What Is Meant By Relation In Relational Data Model ?


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Muhammad Azhar answered
The relational model is based on the concept of a relation. A relation is a table, which consists of a set of named columns and a set of unnamed rows. The tables are the most important part of the database and hold information about objects. A relation is represented as a two dimensional table, in which rows of the table indicate the individual records and the table columns indicate the attributes.

For example, to store the data of student, a relation is represented by the 'Student' table. This table has column attributes Roll-No, Name and city. Each column contains values of the attribute e.g., the Name column contains only the names of students. There are four rows in the table. It means that records of four students are represented.
Therefore, in relational model terminology, we define the following terms as:

Each column of a relation has a heading, called 'field name'. The named columns of the relation are called attributes. In the above 'Student' relation, the attribute names are Roll-No, Name and City.
Degree: The number of attributes in a relation is called its degree. In the
"Student" relation, the degree is 3. A relation with only one column would have degree one and is referred to as unary relation. Similarly, a relation with two columns is called binary and a relation with three columns is called ternary relation.
Tuple:Each row of the relation is called tuple. Each tuple consists of all attribute values that are related to a particular entity.

Cardinality:The number of tuples in the relation is called cardinality. In the 'Student' relation, there are 4 tuples (rows or records), so the cardinality of the relation is 4.
Domain: The range of acceptable values for an attribute is called the domain for the attribute. Two or more attributes may have the same domain.
A relation schema is used to describe a relation. It is made up of a relation name and a list of attributes..

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