Entity-Relationship Model

• 'Owned by' another entity

• Can be uniquely identified in conjunction with owning entities

• Essentially a nested element

• Eg Assignments belong to courses, courses belong to departments

Don't use weak entities if you can use simpler constructs

Not to be confused with Instruction Set Architecture

ISA relationship in databases

• Allows for hierarchical relationships
• Constraints not represented on the diagram must be documented separately
  • Overlap constraints
    • Entity belonging to multiple subclasses
    • Eg an entity being both Faculty and Student (a TA)
  • Covering constraints
    • Must every superclass entity also belong to a subclass?
    • Eg must every Person entity also be a Student, Faculty, and/or Staff?