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Extensions and intensions in Relational Database Model

A relational in a relational database has two components, an extension and an intension.

Extension

The extension of a given relation is the set of tuples appearing in that relation at any given instance. The extension thus varies with time. It changes as tuples are created, destroyed, and updated.

Relation: Employee at time= t1
EmpNo EmpName Age Dept
1001 Jason 23 SD
1002 William 24 HR
1003 Jonathan 28 Fin
1004 Harry 20 Fin

Relation: Employee at time= t2 after adding more records
EmpNo EmpName Age Dept
1001 Jason 23 SD
1002 William 24 HR
1003 Jonathan 28 Fin
1004 Harry 20 Fin
1005 Smith 22 HR
1006 Mary 19 HR
1007 Sarah 23 SD

Relation: Employee at time= t2 after adding more records
EmpNo EmpName Age Dept
1001 Jason 23 SD
1002 William 24 HR

Intension

The intension of a given relation is independent of time. It is the permanent part of the relation. It corresponds to what is specified in the relational schema. The intension thus defines all permissible extensions. The intension is a combination of two things : a structure and a set of integrity constraints.

The naming structure consists of the relation name plus the names of the attributes (each with its associated domain name).

The integrity constraints can be subdivided into key constraints, referential constraints, and other constraints.

For example,

Employee(EmpNo Number(4) Not NULL, EName Char(20), Age Number(2), Dept Char(4) )

This is the intension of Employee relation.

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