Database Schema and Instance | Database Management System

Database Schema and Instance | Database Management System

Database Schema and Instance : The view at each level is described by a scheme. A scheme is an outline or a plan that describe the records and relationships existing in the view.

The Overall Set of Such relationships for the entire database is known as database Schema. It includes all tables, their constraints and relationships.

“The overall design of the database is called the database schema”.

“The collection of information stored in the database at particular moments is called an instance of the database”.

There are three types of schema:

  1. Physical schema
  2. Conceptual Schema
  3. External schema

 

Physical Schema: Physical Schema describes the database design at the physical level. It specifies additional storage details.The physical schema summaries how the relations described in the Conceptual Schema are actually Stored on Secondary devices such as disks and tapes.

Conceptual Schema: The conceptual schema (sometimes called logical schema) describes the stored data in terms of the data model of the DBMS. In a relational DBMS, the conceptual schema describes all relations that are stored in the database.

External Schema: External Schema, which usually are also in terms of the data model of the DBMS allow data access to be customized at the level of individual users or groups of users.