The Entity-Relationship Model

1
Chapter 3

• 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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Problems with ER Models

• Problems may arise when designing a conceptual
  data model called connection traps.
• Often due to a misinterpretation of the meaning
  of certain relationships.
• Two main types of connection traps are called fan
  traps and chasm traps.

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Problems with ER Models

• Fan Trap
• Where a model represents a relationship between
  entity types, but pathway between certain entity
  occurrences is ambiguous.
• Chasm Trap
• Where a model suggests the existence of a
  relationship between entity types, but pathway
  does not exist between certain entity
  occurrences.

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An Example of a Fan Trap

• At which branch office does staff number SG37
  work?

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Restructuring ER model to remove Fan Trap
SG37 works at branch B003.
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An Example of a Chasm Trap

• At which branch office is property PA14
  available?

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ER Model restructured to remove Chasm Trap
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Semantic Net of Restructured ER Model with Chasm
Trap Removed
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Entity vs. Attribute

• Should address be an attribute of Employee or an
  entity (connected to Employee by a relationship)?
• Depends upon the use we want to make of address
  information, the physical data model, and the
  semantics of the data
• If we have several addresses per employee,
  address must be an entity (since attributes
  cannot be set-valued).
• If the structure (city, street, etc.) is
  important, e.g., we want to retrieve employees in
  a given city, address must be modeled as an
  entity (since attribute values are atomic).

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Entity vs. Attribute

• Works_In1 does not allow an employee to
  work in a department for two or more
  periods.
• Similar to the problem of wanting to record
  several addresses for an employee.
• Solution Place periods in a separate entity
  type.

from to
Department did dname budget
0
11
Works_In1
Department did dname budget
0
11
Works_In2
1
Duration from to
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Entity vs. Relationship

• First ER diagram OK if a manager gets a separate
  budget for each dept.
• What if a manager gets a budget that covers all
  managed depts?
• Redundancy of dbudget, which is stored for each
  dept managed by the manager.
• Misleading suggests dbudget tied to managed
  dept.
• Solution Create a separate entity type Mgr_Appts.

since dbudget
Department did dname budget
11
01
Manages1
Department did dname budget
11
01
Manages2
01
Mgr_Appts apptnum since dbudget
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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.