Database 3-Level Architecture

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Database 3-Level Architecture

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.

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

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.

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