Database Reference
In-Depth Information
Mathematical set theory.
Applies the mathematical set theory for manipulation of
data in the database. Data manipulation uses a proven approach.
Disciplined data organization.
Resting on solid mathematical foundation, data
organization and manipulation are carried out in a disciplined manner.
Simple and familiar view.
Provides a common and simple view of data in the form
of two-dimensional tables. Users can easily perceive how data are organized; they
need not be concerned with how data are actually stored in the database.
Logical links for relationships.
As you will recall, hierarchical and network
data models use physical links to relate entities. If two entities such as CUSTOMER
and ORDER are related, you have to establish the relationship by means of
physical addresses embedded within the stored data records. In striking contrast,
relational model uses logical links to establish relationships. This is a major
advantage.
Relation: The Single Modeling Concept
As mentioned above, a relation or table is the primary data modeling concept in
the relational mode. A table is collection of columns that describe a thing of inter-
est to the organization. For example, if COURSE is a conceptual thing of interest
in a university, then a two-dimensional table or relation will represent COURSE
in the relational data model for the university. Figure 8-1 shows a plain two-
dimensional table whose format represents an entity or object.
Note the following features about a relation or two-dimensional table:
•
The relation is a table, representing data about some entity or object.
•
The relation is not just any random table, but one that conforms to certain
relational rules.
•
The relation consists of a specific set of columns that can be named and an
arbitrary number of rows.
•
Each row contains a set of data values.
RELATION
Columns
.
.
Figure 8-1
Relation or two-dimensional table.
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