Database Reference
In-Depth Information
It also allows users to create queries that are more complex than those they created using
a hierarchical database.
Disadvantages
A user has to be familiar with the structure of the database in order to work through the
set structures.
Change in the database structure is difficult without affecting the application programs
that interact with it.
2.1.4 Relational Database Model
It is a data model in which the data is stored in
tables
(also referred to as
entities
).
Each table has
rows
(also referred to as
records or tuples
) and columns (also referred to
as attributes). In relational database model any tables can be linked together. The relational
model is based on a collection of mathematical principles drawn primarily from set
theory and predicate logic. These principles were first applied to the field of data modeling
in the late 1960s by Dr. E. F. Codd. Each row in a table has a unique identifier (UID). In
this database model relationships between tables are defined using the UID of one table
and joining them with the UID of another table. Pointers are not used. Data redundancy
is reduced via a process called normalization.
The relational model presents data in familiar two-dimensional tables, much like
a spreadsheet does. Unlike a spreadsheet, the data is not necessarily stored in tabular
form and the model also permits combining (joining in relational terminology) tables to
form views, which are also presented as two-dimensional tables.
The entire structure is, as we've said, a
relation
. The relation is divided into two
sections, the
heading
and the
body
. The tuples make up the body, while the heading is
composed of, well, the heading. The body of the relation consists of an unordered set of
zero or more tuples. A relation is a relation provided that it's arranged in row and
column format and its values are scalar. Its existence is completely independent of any
physical representation. As such, a user isn't required to know the physical location of
a record in order to retrieve its data. This is unlike the hierarchical and network database
models, in which knowing the layout of the structures is crucial to retrieving data. As
long as a user is familiar with the relationships among the tables in the database, he can
access data in an almost unlimited number of ways. He can access data from tables that
are directly related and from tables that are indirectly related.
Example 3:
The relational model categorizes relationships as one-to-one, one-to-many, and many-to-
many. A relationship between a pair of tables is established implicitly through matching
