Three Schema Architecture of Database system
The goal of the three schema architecture is to separate the user application from the physical database.Database schema can be defined in three levels
- Internal schema.
- Conceptual schema.
- External schema.
Internal schema
Internal level has an internal schema which describes the physical storage structures of the database.That describes the complete details of data storage and access paths for the database.
Conceptual schema
The conceptual schema hides the details of the physical storage structures and concentrates on describing entities,data types,relationships user operations etc.
External Schema
The external schema includes a number of external schema.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.
The stored data that actually exists is at the physical level only.DBMS must transform a request specified on an external schema into a request against the conceptual schema and then into a request on the internal schema for processing over the stored database.If the request is a database retrieval the data extracted from the stored database must be reformatted to match the user external view,The processes of transforming requests and results between levels are called mappings.These mappings may be time consuming.
Data independence which as the capacity to change the schema at one level of a database system without having to change the schema at the higher level.
In Short
- An internal schema describes the physical storage structure of the database.
- A conceptual schema is high level description of the whole database.
- External schema describe the views of different user groups.
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