Education logo

Database Management Systems|Enhanced Entity Relationship and Object Modeling

6.0 Enhanced Entity Relationship and Object Modeling

By SHD TECHPublished about a year ago 4 min read

6.1 Super Classes/ SubClasses

• Entity type represents

o Type of Entity

o Entity Set (collection of entities of that type that exist in the database)

• Entity type may have many subgroups

o Sub groups need to be represented explicitly (due to the significance to

the database)

• Each sub group is called a Sub Class

• Main entity type, which belongs the subgroups is called Super Class

• Member entity of sub class represents the same real world entity as some entity of the super class

• At the same time, that particular entity of sub class plays a distinct specific role, by which the specialty is identified

• Each member of sub class is a member of its super class

• Super Class – Sub Class relationship is called an IS-A relationship


6.2 Type Inheritance

• Member of the sub class

o Inherits all the attributes and the relationships of the super class

o In addition, it has its own specific attributes and relationships

• Sub Class with these inherited and specific attributes and relationships is

considered as an individual entity type

6.3 Specialization/Generalization

6.3.1 Specialization

• Process of

o Defining a set of sub classes of an entity type

o Establish additional attributes with each sub class

o Establish additional specific relationships between each sub class and

other entity types

• Based on some distinguishing characteristics of the entities in the super class

• The specific attributes (those apply only to entities of a particular sub class) are called specific attributes or local attributes

• ⊂: class/subclass relationship

• EngType: specific or local attribute

• Manage: specific relationship to “Manager”

6.3.2 Generalization

• Process of

o Minimizing the differences between entities by identifying common


o Generalizing them into a typical superclass

o Defining a generalized entity type from given entity types

• Suppress the differences among different entity types

• Functionally, the inverse of specialization

• Original entity types are special subclasses

• Identify common features

• Group them to form a superclass

6.4 Characteristics and Constraints of Specialization and Generalization

6.4.1 Predicate-Defined Sub Classes

• Also called Condition-Defined sub class

• Determine the sub class to which an entity is belong to, by applying a

condition on the value of a given attribute of the super class

• Condition is called defining predicate

• Defining Predicate is the constraint which specifies the entity with which

value of the particular attribute should go the given sub-class

• Attribute –Defined Specialization

• All the sub classes have their specialization, depending on the conditions

formed on the same attribute, of the super class

• Such attribute is called defining attribute of the specialization

6.4.2 User-Defined Sub Classes

• Membership to the sub class is specified by the database users at the time of operation

• Membership is specified individually for each entity by the user

• No conditions are used to evaluate the entities automatically

6.4.3 Disjointness Constraint

• An entity can be a member of at most one sub class

• Attribute defined specialization implies the Disjointness, if defining attribute is a single valued

• In EER diagram, disjointness is denoted by a letter d in a circle

6.4.4 Overlapping Specialization

6.4.5 Completeness Constraint

Partial Specialization

o Allows an entity not to belong to any sub class

o Denoted by a single line connecting the super class to the circle

6.4.6 Insertion and Deletion Rules

• Deleting an Entity from super class automatically deletes the entity from all the sub classes to which it belongs

• Inserting an entity in a super class implies that the entity is mandatorily inserted to the predicate-defined sub classes, if the entity satisfies the defining predicate

• Inserting an entity in a super class of total specialization implies that the entity is mandatorily inserted in at least one sub class

6.4.7 Union Types

• Considered with multiple inheritance

• Used to represent sub classes which inherit from more than one disjoint super classes

• The domain for sub class entity is the UNION of all its distinct super class entity types

• Such sub class is called a Union Type or a Category

• Super classes of the category may have different key attributes

• Total Category

o Holds union of all entities in its super class

o Represented by a double line connecting the category and the circle

• Partial Category

o Can hold a subset of union

o Represented by a single line connecting the category and the circle

6.4.8 Specialization Vs Categorization

• If the categorization is total, it may also be alternatively represented as Total Specialization

• Use Specialization if the two classes

o represent the same set of entities

o Share many attributes including the same key attribute

• Otherwise, categorization (Union Type) is more appropriate

6.4.9 Specialization Lattice

• Occurs when a sub class has more than one parent classes (multiple inheritance)

• If its parent class is also a subclass of another super class, the level 2 sub class inherits from both:

o Direct super class

o Predecessor super class

teacherstudenthigh schooldegreecoursescollege

About the Creator


Reader insights

Be the first to share your insights about this piece.

How does it work?

Add your insights


There are no comments for this story

Be the first to respond and start the conversation.

Sign in to comment

    Find us on social media

    Miscellaneous links

    • Explore
    • Contact
    • Privacy Policy
    • Terms of Use
    • Support

    © 2024 Creatd, Inc. All Rights Reserved.