星期六, 10月 02, 2010

The Enhanced E-R Model

Objectives
  • Define terms
  • Understand use of supertype/subtype relationships
  • Understand use of specialization and generalization techniques
  • Specify completeness and disjointness constraints
  • Develop supertype/subtype hierarchies for realistic business situations
  • Develop entity clusters
  • Explain universal (packaged) data model
  • Describe special features of data modeling project using packaged data model

 

 

 
Supertypes and Subtypes
  • Enhanced ER model: extends original ER model with new modeling constructs
  • Subtype: A subgrouping of the entities in an entity type that has attributes distinct from those in other subgroupings
  • Supertype: A generic entity type that has a relationship with one or more subtypes
  • Attribute Inheritance:
    • Subtype entities inherit values of all attributes of the supertype
    • An instance of a subtype is also an instance of the supertype

 

 

 
Relationships and Subtypes
  • Relationships at the supertype level indicate that all subtypes will participate in the relationship
  • The instances of a subtype may participate in a relationship unique to that subtype. In this situation, the relationship is shown at the subtype level

 

 

 
Generalization and Specialization
  • Generalization: The process of defining a more general entity type from a set of more specialized entity types. BOTTOM-UP
  • Specialization: The process of defining one or more subtypes of the supertype and forming supertype/subtype relationships. TOP-DOWN

 

 

 
Constraints in Supertype/ Completeness Constraint
  • Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype
  • Total Specialization Rule: Yes (double line)
  • Partial Specialization Rule: No (single line)

 

 

 
Constraints in Supertype/ Disjointness constraint
  • Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes
  • Disjoint Rule: An instance of the supertype can be only ONE of the subtypes
  • Overlap Rule: An instance of the supertype could be more than one of the subtypes

 

 

 
Constraints in Supertype/ Subtype Discriminators
  • Subtype Discriminator: An attribute of the supertype whose values determine the target subtype(s)
  • Disjoint – a simple attribute with alternative values to indicate the possible subtypes
  • Overlapping – a composite attribute whose subparts pertain to different subtypes. Each subpart contains a Boolean value to indicate whether or not the instance belongs to the associated subtype

 

 

 
Entity Clusters
  • EER diagrams are difficult to read when there are too many entities and relationships
  • Solution: Group entities and relationships into entity clusters
  • Entity cluster: Set of one or more entity types and associated relationships grouped into a single abstract entity type

 

 

 
Packaged Data Models
  • Predefined data models
  • Could be universal or industry-specific
  • Universal data model = a generic or template data model that can be reused as a starting point for a data modeling project (also called a “pattern”)

 

 

 
Advantages of Packaged Data Models
  • Use proven model components
  • Save time and cost
  • Less likelihood of data model errors
  • Easier to evolve and modify over time
  • Aid in requirements determination
  • Easier to read
  • Supertype/subtype hierarchies promote reuse
  • Many-to-many relationships enhance model flexibility
  • Vendor-supplied data model fosters integration with vendor’s applications
  • Universal models support inter-organizational systems

 

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