August 1999
DB212 : DATABASE MANAGEMENT SYSTEMS

QUESTION 2

Total Marks: 20 Marks

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(a) Describe the DBMS architecture known as the three-schema architecture. Your answer should include descriptions of each level of architecture (3 marks for each level), as well as a diagram. (3 marks). [12]
External level
  • one or more external view, each with corresponding external schema
  • present a predetermined subset of the database to the user
  • concerned with the way data is presented to the user

Conceptual level

  • only one conceptual view with a corresponding conceptual schema
  • present the complete databases structure to the DBA
  • concerned with all database objects, security and integrity rules

Internal rules

  • only one internal view with a corresponding internal schema
  • present low level implementation details
  • concerned with physical constructs like indexes, points, hashing schemas, access techniques etc.

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(b) (i) What is data independence?

(ii) In the three-schema architecture, define the two forms of data independence, and explain which parts of the architecture make these two forms possible.

 [6]
(i) Data definitions are separated from applications, they are independent of each other.

(ii) Physical data independence: any change made in the internal level does not require an associated change at the conceptual level and the external level.
Conceptual-internal mapping makes physical data independence possible.

Logical data independence: any change made in the conceptual level does not require an associated change at the external level.

External-conceptual mapping makes logical data independence possible.