The Entity-Relationship Model

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Lecture 2

• The Entity-Relationship Model

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ER Model

• Conceptual Design answers
• What are the entities and relationships?
• What information about these entities and
  relationships should we store in the database?
• What are the integrity constraints or business
  rules?
• ER Model Concepts
• Entity types
• Relationship types
• Attributes

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Diagrammatic notation

• Use of UML class diagram for displaying ER
  concepts.
• UML class represents an entity type.
• First section include entity type name.
• Second section lists attributes.
• Third section is not displayed (or used for
  derived attributes).
• Relation types represented as lines connecting
  entity types (classes).
• Some UML terminology differs from ER terminology
  (discussed later).

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Entity Type

• Entity type
• Define a group of objects with same properties or
  attributes.
• Examples EMPLOYEE, CUSTOMER, SUPPLIER entity
  types.
• Entity occurrence (or entity instance)
• Uniquely identifiable object of an entity type.
• Examples Employee John Doe, Customer Mike
  Jordan, Supplier Office Depot.
• Entity set is the set of entity occurrences.

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ER diagram of Staff and Branch entity types
An entity type is displayed in a rectangular box.
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Relationship Types

• Relationship type
• Define a set of meaningful associations among
  entity types.
• Examples A Branch HAS some Staff, An Employee
  WORKS ON a Project.
• Relationship occurrence
• Uniquely identifiable association, which includes
  one occurrence from each participating entity
  type.
• Relationship set is the set of relationship
  occurrences (current state of a relationship).

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Semantic net of Has relationship type
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ER diagram of Branch Has Staff relationship
Another notation
Has
Staff
Branch
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Relationship Types

• Degree of a Relationship
• Number of participating entities in
  relationship.
• Relationship of degree
• two is binary
• three is ternary
• four is quaternary.

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Binary and ternary relationships
Binary
Ternary
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Quaternary relationship called Arranges
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Relationship Types

• Recursive Relationship
• A relationship type where same entity type
  participates more than once in different roles.
• Relationships may be given role names to indicate
  purpose that each participating entity type plays
  in a relationship.

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Recursive relationship called Supervises with
role names
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Entities associated through two distinct
relationships with role names
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Attributes

• Attribute
• Property of an entity or a relationship type.
• Attribute Domain
• Set of allowable values for one or more
  attributes.
• Simple Attribute (or single-valued Attribute)
• An entity has a single atomic value for the
  attribute.
• Example Employee with SSN 123-45-6789
• Composite Attribute
• Attribute composed of multiple components.
• Examples Address, Name.
• It can be nested.

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Attributes

• Multi-valued Attribute
• Attribute that holds multiple values for each
  occurrence of an entity type.
• Example Previous jobs for an employee.
• Derived Attribute
• Attribute that represents a value that is
  derivable from value of a related attribute, or
  set of attributes, not necessarily in the same
  entity type.
• Example Age derived from DOB and Todays date.

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Keys

• Candidate Key
• Minimal set of attributes that uniquely
  identifies each occurrence of an entity type.
• Primary Key
• Candidate key selected to uniquely identify each
  occurrence of an entity type.
• Composite Key
• A candidate key that consists of two or more
  attributes.

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ER diagram of Staff and Branch entities and
their attributes
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Entity Type

• Strong Entity Type
• Entity type that is not existence-dependent on
  some other entity type.
• Weak Entity Type
• Entity type that is existence-dependent on some
  other entity type.
• It does have a key attribute.
• Identified by a partial key of the weak entity
  type and by the primary of the strong entity type
  they are related to.

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Strong entity type called Client and weak entity
type called Preference
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Relationship called Advertises with attributes
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Structural Constraints

• Main type of constraint on relationships is
  called multiplicity.
• Multiplicity - number (or range) of possible
  occurrences of an entity type that may relate to
  a single occurrence of an associated entity type
  through a particular relationship.
• Represents policies (called business rules)
  established by user or company.

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Structural Constraints

• The most common degree for relationships is
  binary.
• Binary relationships are generally referred to as
  being
• one-to-one (11)
• one-to-many (1 or 1M)
• many-to-many ( or MN)

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Semantic net of Staff Manages Branch relationship
type
One-to-one (11) relationship
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Multiplicity of Staff Manages Branch (11)
relationship
11
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Semantic net of Staff Oversees PropertyForRent
relationship type
One-to-many (1) relationship
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Multiplicity of Staff Oversees PropertyForRent
(1) relationship type
1M
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Semantic net of Newspaper Advertises
PropertyForRent relationship type
Many-to-many () relationship
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Multiplicity of Newspaper Advertises
PropertyForRent () relationship
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Structural Constraints

• Multiplicity for Complex Relationships
• Number (or range) of possible occurrences of an
  entity type in an n-ary relationship when other
  (n-1) values are fixed for the other
  participating entity types.

