Three Schema Architecture

Three schema Architecture

• It is also called ANSI (American National Standards Institute) schema Architecture, or, SPARC (Standards Planning and Requirements Committee) schema Architecture, or, three level architecture.

• It describes the structure of a specific database system.

• It separates the user applications and physical database.

• It contains three levels. It breaks the database down into three different categories.

• It allows a clear separation of the information meaning (conceptual view) from the external data representation and from the physical data structure layout.

• A database system that is able to separate the three different views of data is likely to be flexible and adaptable. It enables data independence in a database system.

Three Schema Architecture Objectives

• Insulation of application programs and data

• Support of multiple user views

• Use of schema to store the DB description (mete-data).

Three schema Architecture Levels

• External Level/schema (View Level) - Individual user view.

• Conceptual Level/schema (Logical Level) - Community user view.

• Internal Level/schema (Storage Level) - Physical or Storage view.

External Schema

• At the external level, a database contains several schema /sub-schema.

• The sub-schema is used to describe the different view of the database.

• An external schema is also known as view schema.

• Often a restricted view of database is presented in this level. Same database may provide different views for different class of users.

• The view schema describes the end user interaction with database systems.

Conceptual Schema

• It describes the design of a database at the conceptual level. It is also known as logical level.

• It describes the structure of the whole database.

• It describes what data are to be stored in the database and also describes what relationship exists among those data. Internal details such as an implementation of the data structure remain hidden in this level.

• Programmers and database administrators work at this level.

• The conceptual view represents: entities, attributes, relationships, constraints on the data, Semantic information about the data, Security & integrity information.

Internal Schema

• The internal schema describes the physical storage structure of the database. It describes how the data is stored in the database.

• It is also known as a physical schema.

• It uses the physical data model. It is concerned with how the data are physically stored on the hardware. It describe complex low-level data structures in detail.

The internal view represents:

• Storage space allocation for data.

• Record descriptions for storage.

• Storage path of the database.