Database 3-Level Architecture

Objective : Separate each user’s view of database from way it is physically represented.

 

External Level :

• The User’s view of the database

• Consists of a no. of different external views of the DB

• Describes part of the DB for particular group of users

• Provides a powerful & flexible security mechanism by hiding part of DB from certain users. The user is not aware of any existing attributes that are missing from the view.

• It permits users to access data in a way that is customized to their needs, so that the same data can be seen by different users in ways, at the same time.

 

Conceptual View

• The Logical Data Structure of the entire database as seen by DBA

•  What data is stored in database

• The relationships among the data

• Complete view of the data requirements of the organization, independent of any storage consideration

Represents

•             Entities, Attributes, Relations

•             Constraints on data

•             Semantic info. on data

•             Security, Integrity info.

Supports each external view: any data available to a user must be contained in, or derivable from the conceptual level.

  

External Level

• Physical representation of the DB on the comp.

• How the data is stored in database

• Physical implementation of the DB to achieve optimal run-time performance & storage space utilization

            Storage space allocation for data & indexes

            Record description for storage

            Record placement

            Data Compression, Encryption

 

Physical Level

            Managed by the OS under the direction of the DBMS.