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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
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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

Eg:

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

features.

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

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SHD TECH

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