THREE SCHEMA ARCHITECTURE

THREE SCHEMA ARCHITECTURE:

 The overall design of the database is called the “Schema” or “Meta Data”. A database schema corresponds to the programming language type definition. The value of a variable in programming language corresponds to an “Instance” of a database Schema.

Three Schema Architecture:

 The goal of this architecture is to separate the user applications and the physical database. In this architecture, schemas can be defined at the following three levels:

1.The internal level has an internal schema, which describes the physical storage structure of the database. The internal schema uses a physical data model and describes the complete details of data storage and access paths for the database.

 2.The conceptual level has a conceptual schema, which describes the structure of the whole database for a community of users. The conceptual schema hides the details of physical storage structures and concentrates on describing entities, data types, relationships, user operations, and constraints. A high-level data model or an implementation data model can be used at this level.

 3. The external or view level includes a number of external schemas or user views. Each external schema describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group. A high-level data model or an implementation data model can be used at this level.