ENHANCED ENTITY RELATIONSHIP EER MODELING

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ENHANCED ENTITY RELATIONSHIP (EER) MODELING
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• EER model attempts to include the concepts of
  semantic data models in the ER model.
• Such concepts include class/sub class
  relationship, attribute inheritance,
  specialization and generalization, category or
  union type.
• The resulting schema diagrams are called Enhanced
  ER or EER diagrams.

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SUBCLASSES, SUPERCLASSES, AND INHERITANCE

• An entity type is used to represent a type of
  entity and the entity set or collection of
  entities of that type that exist in the database.
  
• The entity EMPLOYEE in the COMPANY database may
  have some sub groupings, such as SECRETARY,
  ENGINEER, MANAGER, TECHNICIAN, SALARIED_EMPLOYEE,
  and HOURLY_EMPLOYEE.
• The set of entities in each of these sub groups
  is a subset of the EMPLOYEE entity set.
• Every entity in any of these groupings belongs to
  the EMPLOYEE entity.

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SUBCLASSES, SUPERCLASSES, AND INHERITANCE

• Each of these subgroupings is called a subclass
  of the EMPLOYEE entity type
• The EMPLOYEE entity is called the superclass for
  each of these subclasses.
• The relationship is called superclass/ subclass
  relationship or class/ subclass relationship

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SUBCLASSES, SUPERCLASSES, AND INHERITANCE

• An Entity can not exist by just being a member of
  the subclass. It must also be a member of the
  superclass, but not all entities of the
  superclass may belong to a subclass.
• A member of the superclass can be optionally
  included as a member of any number of its
  subclasses.
• An entity can be a member of more than one
  subclass.
• An entity that is member of a subclass represents
  the same real-world entity as some member of the
  superclass
• The Subclass member is the same entity in a
  distinct specific role

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Type Inheritance

• An entity that is member of a subclass inherits
  all attributes of the entity as a member of the
  superclass.
• It also inherits all the relationships in which
  the superclass participates.
• The subclass with its own (local) attributes and
  relationships, together with the attributes and
  relationships inherited from the superclass, can
  be considered an entity type in its own rights

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SPECIALIZATION

• This is the process of defining a set of
  subclasses of a superclass.
• The set of subclasses is based upon some
  distinguishing characteristics of the entities in
  the superclass.
• Example SECRETARY, ENGINEER, TECHNICIAN is a
  specialization of EMPLOYEE based upon job type.

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SPECIALIZATION

• It is possible to have several specializations of
  the same entity type. Another specialization
  could be SALARIED_EMPLOYEE, HOURLY_EMPLOYEE,
  which is based on method of pay.
• In EER diagrams, the subclasses that defines a
  specialization are attached by lines to a circle
  that represents the specialization, which is
  connected to the superclass.
• The subset symbol on each line connecting a
  superclass to the circle indicates the direction
  of the superclass/ subclass relationship.

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SPECIALIZATION

• Attributes that apply only to entities of a
  particular subclass are called specific
  attributes or local attributes. For example,
  TypingSpeed of SECRETARY
• The subclass can participate in specific
  relationship types. For example, HOURLY_EMPLOYEE
  subclass can participate in BELONGS_TO
  relationship

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SPECIALIZATION

• Define a set of subclasses for an entity type
• Establish additional specific attributes with
  each subclass
• Establish additional specific relationship types
  between each subclass and other entity types or
  other subclasses.

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GENERALIZATION

• process of defining a generalized entity type
  from given entity types.
• It is a reverse of the specialization process.
• The differences among several entity types are
  suppressed, while identifying their common
  features, to generalize them into a single
  superclass of which the original entity types are
  special subclasses.
• Several classes with common features are
  generalized into a superclass original classes
  become its subclasses.

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GENERALIZATION

• Example CAR, TRUCK generalized into VEHICLE
• CAR, TRUCK become subclasses of the superclass
  VEHICLE.
• We can view CAR, TRUCK as a specialization of
  VEHICLE. Alternatively, we can view VEHICLE as a
  generalization of CAR and TRUCK.
• Also EMPLOYEE is a generalization of SECRETARY,
  TECHNICIAN, and ENGINEER.

