mod 4 advanced data modeling

• Extended Entity-Relationship (EER) model


  • Result of adding more semantic constructs to the original entity-relationship (ER) model
  • Diagram using this model is called an EER diagram (EERD)
• Entity supertype


  • Generic entity type related to one or more entity subtypes
  • Contains common characteristics
• Entity subtype


  Contains unique characteristics of each entity subtype
• Relationships at the supertype level


  Indicate that all subtypes will participate in the relationship
• instances of a subtype

  
  

  may participate in a relationship unique to that subtype
• Inheritance


  • Enables entity subtype to inherit attributes and relationships of the supertype
  • All entity subtypes inherit their primary key attribute from their supertype
  • At the implementation level, the supertype and its subtype(s) maintain a 1:1 relationship
  • Entity subtypes inherit all relationships in which the supertype entity participates
  • Lower-level subtypes inherit all attributes and relationships from all upper-level supertypes
• Specialization


  • Identifies more specific entity subtypes from higher-level entity supertype
  • Top-down process
  • Based on grouping unique characteristics and relationships of the subtypes

  
• Generalization

  • Identifies more generic entity supertypes from lower-level entity subtypes
  • Bottom-up process
  • Based on grouping common characteristics and relationships of the subtypes
• Specialization hierarchy


  • Depicts arrangement of higher-level entity supertypes and lower-level entity subtypes
  • Relationships described in terms of "IS-A" relationships
  • Subtype exists only within the context of the supertype
  • Every subtype has only one supertype to which it is directly related
  • Can have many levels of supertype/subtype relationships

  
• Partial completeness


  Some supertype occurrences are not members of any subtype
• Total completeness


  Every supertype occurrence must be a member of at least one subtype
• 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
• Subtype discriminator


  • An attribute in the supertype entity that determines to which entity subtype each supertype occurrence is related
  • Default comparison condition for subtype discriminator attribute is equality comparison
  • Subtype discriminator may be based on other comparison condition
• Disjoint subtypes


  Subtypes that contain a unique subset of the supertype entity set
• Overlapping subtypes


  Subtypes that contain non-unique subsets of the supertype entity set
• Primary key


  • Most important characteristic of an entity
  • Single attribute or some combination of attributes
  • Primary key's function is to guarantee entity integrity
  • Primary keys and foreign keys work together to implement relationships
  • Properly selecting primary key has direct bearing on efficiency and effectiveness
• Natural key


  • A real-world identifier used to uniquely identify real-world objects
  • Familiar to end users and forms part of their day-to-day business vocabulary
  • Generally, data modeler uses natural identifier as primary key of entity being modeled