Week 4 Lecture 2
Generalisation/specialisation induces IS-A relationships between entity sets in an ER diagram. The entities and IS-A relationships often evolve together to make a complete ER diagram. IS-A relationships are of a different character than relationship sets seen so far.
Relationship sets also get inherited through ISA relationships. Thus, each staff and student entity can participate in the ASSOC_WITH relationship.
Overlapping generalisation is one where an entity of the general entity type can be part of multiple specialised entity types.
Disjoint generailsation is one where an entity of the general entity type can be part of at most one specialised entity type.
A total generalisation is one where an entity of the general entity type should necessarily be part of oe specialised entity type. Also called a completeness constraint.