Constraints in ER modeling

Generalization

* similarities are generalized to a super-class entity, differences are specialized to a subclass e called an “ISA” relationship (“specialization” is the inverse relationship)

* disjointness constraint - there is no overlap among subclasses

* completeness constraint - constrains subclasses to be all-inclusive of the super-class or not total or partial coverage of the superclass)

* special property: hierarchical in nature

* special property: inheritance - subclass inherits the primary key of the super-class, super-cla common nonkey attributes, each subclass has specialized non-key attributes

Aggregation

* “part-of” relationship among entities to a higher type aggregate entity (“contains” is the inve relationship)

* attributes within an entity, data aggregate (mo-day-year)

* entity clustering variation: membership or “is-member-of” relationship

Constraints in ER modeling

* role - the function an entity plays in a relationship

* existence constraint (existence dependency) - weak entity

* exclusion constraint - restricts an entity to be related to only of several other

* entities at a given point in time

- mandatory/optional

- specifies lower bound of connectivity of entity instances

- participating in a relationship as 1 or 0

* uniqueness constraint – one-to-one functional dependency among key attributes in a relationship: binary, ternary, or higher n-ary

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Дата добавления: 2015-11-16