Database Schema and Instance

Database Schema

The overall design of the Database is called schema. Database system have several schema divided accordingly the level of the abstraction.
There are 3 level of schema: -

1. Physical Schema: -

Physical Schema describe the Database design at physical schema summarizes how the relation describe in the conceptual schema are actually stored on devices.

2. Conceptual Schema: -

The conceptual schema describes the stored data in terms of the data model of  Database Management System (DBMS). In a Relational Database Management System (RDBMS) the conceptual describe all relations that are stored in the Database.

3. External Schema: -

Which is usually also in term of the data model of the Database Management System (DBMS).
Allow data access to be customize at the level of individual user or group of users.

Database Instances

The collection of information stored in the Database. At a particular moment is called an instance of the Database in a program.
A database refers to a complete database environment. It includes Relational database management system (RDBMS) software, table structure, stored procedures and other function. It is user to describe multiple instances of same database.

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