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Data Modeling with Specialization and
Generalization

• A superclass or subclass represents a set of
  entities
• Shown in rectangles in EER diagrams (as are
  entity types)
• Sometimes, all entity sets are simply called
  classes, whether they are entity types,
  superclasses, or subclasses

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Constraints on Specialization and Generalization

• The following constraints may apply to
  specialization
• Defining predicate constraint
• Disjointness constraint
• Completeness constraint

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Defining Predicate Constraint

• In some specializations, it is possible to
  determine the exact entities that will become
  members of each subclass by imposing a condition
  on the value of the attributes of the subclass.
• Such subclasses are called predicate-defined (or
  condition-defined) subclasses.
• The predicate condition is defined as a
  constraint on the subclass. For example, the
  condition (JobTypeSecretary can be defined as
  a constraint on the SECRETARY subclass in the
  EMPLOYEE superclass.
• A predicate-defined subclass is defined by
  writing the predicate condition next to the line
  attaching the subclass to its superclass .

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Defining Predicate Constraint

• If all subclasses in a specialization have
  membership condition on same attribute of the
  superclass, specialization is called an attribute
  defined-specialization
• The attribute is called the defining attribute of
  the specialization.
• The attribute defined specialization is displayed
  by placing the defining attribute name next to
  the arc from the circle to the superclass
• Example JobType is the defining attribute of the
  specialization SECRETARY, TECHNICIAN, ENGINEER
  of EMPLOYEE
• When there is no condition for determining
  membership in a subclass, the membership is
  specified individually for each entity by the
  user. Such a subclass is said to be user defined
  

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EER diagram notation for an attribute defined
specialization on JobType
-
.
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Disjointness Constraint

• This constraint specifies that the subclasses of
  the specialization must be disjoint
• An entity can be a member of at most one of the
  subclasses of the specialization.
• The notation d is used to specify that user
  defined subclass of a specialization must be
  disjoint.
• If the subclasses are not constrained to be
  disjoint, then their set of entities may overlap,
  that is, the same real world entity may be a
  member of more than one subclass in the same
  specialization.
• The letter o is placed in the circle to indicate
  that the subclasses can overlap and this is the
  default.

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Completeness Constraint

• This constraint can either be total or partial.
• The total completeness constraint specifies that
  every entity in the superclass must be a member
  of at least, one specialization.
• E.g. every EMPLOYEE must be HOURLY_EMPLOYEE or
  SALARIED_EMPLOYEE
• The specialization HOURLY_EMPLOYEE,
  SALARIED_EMPLOYEE is a total specialization of
  EMPLOYEE.
• Double line is used to connect the superclass to
  the circle.

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Completeness Constraint

• Partial completeness constraint allows an entity
  in the superclass not to belong to any subclass.
• E.g. if any of the EMPLOYEES does not belong to
  any of the subclasses SECRETARY, TECHNICIAN,
  ENGINEER, then that specialization is partial.
• A single line is used to connect the superclass
  to the circle in the EER diagram.

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Insertion and deletion rules that apply to
specialization and generalization

• Deleting an entity from a superclass implies that
  it is automatically deleted from all the
  subclasses to which it belongs.
• Inserting an entity in the superclass implies
  that the entity is automatically inserted in all
  subclasses that are attribute defined (or
  predicate defined), for which the entity
  satisfies the defining predicate.
• Inserting an entity in a total specialization
  implies that the entity is automatically inserted
  in at least one of the subclasses in the
  specialization.

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Specialization and Generalization Hierarchies and
Lattices

• A subclass may itself have further subclasses
  specified on it, thereby forming a hierarchy or
  a lattice.
• E.g. ENGINEER could be a subclass of EMPLOYEE,
  and a superclass of ENGINEERING_MANAGER.
• The real world constraint is that every
  engineering manager must be an engineer.

