EntityRelationship Modeling

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Entity-Relationship Modeling
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Outline

• Basic Concept
• Advanced Concept
• Examples and Exercises

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Enterprise schema-an intermediate step in
logical data base design
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Entity Relationship Model

• E/R approach
• E/R concept
• Entity
• Relationships
• Attributes

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An Example EER Model
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Entity

• An object that exists and distinguishable from
  other objects
• Concrete Entity Vs. Abstract Entity
• Weak Entity Vs. Regular Entity

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Basic Concepts

• Entity Sets Group of similar entities
• Attribute / Property A function that maps an
  entity set into a domain ( value set )
• Domain Set of permissible values for an
  attribute

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Keys

• Attribute whose values uniquely identify each
  entity in an entity set
• Candidate key
• Primary key

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Relationship

• Association among several entities, ordered list
  of entity sets
• Relationship set set of relationships of the
  same type

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E-R Diagrams

• Graphical representation of the logical design of
  a database
• Rectangles represent entity sets
• Circles or ellipses represent attributes
• Diamonds represent relationship sets
• Links connect attributes to entity sets and
  entity sets to relationships

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Functionality of Relationships

• Mapping Cardinality Number of entities of one
  set that can be associated with another entity
  via relationship.
• Indicate functionality through the use of
  directed arcs
• One-to-one
• One-to-many
• Many-to-one
• Many-to-many

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Many-to many an entity in A is associated with
any number of entities in B and an entity in B is
associated with any number of entities in A.
Many-to-many relationship
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• All entities in a relationship need not to be
  distinctive
• recursive relationship sets
• two relationship sets between the same two entity
  sets
• Relationship may have descriptive attributes
• relationship sets have primary keys
• A relationship may be n-ary

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All entities in a relationship need not be
distinct

• E.g. Recursive relationship set

Role function played by an entity in a
relationship More than one relationship set
possible between 2 entity sets
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Two relationship sets between the same entity sets
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Relationship may have descriptive attributes
Relationship sets have primary keys
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Ternary Relationship

• Doctors-patients-tests
• Customers-accounts-branches

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Abstraction

• ability to hide detail and concentrate on
• general, common properties of a set of
• objects
• Generalization/Specialization Abstraction
• captured by generalization hierarchies or subsets
• applied only to entities
• take the union of two or more lower level entity
  sets to produce a higher level entity set.

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