Chapter 5 -- Entity model Flashcards
Extended entity relationship model (EERM)
The enhanced entity-relationship model
The result of adding more semantic constructs such as entity supertypes, entity subtypes, and entity clustering, to the original entity-relationship model.
EER diagram (EERD)
The entity relationship diagram resulting from the application of extended entity relationship concepts that provide additional semantic content in the ER model.
Entity supertype
A generic entity type that contains the common characteristics of entity subtypes
Entity subtype
A subset of an entity supertype
Specialization hierarchy
A hierarchy based on the top-down process of identifying lower-level, more specific entity subtypes from a higher-level entity supertype. It is based on grouping unique characteristics and relationships of the subtypes.
To support attribute inheritance
Define a special supertype attribute known as the subtype discriminator
Define disjoint or overlapping constraints and complete or partial constraints.
Inheritance
The property that enables an entity subtype to inherit the attributes and relationships of the entity supertype.
All entity subtypes inherit their primary key attribute from the supertype.
Entity subtypes inherit all relationships that the supertype entity participates.
Subtype discriminator
The attribute in the supertype entity that determines to which entity subtype each supertype occurence is related.
Disjoint subtypes
Unique subset of the supertype entity set
non-overlapping subtypes.
Overlapping subtype
Subtypes that contain nonunique subset of the supertype entity set which means each entity instance of the super type may appear in more than one subtype
Completeness constraint
A constraint that specifies whether each entity supertype occurrence must also be a member of that one subtype
It can be partial or total.
Partial completeness
A condition that some supertype occurrences might not be members of any subtype.
Total completeness
A condition that every supertype occurrence must be a member of at least one subtype.
Partial constraint
Disjoint constraint
Supertype has optional subtypes.
Subtype discriminator can be null.
Subtype sets are unique.
Overlapping constraints
Supertype has optional subtypes.
Subtype discriminator can be null.
Subtype set are not unique.
Total constraint
Disjoint constraint
Every supertype occurrence is a member of only one subtype.
Subtype discriminator cannot be null.
Subtype sets are unique.
Overlapping constraint
Every supertype occurrence is a member of at least one subtype.
Subtype discriminators cannot be null.
Subtype sets are not unique.
Specialization
The top-down process of identifying lower-lowel, more specific entity subtypes from a higher-level entity supertype.
Generalization
The bottom-up process of identifying a higher-level, more generic entity supertype from lower-level entity subtypes.
Entity cluster
A ‘virtual’ entity type used to represent multiple entities and relationships in the ERD.
