Database Reference
In-Depth Information
The body consists of a time-varying set of tuples where each tuple consists of a set of
attribute-value pairs
{(A1:Vi1), (A2:Vi2), . . . (An:Vin)} (i = 1 . . . m)
where m is the number of tuples
(cardinality)
in the set. The body of the relation is also
sometimes referred to as the
proposition
of the relation. The proposition defines a set of
tuples whereby for each row in the relation, the respective attributes take on legal values
from their respective domains.
Observe that the definition of a relation appears to be similar to that of an entity.
There are two subtle differences:
•
The term
relation
as used, belongs to the field of relational
systems. We talk about relations because we are discussing the
relational model. Entities on the other hand, describe identifiable
objects and/or situations.
•
Entity
, as defined does not account for relationships. Relation
on the other hand, accounts for entities as well as relationships.
Thus in the relational model, we represent entities as relations
and (M:M) relationships (between entities) as relations. A binary
relation for instance, has two attributes. If both attributes are
foreign keys and they both constitute the primary key, this binary
relation actually represents a
many-to-many relationship
between two referenced relations; otherwise it is (a relation that
can be construed as) an entity. This point will become clear as
we proceed.
The foregoing underscores the point that entities can be construed as special
kinds of relations. In designing a database, the software engineer or database designer
commences by identifying entities during the requirements specification. After further
analysis, these entities are eventually implemented by normalized relations.
Note:
A unary relation differs from a domain in the sense that former is dynamic and
the latter static.
3.3.1 Properties of a Relation
Based on the relational model, all relations have the following properties:
•
No duplicate tuples (records)
•
Records are unordered
•
Attributes are unordered
•
Attribute values are atomic
