Attribute inheritance is the gaining of attributes by a subtype entity from a supertype entity This inheritance of common pieces of data from general to more specific definitions of entities is what enables subtypes to be further defined from ID: 602570
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Slide1
Question 11
Attribute inheritance is the gaining of attributes by a subtype entity from a
supertype
entity. This inheritance of common pieces of data from general to more specific definitions of entities is what enables subtypes to be further defined from
supertypes
. By separating entities out into subtypes that have their own data that doesn’t need to be defined in the more general
supertype
of the entity, the data stored is less redundant and the relationships among it are more flexible.Slide2
Question 12
a. For the disjoint rule to apply, an entity instance that corresponds to the
supertype
cannot correspond to more than one of its subtypes. An example would be a
supertype
entity FURNITURE that has relationships with subtype entities CHAIR and TABLE. An entity instance cannot be both a CHAIR and a TABLE according to the business rule and the standard definitions of chairs and tables.
b. For the overlap rule to apply, an entity instance that corresponds to the
supertype
must be able to correspond to more than one of its subtypes at a time. An example would be a
supertype
entity PATRON and related subtype entities DONOR and BOARDMEMBER for an art gallery. All entities participating are instances of type PATRON, can be instances of type DONOR, and those of type DONOR can be of type BOARDMEMBER as well, depending on the business rule.Slide3
Question 13
Enhanced Entity-Relationship diagrams are designed to represent the inheritance of attributes that occurs between instances of the same entities that should not all use the same attributes. These types of entities arise from business rules that specify entities that are related but not identical.
Such rules are those where certain instances of an entity don’t use one or more of their given attributes and may have invalid null values for those attributes, or may share only several of their attributes and can be related but not lumped together under one definition.
Business
rules that specify different types of employees that have some shared attributes and some different employees have extra non-uniform attributes should be modeled with EER diagrams to allow for this incongruence of data between different types.