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Semantic net of ternary Registers relationship
with values for Staff and Branch entities fixed
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Multiplicity of ternary Registers relationship
A staff/branch value pair registers zero or more
clients
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Summary of multiplicity constraints
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Structural Constraints

• Multiplicity actually consists of two types of
  restrictions on relationships cardinality and
  participation.
• Cardinality
• Describes maximum number of possible relationship
  occurrences for an entity participating in a
  given relationship type.
• Participation
• Determines whether all (total) or only some
  (partial) entity occurrences participate in a
  relationship.

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Multiplicity as cardinality and participation
constraints
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Enhanced Entity-Relationship Model

• Basic concepts of ER modeling are not sufficient
  to represent more complex systems.
• Semantic concepts incorporated into the original
  ER model are called the Enhanced
  Entity-Relationship (EER) model.
• Examples of additional EER concepts are
• subclass / superclass
• specialization / generalization
• Categories (UNION types)
• aggregation

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Subclasses and Superclasses

• Superclass
• An entity type that includes one or more distinct
  subgroupings of its occurrences.
• Example Employee entity may be further grouped
  into Secretary, Mechanic, Manager, etc.
• Subclass
• A distinct subgrouping of occurrences of an
  entity type.
• Example Secretary is a subclass of Employee

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Subclasses and Superclasses (2)

• Superclass/subclass relationship is one-to-one
  (11).
• These are called IS-A relationships
• Superclass may contain overlapping or distinct
  subclasses.
• An engineering manager may become member of two
  subclasses Manager and Engineer.
• Not all members of a superclass need be a member
  of a subclass.
• Some employees may be unclassified.

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Specialization / Generalization

• Attribute Inheritance
• An entity in a subclass may possess
  subclass-specific attributes, as well as those
  associated with the superclass.
• Specialization Top down process of defining a
  set of subclasses of a superclass.
• Identify unique characteristics of each subclass.
• Generalization Bottom up process that is the
  reverse of the specialization process.
• Identify common characteristics of the subclasses.

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AllStaff relation holding details of all staff
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Specialization/generalization of Staff entity
into subclasses representing job roles
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Specialization/generalization of Staff entity
into job roles and contracts of employment
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EER diagram with shared subclass and subclass
with its own subclass
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Constraints on Specialization / Generalization

• Two constraints that may apply to a
  specialization/generalization
• participation and disjoint constraints
• Participation constraint
• Determines whether every member in a superclass
  must participate as a member of a subclass.
• May be mandatory or optional.
• Also called total (for mandatory) or partial (for
  optional)
• Disjoint constraint
• Describes whether member of a superclass can be a
  member of one, or more than one, subclass.
• May be disjoint (Or) or nondisjoint (And).

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Constraints on Specialization / Generalization

• There are four categories of constraints of
  specialization and generalization
• mandatory and disjoint
• optional and disjoint
• mandatory and nondisjoint
• optional and nondisjoint.
• Reasons for using Specialization/Generalization
• To add descriptive attributes specific to a
  subclass
• To identify entities that participate in a
  relationship

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EER diagram for a Real Estate organization with
specialization/generalization
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Categories (UNION type)

• A shared subclass is a subclass participating in
  relationships with more than one superclass.
• Shared class may have its own attributes
• This leads to multiple inheritance.
• A member of a shared subclass must be a member of
  the associated superclasses.
• A UNION type is a special shared subclass
• A UNION subclass must exist in at least one of
  its superclasses (not in all).
• Participation is usually optional

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Aggregation

• It is an abtraction concept.
• Similar to the is-part-of relationship in UML
• Use a box to include its components.
• Used when we have to model a relationship
  involving a relationship type
• Aggregation allows us to treat a relationship
  type as a high level entity for purpose of
  participation in other relationships.

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Example of Aggregation
Contact_Name Contact_Phone Date
Job_Applicant SSN Name Address Phone
Company Cname Caddress
1
11
Interview
11
Starting_Date Starting_Salary
Results_in
01
Job_Offer Position
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Example of incorrect design
Job_Applicant SSN Name Address Phone
Company Cname Caddress
11
11
Contact_Name Contact_Phone Date Starting_Date Star
ting_Salary
If an interview does not result in an offer, then
Starting_Date and Starting_Salary are
NULL. Multiplicities are different than those on
previous diagram. May show a wrong interview
instance that is not allowed by the aggregation.
01
Job_Offer Position
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Data Modeling Tools

• Several popular tools exist for conceptual design
  and mapping to relational schema.
• Examples ERWin, S-Designer, ER-Studio, MS Visio.
• Good for documenting database design.
• But, it may not follow ER notation.
• Original ER model did not support other concepts
• Such as specializations/generalizations and
  aggregations.
• They are discussed in next chapter.