Week 4 Lecture 2

Generalisation/Specialisation

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.

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Relationship sets also get inherited through ISA relationships. Thus, each staff and student entity can participate in the ASSOC_WITH relationship.

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Overlapping generalisation

Overlapping generalisation is one where an entity of the general entity type can be part of multiple specialised entity types.

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Disjoint generalisation

Disjoint generailsation is one where an entity of the general entity type can be part of at most one specialised entity type.

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Total generalisation

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.

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