EERD Flashcards
Brief Explanation of EERD
a response to the shortcomings of the basic E-R
model
not universally agreed upon in some respects
introduced the supertype/subtype relationship
extends original
ER model with new modeling constructs
Subtype
subgrouping of the entities in an entity type that has attributes distinct from those in other subgroupings
Super Type
generic entity type that has a relationship with one or more subtypes
Attribute Inheritance
Subtype entities inherit values of all
attributes of the supertype
An instance of a subtype is also an instance
of the supertype
Generalization
Defining a set of entities to a much more general entity type from bottom to up approach
Specialization
Opposite of generalization. Top-Down Defining one or more subtypes of a supertype and forming new supertype/subtypes
Total Specialization Rule
A <supertype> May be Either be <subtype> Or <Subtype>
Double lines</Subtype></subtype></supertype>
Partial Specialization
A <supertype> May Either be <Subtype> Or <subtype> but does not have to be either
Single Line</subtype></Subtype></supertype>
Disjoint Rule
Can Either be Or (may depend on what type of specialization) , But not both at the same time
and has a letter d on the circle
Overlap Rule
A <supertype> May be both a <subtype> and a <subtype>
has a letter O</subtype></subtype></supertype>
If EER diagrams are difficult to read when there are too many entities and relationships you can ??? and what is it called???
Group entities and relationships into entity clusters
Entity clusters
Set of one or more entity
types and associated relationships
grouped into a single abstract entity
type
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Subtype discriminators
Attribute of the supertype whose value determines wo which subvtype an instance belongs
Universal data model
a generic or
template data model that can be
reused as a starting point for a data
modeling project (also called a
“pattern”)