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Specialization and Generalization Hierarchies and
Lattices

• Specialization Hierarchy has a constraint that
  every subclass has only one superclass (called
  single inheritance), thereby forming a tree
  structure.
• On the other hand, in a lattice, a subclass can
  be subclass of more than one superclass (called
  multiple inheritance)

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MODELING UNION TYPES USING CATEGORIES

• There may be the need to model a single
  superclass/subclass relationship with more than
  one superclass, where the superclasses represent
  different entity types.
• In this case, the subclass will represent a
  collection of objects that is a subset of the
  UNION of different entity types.
• Such a subclass is called a category or UNION
  TYPE

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MODELING UNION TYPES USING CATEGORIES

• Database for vehicle registration, vehicle owner
  can be a person, a bank (holding a lien on a
  vehicle) or a company.
• Category (subclass) OWNER is a subset of the
  union of the three superclasses COMPANY, BANK,
  and PERSON
• A category member must exist in at least one of
  its superclasses
• The superclasses COMPANY, BANK, and PERSON are
  connected to the circle with U symbol (U stands
  for UNION operation). An arc, with the subset
  symbol connects the circle to the subclass
  (OWNER) category

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MODELING UNION TYPES USING CATEGORIES

• A category can either be total or partial.
• Total category holds the union of all entities in
  its superclasses
• Partial category can hold a subset of the union.
• A total category is represented by a double line
  connecting the category and the circle, whereas,
  a partial category is represented using single
  lines.

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CONSTRAINTS ON TERNARY (OR HIGHER DEGREE)
RELATIONSHIPS

• Two constraints are defined for the n-ary
  relationships, based on earlier defined
  notations.
• The first constraint is based on the cardinality
  ratio notation of binary relationships.
• Here, 1, N, or M is specified on each
  participating arc.
• 1 specifies that a particular relationship
  combination can only appear at most once in the
  relationship set (e.g. for a project-part
  combination, if only one supplier supplies part
  to a particular project).
• Both M and N stand for many and any number of
  appearances in the relationship set.

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CONSTRAINTS ON TERNARY (OR HIGHER DEGREE)
RELATIONSHIPS

• The second constraint is based on the (min, max)
  notation for binary relationships
• This specifies that each entity participates in
  at least min and at most max relationship
  instances in the relationship set.
• places restrictions on how many relationship
  instances each entity can participate in.

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DATA ABSTRACTION CONCEPTS

• Classification and Instantiation
• Classification involves assigning similar
  entities/objects to object classes/ entity types.
  
• Collections of objects share same types of
  attributes, relationships and constraints.
• For example, John Smith, Sheela Patel, and Peter
  Wang are all employees. They are all members of a
  class EMPLOYEE class.

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DATA ABSTRACTION CONCEPTS

• Classification and Instantiation
• Instantiation is the reverse of classification.
• It involves the generation of distinct objects of
  a class.
• An object instance is related to its object class
  by the IS-AN-INSTANCE-OF or IS-A-MEMBER-OF
  relationship.
• An instance may itself, be another class,
  allowing multiple level classification schemes.

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DATA ABSTRACTION CONCEPTS

• Classification and Instantiation

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DATA ABSTRACTION CONCEPTS

• Identification
• Identification is an abstract process that makes
  objects and classes uniquely identifiable, by
  means of an identifier.
• The identifier identifies one concept (an
  instance of it) from another concept.
  Identification is needed at two levels
• To distinguish among database objects and classes
• To identify database objects and to relate them
  to their real world counterparts

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DATA ABSTRACTION CONCEPTS

• Specialization and Generalization
• Specialization classifies a class of objects into
  more specialized sub-classes.
• Generalization on the other hand, is the inverse
  process of generalizing several classes into a
  higher level abstract class that includes all the
  objects in all the sub-classes.
• Generalization defines a set-subset relationship
  between a class and a set of member classes.
• Specialization establishes a mapping (or a
  relationship) from the generic class to the
  member class (or subclass, or subset class).

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GENERIC CLASS EMPLOYEEMEMBER CLASS Engineer,
Staff, ManagerThis implies that all properties
associated with the Employee class are inherited
by the three leaf classes.
Generalization establishes an IS-A relationship,
or IS-A-SUBCLASS-OF relationship(e.g. An Engineer
IS-A-SUBCLASS-OF Employee)
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DATA ABSTRACTION CONCEPTS

• Aggregation and Association
• Aggregation is the process of building composite
  objects from their component objects.
• It defines a new class from a set of classes
  which are identified as components of the root
  class.