EER Enhanced Entity Relationship Model

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• EER Enhanced Entity Relationship Model
• Chapter 4.1-4.3

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Why EER?

• There are limitations to the ER model
• ER model enhanced (EER) to include 
• superclass/subclass relationships
• type inheritance
• specialization and generalization
• constraints

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Subclasses

• Subclass of an entity type
• relationship is-a
• derive subclass from superclass
• manager is a(n) employee
•    subset symbol in connecting line Fig
  4.1
• member of a subclass must also be a member of
  some superclass

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Type inheritance

• inherit all attributes of superclass
• useful for modeling, convenience

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Specialization

• Specialization
• salaried and hourly specialization of
  employee
• specific attributes attached to subclass
• subclasses can also participate in specific
  relationship types
• circle connecting all specialized subclasses 
• still use subset symbol to connect subclass to
  circle Fig 4.4

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Generalization

• Generalization of entity types into a single
  superclass
• vehicle is generalization of car, truck
• Fig 4.3

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Specialization Constraints

• Predicate defined (condition-defined) subclasses
• manager True
•  has a defining predicate for the subclass
• Attribute defined Fig 4.4
• if all subclasses have membership based on same
  attribute
• User defined - if no condition

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Constraints contd

• Disjointedness constraint
• subclasses disjoint
• member of at most 1 subclass
• use d in circle to indicate
• Otherwise,
• Overlapping constraint
• sets of entities may overlap
• member of more than 1 subclass
• use o in circle to indicate

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Constraints contd

• Completeness constraint
• total specialization 
• every entity in superclass must be member of some
  subclass of specialization
• indicate with double line from superclass to
  circle 
•  partial specialization
• entity does not have to belong to any subclass
• indicate with single line Fig 4.1 
• Disjoint, total Overlapping, total
• Disjoint, partial Overlapping, partial
•  

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  Constraints contd

• insertion and deletion rules
• If delete from superclass, delete from subclass
• If insert into superclass, inserted into all
  predicate or attribute define subclasses it
  satisfies
• If insert into total specialization, must be
  inserted into at least one subclass

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Specialization/Generalization Lattice

• a class can be a subclass in more then one
  class/subclass relationship
• an entity may exist in several leaf nodes
• multiple inheritance Fig 4.6
• Engineer is one distinct entity at the instance
  level, not 2 distinct entities  
• Can have disjoint total, disjoint partial,
  overlapping total, overlapping partial Fig 4.7

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UML notation

• Aggregation relationship between object and
  component parts (has-a relationship)
• Diamond indicates whole vs. part
• Subclasses
• specialization/generalization
• blank triangle is disjoint, filled triangle is
  overlapping (Fig 4.10)

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UML Notation

• Section 4.6 discusses UML conceptual schema
• Objects are classes
• box contains name, attributes method names
• Can specify domain of attribute by using
• Composite attribute modeled as structured domain
• Multivalued attribute modeled as separate class
• Weak entity qualified association
• placed in box attached to owner class

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UML Contd

• Relationship types are associations (lines)
• Relationship instances are links
• (min, max) notation used - opposite to EER
• indicates no limit
• Box/dashed line for relationship (link)
  attributes (Fig 4.11)
• Aggregation relationship between object and
  component parts (has-a relationship)
• Diamond indicates whole vs. part

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UML Contd

• Subclasses
• specialization/generalization
• blank triangle is disjoint, filled triangle is
  overlapping (Fig 4.12)