The relational model in DBMS is based on the concept of relations and set theory proposed by Dr. Edgar F. Codd in 1970. A relation is defined as a two-dimensional table with rows representing tuples (records) and columns representing attributes (properties). A domain defines a set of atomic values for an attribute such as data type and value range. A relation schema specifies the structure of a table with attributes. A relation instance represents the actual data in a relation at a given time which can change with inserts, deletes or updates. Other concepts include degree as the number of columns, cardinality as the number of rows, and NULL values to represent unknown data.
2. Relational Model in DBMS
The relational database model derived from the mathematical concept of relation and set theory. It was proposed as
a technique to data modeling by Dr Edgar F. Codd of IBM Analysis in 1970 in his document entitled “A Relational
Technique of Information for Huge Shared Data Banks.” This document marked the start of the field of a relational
database.
The relational model is based on concept of a relationship that is physically defined as a table. In this model, tables
are used to hold data about the object to be set in the database. A relation is defined as a two-dimensional table in
which rows of the table correspond to single records, and the columns correspond to attributes. Each row is known
as a tuple, and each column is known as an
attribute.
3. Domain
A domain is a collection of atomic values. Atomic defines that each value in the domain is hidden to the relational
model. A domain is generally specified by name, data type, format, and constrained range of values. For example, as
shown in figure attribute EMP-NO, it is a domain whose data type is an integer with a value ranging between 1, 00,000
and 2, 00, 00. Attributes are the properties that specify a relationship. For example, EMP-NO, NAME.
4. Attributes
Attributes are the properties that prescribe a relation. For example, EMP-NO, EMP-NAME.
Relation Schema
The relation schema refers to the mechanism of a table.
Example: EMPLOYEE (EMP-NO, EMP-NAME, DATE-OF-BIRTH, SEX, TEL-NO, and SALARY) is a relation
schema for EMPLOYEE. If a schema has greater than one relation, it is known as Relational Schema.
Tuple
106510
Rajesh
Sharma
10-Feb-
1993
Male 9813245678 50,000
Each sequence in the relation is called a tuple. The relation includes 6 tuples, one of which is shown as:
5. Relation Instance
The set of records of a relation at a specific instance of time is known as relation instance. The table shows the relation
example of EMPLOYEE at a particular time. It can modify every time there is an insertion, deletion, or updating in the
database.
Degree
The number of columns in the table is known as the degree of the relation. EMPLOYEE relation has degree 6.
Cardinality
The number of rows in a table is called the cardinality of the relation. EMPLOYEE relation has cardinality 6.
NULL Values
The value which is anonymous or not available is known as a NULL Value. It is described by blank space.
