Binary
versus n-ary Relationship Sets
Relationships
in databases are often binary. Some relationships that appear to be nonbinary
could actually be better represented by several binary relationships. For
instance, one could create a ternary relationship parent, relating a child to his/her
mother and father. However, such a relationship could also be represented by
two binary relationships, mother and father, relating a child to his/her mother
and father separately. Using the two relationships mother and father provides
us a record of a child’s mother, even if we are not aware of the father’s
identity; a null value would be required if the ternary relationship parent is
used. Using binary relationship sets is preferable in this case.
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| (a) ternary relationship (b) three binary relationship |
Specialization
An
entity set may include subgroupings of entities that are distinct in some way from
other entities in the set. For instance, a subset of entities within an entity
set may have attributes that are not shared by all the entities in the entity
set. The E-R model provides a means for representing these distinctive entity
groupings. As an example, the entity set person may be further classified as
one of the following :
employee.
student.
Each
of these person types is described by a set of attributes that includes all the
attributes of entity set person plus possibly additional attributes. For
example, employee entities may be described further by the attribute salary,
whereas student entities may be described further by the attribute tot cred.
The process of designating subgroupings within an entity set is called
specialization. The university could create two specializations of student, namely
graduate and undergraduate. As we saw earlier, student entities are described
by the attributes ID, name, address, and tot cred. The entity set graduate
would have all the attributes of student and an additional attribute office
number. The entity set undergraduate would have all the attributes of student,
and an additional attribute residential college. We can apply specialization
repeatedly to refine a design. For instance, university employees may be
further classified as one of the following :
instructor.
secretary.
The
way we depict specialization in an E-R diagram depends on whether an entity may
belong to multiple specialized entity sets or if it must belong to at most one
specialized entity set. The former case (multiple sets permitted) is called overlapping
specialization, while the latter case (at most one permitted) is called disjoint
specialization. For an overlapping specialization (as is the case for student and
employee as specializations of person), two separate arrows are used. For a disjoint
specialization (as is the case for instructor and secretary as specializations of
employee), a single arrow is used. The specialization relationship may also be referred
to as a superclass-subclass relationship.
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| Specialization/Generalization |
Generalization
The
refinement from an initial entity set into successive levels of entity
subgroupings represents a top-down design process in which distinctions are
made explicit. The design process may also proceed in a bottom-up manner, in
which multiple entity sets are synthesized into a higher-level entity set on
the basis of common features. The database designer may have first identified :
instructor
entity set with attributes instructor id, instructor name, instructor salary,
and rank.
secretary
entity set with attributes secretary id, secretary name, secretary salary, and
hours per week.
Attribute
Inheritance
A
crucial property of the higher- and lower-level entities created by
specialization and generalization is attribute inheritance. The attributes of
the higher-level entity sets are said to be inherited by the lower-level entity
sets. Whether a given portion of an E-R model was arrived at by specialization
or generalization, the outcome is basically the same :
A
higher-level entity set with attributes and relationships that apply to all of its
lower-level entity sets.
Lower-level
entity sets with distinctive features that apply only within a particular
lower-level entity set.
Constraints
on Generalizations
Condition-defined.
In condition-defined lower-levelentity sets, membership is evaluated on the
basis of whether or not an entity satisfies an explicit condition or predicate.
User-defined.
User-defined lower-level entity sets are not constrained by a membership
condition; rather, the database user assigns entities to a given entity set.
Disjoint.
A disjointness constraint requires that an entity belong to no more than one
lower-level entity set.
Overlapping.
In overlapping generalizations, the same entity may belong to more than one
lower-level entity set within a single generalization.
The
completeness constraint on a generalization or specialization, specifies
whether or not an entity in the higher-level entity set must belong to at least
one of the lower-level entity sets within the generalization / specialization. This
constraint may be one of the following :
Total
generalization or specialization. Each higher-level entity must belong to a
lower-level entity set.
Partial
generalization or specialization. Some higher-level entities may not belong to
any lower-level entity set.
Source : Silberchatz, Korth & Sudarshan (2010) Database Systems Concepts, 6th Edition, McGraw-Hill
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