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Extensions and intensions in Relational Database Model

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.

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.

See Also

• Relational Database Model
• Different Types of Keys in Relational Database Model
• Integrity Rules
• Extensions and intensions
• Key Constraints
• Relational Algebra
• Traditional set operators
• Special Relational Operators
• Object oriented Model
• Encapsulation of Operations, Methods, and Persistence
• Comparison Between Relational Database Model and Object Oriented Model

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