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Enhanced Entity Relationship EER

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Subclass represent the same mini-world entity of the ... Example: truck & car can be generalized into VEHICLE. Inverse of the ... with Car and Trucks ... – PowerPoint PPT presentation

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Title: Enhanced Entity Relationship EER


1
Chapter 4
  • Enhanced Entity Relationship (EER)
  • Elmasri et al 2000

2
Subclass
  • In chap 3, entity type describes
  • Type of entity
  • The entity set
  • Example EMPLOYEE
  • Employee can be sub-grouped into
  • Secretary
  • Engineer
  • Technician
  • These are called the subclass of EMPLOYEE entity
    type.

3
Super-class
  • EMPLOYEE entity type is the super class of
    engineer, secretary technician class.
  • Subclass represent the same mini-world entity of
    the superclass, but in a distinct specific role.
  • Entity in a subclass must be a member of a
    superclass, but not vice-versa! Example…?

4
Type inheritance
  • Subclass inherits all attributes of the
    superclass.

5
Specialization
  • Is a process of defining a set of subclasses of
    an entity type (the superclass)
  • Secretary, engineers, technician are
    specialization of EMPLOYEE based on job type
    attribute

6
Contd
  • May have several specialization based on
    different characteristics
  • Example, EMPLOYEE can be subclass-ed into
    Salaried_Employee and Hourly_Employee

7
Fig 4.1
8
Notations from the EER diagram
  • Subset symbol ?
  • Specific attributes, or local attributes
  • Specific relationships
  • Superclass/subclass EMPLOYEE/Secretary resembles
    11 relationship at the instance level, of one
    entity.
  • Different from 11 relationship of 2 entities!

9
Fig 4.2
10
Benefit of Specialization
  • Define a set of subclasses of 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
  • Refer to the EER diagram…!

11
Generalization
  • Identify common features (attributes), and
    generalize into a superclass
  • Example truck car can be generalized into
    VEHICLE
  • Inverse of the specialization process

12
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13
Constraints
  • Constraints in Specialization
  • Hierarchies Lattices
  • Specialization in conceptual data modeling

14
Predicate defined subclasses
  • Also called condition-defined subclasses
  • Conditioned by a defining predicate at the
    superclass
  • Example, JobType Secretary
  • Place attribute name on the arc

15
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16
User defined subclass
  • Membership is specified individually for each
    entity by the user
  • Not by any condition that may be evaluated
    automatically
  • Example Manager subclass

17
Disjointness constraints
  • Subclasses of a specialization must be disjoint
  • An entity can only be at most one of the subclass
  • Look at the EER diagram
  • Use (d)

18
Overlap
  • The same entity may be a member of more than one
    subclass of the specialization
  • Use the (o)
  • Example, a person can be
  • A student
  • A faculty member
  • An alumni

19
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20
Completeness constraints
  • Total specialization
  • Every entity in the superclass must be a member
    of some subclass
  • Example, the Salaried_Employee and
    Hourly_Employee
  • Shown using double line

21
  • Partial specialization
  • Allows an entity not to belong to any subclass
  • Example
  • Manager
  • Job type
  • Use single line

22
Rules
  • Deleting entity from a superclass ? deletes it
    also from the subclasses
  • Inserting in a superclass, when attribute defined
    is filled ? must insert to the proper subclass as
    well
  • Inserting in superclass of total specialization ?
    must insert into at least one subclass

23
Hierarchy Lattice
  • Hierarchy a subclass only participates in one
    class/subclass relationship
  • Example Vehicle with Car and Trucks
  • Lattice a subclass can participate in more than
    one class/subclass relationship
  • Example an Engineering Manager, must be an
    Engineer, and also a Manager!
  • The concept of multiple inheritance

24
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25
Figure 4.7
26
More Explanation of the EER Diagram
  • Leaf node class that has no subclasses
  • An entity may exist in several leaves
  • Example, a student as Graduate_Student and a
    Teaching_Assistant
  • Multiple inheritance
  • Student_assistant
  • But the Person attribute is only inherited once

27
What if system do not allow multiple inheritance?
  • Use additional multiple subclasses
  • (E)mployee, (A)lumnus, (S)tudent
  • E, A, S, EA, ES, AS and EAS!

28
UNION Types Using Categories
  • Engineering_Manager has 3 distinct relation, each
    relation has 1 superclass
  • In our new case, a subclass has a single
    relationship with 3 distinct superclass.
  • The subclass represent collection of objects,
    which we call union type or category

29
Figure 4.8
30
Explanation
  • A category OWNER is a union subclass of COMPANY,
    BANK and PERSON
  • Use the (U) symbol
  • Registered_Vehicle is a union subclass of Car
    Truck

31
The difference?
  • Engineering_Manager must exist in all three
    superclass Manager, Engineer, Salaried_Employee
  • Owner, must exist in only one superclasses
  • Engineering_Manager inherited all superclasses
    attributes
  • Owner, selective attribute inheritance, depending
    on the superclass

32
Partial Category
  • Partial category, may or may not participate in
    the relation

33
Total Category
  • Must be one of the superclasses
  • Example A building and a lot must be a member of
    PROPERTY
  • May be represented as a generalization (d),
    especially when the similarity is numerous

34
University Enhanced ER Schema
35
Fig 4.10
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