ASSOCIATIONS (ENTITY RELATIONSHIPS)

• ASSOCIATIONS: ER OR RELATIONAL DATA MODEL RELATIONSHIPS BETWEEN ENTITIES

• ASSOCIATIONS CAN HAVE THEIR OWN ATTRIBUTES

• IN MOST OO DATA MODELS THERE IS NO EXPLICIT CONCEPT OF ASSOCIATION

• ASSOCIATIONS ARE REPRESENTED BY REFERENCES BETWEEN OBJECTS - DIFFICULTIES IN REPRESENTING:
  
  
  • TERNARY RELATIONSHIPS
  • RELATIONSHIPS WHICH HAVE THEIR OWN ATTRIBUTES
  • ASSOCIATIONS ARE TWO-WAY - OBJECT REFERENCES ARE ONE WAY I.E. TRANSVERSAL IS EASY ONLY IN ONE DIRECTION
  
  
• OO DATA MODEL ASSOCIATION EXTENSIONS NOT SUPPORTED BY CURRENT ODBMSs BECAUSE:
  
  
  • INCREASED COMPLEXITY OF IMPLEMENTATION & REDUCED SYSTEM'S PERFORMANCE
  • TOO MANY MODELING CONSTRUCTS INCREASE THE USER DIFFICULTY IN DESIGNING THE DATABASE
  • USE "ASSOCIATION" CLASSES THAT HAVE THE OIDs OF THE RELATED ENTITIES (FOREIGN KEYS) & OWN ATTRIBUTES
  • ASYMMETRY OF REFERENCE CAN BE ELIMINATED BY USE OF REVERSE REFERENCES
  
  

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Ugo O. Gagliardi
GSD-6306 Spring 2003-2004