UNIT 5 -Three Levels of Schema (Three-Tier Architecture)

pt 2

Physical/Internal Schema

Defines actual storage (files, indexes, access paths).

Lowest level of abstraction.

Example: store relations as unsorted files, create indexes for faster access.

Conceptual/Logical Schema

Describes structure of the database for the whole community of users.

Defines entities, attributes, relationships, security, and integrity.

Example: Students, Faculty, and Courses relations in a university database.

External/View Schema

Describes what a specific user or group sees.

Hides irrelevant details.

Multiple external views possible.

Example: Sales staff see only sales-related data.

Goals of Three-Schema Architecture

Users can view the same data differently (customized views).

Users don’t need to worry about physical storage details.

DBAs can change storage without affecting users.

Ensures independence between levels (data abstraction).