● Generalization : the reverse of taking two or more entity types and clubbing them
under a common super class is called as generalization. Suppose in the UOD , we
have cars, trucks, scooters and cycles, all of these have some common attributes.
So we make them as a specialized class of more general class “vehicle” which
have attributes common among specialized class . It is not a straight forward
process. It depends on the application context.
● PROPERTIES OF SPECIALIZED CLASS: whatever the attributes the general
class have , that attributes should also be inherited by the specialized class.
● Formation of the generalized class. Here the D denotes
that the subclasses are disjoint.
● Predicate defined class: in this the specialized class is
formed on the basis of value of attribute called predicate
attribute which is here it is called job type.
● Here O denotes that the sub class that are not disjoint
are said to be overlap. Disjoint or overlap may be partial
UNION TYPES OR CATEGORIES: an account holder an be an individual or
family or institution. Not every individual or family or institution is an account
holder. In such a case, an account holder is said to be a “partial” union. Usually a
constraint is specified that determines which entity has to participates in the union.
● Aggregation : we called it as knowledge management. In this aggregation
aggregates the schema to a higher level of abstraction. Aggregation brings about
the contain ship. Here the aggregating entity called offers which contains the
atleast an instances of relationship between the instructor, semester and course.
● The difference between the abstraction in the inheritance and abstraction in the
aggregation is that in aggregation, the relationship show the “containment”
whereas in the inheritance the relationship shows the “is a”.
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