Entity Relationship Modeling

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Chapter 11 12
Entity Relationship Modeling (ER) Thomas
Connolly, Carolyn Begg, Database System, A
Practical Approach to Design Implementation and
Management, 4th Edition, Addison Wesley Pg 342
386
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Learning Outcomes

• Entity types
• Relationship types
• Attributes
• Strong and weak entity type
• Attributes on relationship
• Structural constraints

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Learning Outcomes

• Problem with ER Models
• Specialization / generalization
• Aggregation
• Composition

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Characteristics of E-R Model

• Semantic data model
• Express the logical properties of an enterprise
  database
• Design tools and documentation for data base
  structure
• No physical DBMS
• Proposed by Dr. Peter Chen
• Unified Modeling Language (UML)

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Components of E-R Model

• Entity
• Attribute
• Key
• Relationship
• Structural constraints on relationship

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Entity

• Definition
• An object or concept
• Types
• Strong entity (parent, owner, dominant)
• Weak entity (child, dependent, or subordinate)
• Diagram Notation
• Rectangular

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Attribute

• Domain
• Types
• Single
• Composite
• Single-valued
• Multi-valued
• Derived

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Key

• Candidate key
• Primary key
• Composite key

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Relationship

• Definition
• Association among entities
• Diagram Notation
• Line (relation or role name)
• Degree of a relationship
• Number of participating entities
• Types
• Unary (recursive relationship)
• Binary
• Ternary
• Quaternary

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Structural Constraints on Relationship

• Cardinality constraints
• Zero-to-one 0..1
• Zero-to-many 0..
• One-to-many 1..
• Many-to-many ..
• Participation constraints
• Total (mandatory, every one involved)
• Partial (optional, only some involved)
• Improper relationship
• Fan trap (ambiguous pathway)
• Chasm trap (missing pathway)

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Enhanced Entity-Relationship Model

• Additional entity types
• Superclass including one or more distinct
  subgroups in the data model
• Subclass a distinct subgroup of an entity type
  in the data model
• Attribute Inheritance
• Specialization hierarchy (specialization
  maximizing the differences between members of an
  entity by identifying their distinguishing
  characteristics)
• Generalization hierarchy (generalization
  minimizing the differences between entities by
  identifying their common characteristics)
• Is-A hierarchy
• Constraints on specialization/generalization
• Participation (mandatory, optional)
• Disjoint disjoint (or), non-disjoint (and)
• Other
• Aggregation (has a or is part of)
• Composition (strong ownership of aggregation)

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Design Steps

• Identify
• Entity types, relationship types
• Cardinality and participation constraints
• Attributes
• Keys
• Specialize/generalize
• EER diagram
• EER model example

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• Create an enhanced ER diagram for a rental
  management using following entities
• Rental agency
• Staff
• Part time
• Full time
• Owner
• Renter
• Property
• Business
• Home