Although data items
are real attributes of the entities represented, they need not always
be explicitly defined within the formal database structure. A relationship
may be represented in the data structure using one of the three forms.
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1. Explicit data items: as defined by the user and stored in each of
the entity records participating in the relationship, sometimes called
‘symbolic pointers’
2. Physical pointers: chaining or an array of pointers in the many direction
of the relationship, pointer in one direction to an exclusive member
of the related entity class.
3. Physical contiguity: storing member records with parent records.
More than one form
can be used simultaneously, resulting in greater redundancy and greater
reliability.
A One to One relationship
can be represented by a data item value in one entity being equal to
a data item value in the other entity. There can be no relationship
between two entities unless there is some common domain on which to
base the relationship. The second representational form uses a physical
pointer in one record to point to a related record instance. In the
third representational form, one of the two entity types could be designated
as parent with the other entity instance physically stored next to the
parent entity instance. With physical pointers or physical contiguity,
it is no longer necessary to explicitly store the data item values of
the common domain.
The problem with the last two representational forms is that proper
recording of the relationship is dependent upon the correctness of the
physical pointer or physical congruity. If the un-stored attribute value
in the dependent entity instance changes, the person updating the database
must identity both the old and the new instances so that the system
can appropriately update both physical pointers, or the change must
be reflected in a physical move of the entity record.
Also, the pointer and physical contiguity representational forms imply
a directionality and access path which must be known and adhered to
on the part of the user. It is more difficult to access dependent entity
instances independent of parent entity instances. It is generally more
desirable to store the data item explicitly and then let the use of
pointers of physical contiguity be dictated solely by the need for more
rapid access or better utilization of storage. At the very least, the
user should be insulated from these physical considerations. Defining
the database structure should initially be concerned only with the relationship
and not its representational form that is a matter for physical storage
structure design.
A One-to-Many relationship:
The relationship is represented by adding a data item to one record.
It contains the identifier of the related record from the other entity
class. Either the identifier of X is stored in the records of type Y
or vice versa. Doing one or the other has some access efficiency implications.
A one-many relationship between X and Y can be represented by storing
the identifier of X in each instance of Y, storing a set of identifiers
of Y in a repeating group within each instance of X or storing the identifier
for the first Y in X and having the rest of the Y’s chained together.
Another representational choice is to store the Y’s physically
contiguous with their related X. A variation of the third choice would
be to store the Y’s of an X remote from X but physically contiguous
with each other. The first representational form is the bases for the
relational data model while the second and third are the basis of the
network data model.
There is an implicit directionality from many-to-one in the relational
data model and directionality from one-to-many in the network model.
Neither one of these directions is inherent in the relationship to be
defined and captured in the stored database. The particular representational
form chosen depends upon the behavioral characteristics of the database,
how it is used and the more frequent avenue of access. It is obviously
possible to store sufficient information to permit efficient access
in the either direction. With higher degree, One-to-Many relationships,
the extra attributes of the relationship can be added to the repeating
group of foreign identifiers in either the X or the Y record.
Alternatively, the extra attributes can be added to the special linking
record. This last alternative is the only one permitted way in either
the network or the relational data models. Ideally the user should not
be involved with definitions at this level of representation. It should
be sufficient to declare two entity classes, a relationship between
them, the characteristics and criteria of the relationships, any attributes
of the relationship and the behavioral characteristics of the relationship.
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