ER modeling part 2...

(see the posted PDF version/Reading Packet)

*   recursive relationships - amongst instances of the
    same entity class;

    ...the relationship line just begins and ends at the
       same entity class rectangle;

    ex: employee manages employee,
        course is a prerequisite of a course,
        student rooms with a student,
        employee is a spouse of employee

*   weak entities - an entity class VERY strongly dependent
    on another entity class;

    (don't take this too overboard, or too literally --
    it is meant to be a special case...)

    a possible indicator: a weak entity might not have
    any identifying attributes...

   *   another thing to check: the possible weak entity
       had BETTER have a mandatory relationship with
       the entity it is existence-dependent on...!

*   we will indicate a weak entity by drawing a double-border
    on its rectangle...

*   Supertypes and Subtypes (and Unions)

*   part of the extended ER model (EER)

*   POSSIBLE symptoms of supertype-subtype:
*   something IS-A(n) instance of another thing;
    they share some common attributes;
    they share some common relationships;
    an entity class has distinct sets of seemingly-optional
       attributes;

*   see posted PDF notes/reading packet
    for discussion on disjoint supertype/subtypes,
    overlapping supertype/subtypes, and unions
    
    ...and how to depict them in an ER diagram
    ...and notes about their entity-attribute lists