Three Level Database Architecture

Posted By on November 4, 2014

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Introduction to Database Managment System
Database Users

Three Level Database Architecture


Data and Related Structures

Data are actually stored as bits, or numbers and strings, but it is difficult to work with data at this level.

It is necessary to view data at different levels of abstraction.Diagram to show the 3 level of adatabase architecture.


  • Description of data at some level. Each level has its own schema.

We will be concerned with three forms of schemas:

  • physical,
  • conceptual, and
  • external.








Physical Data Level

The physical schema describes details of how data is stored: files, indices, etc. on the random access disk system.  It also typically describes the record layout of files and type of files (hash, b-tree, flat).

Early applications worked at this level – explicitly dealt with details. E.g., minimizing physical distances between related data and organizing the data structures within the file (blocked records, linked lists of blocks, etc.)


  • Routines are hardcoded to deal with physical representation.
  • Changes to data structures are difficult to make.
  • Application code becomes complex since it must deal with details.
  • Rapid implementation of new features very difficult.



Conceptual Data Level

Also referred to as the Logical level

Hides details of the physical level.

  • In the relational model, the conceptual schema presents data as a set of tables.

The DBMS maps data access between the conceptual to physical schemas automatically.

  • Physical schema can be changed without changing application:
  • DBMS must change mapping from conceptual to physical.
  • Referred to as physical data independence.



External Data Level

In the relational model, the external schema also presents data as a set of relations. An external schema specifies a view of the data in terms of the conceptual level. It is tailored to the needs of a particular category of users. Portions of stored data should not be seen by some users and begins to implement a level of security and simplifies the view for these users


  • Students should not see faculty salaries.
  • Faculty should not see billing or payment data.

Information that can be derived from stored data might be viewed as if it were stored.

  • GPA not stored, calculated when needed.

Applications are written in terms of an external schema. The external view is computed when accessed.  It is not stored. Different external schemas can be provided to different categories of users. Translation from external level to conceptual level is done automatically by DBMS at run time. The conceptual schema can be changed without changing application:

  • Mapping from external to conceptual must be changed.
  • Referred to as conceptual data independence.
Introduction to Database Managment System
Database Users

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