Three level of abstraction, along with th e map ping from internal to conceptual and from conceptual to external, provide two distinct levels of data independence: logical data independence and physical data independence.
Logical data independence indicates that the conceptual schema can be changed without affecting the existing external schemes Logical data independence also insulates application programs from operations such as combining two records into two or more records.
Logical data independence is achieved by providing the external level or user view of the database. The application programs or users see the database as described by their respective external views. The DBMS provides a mapping from this view to the conceptual view. The view at the conceptual level of the database is the sum total of the community view of the database there will be many external views, but only conceptual view of database.
Physical data independence is indicates that the physical storage structures or devices used for storing the data could be changed without a change in the conceptual view or any of the external views. The change would be absorbed by the mapping between the conceptual and internal levels.
The following is the external views of the users from the
public relations and payroll departments. Each of these external views is represented in a high level language declaration in accordance with the normal rules such language.
EXTERNAL SCHEMES OF USER IN PUBLIC RELATIONS DEPARMENT AND USER IN PAYROLL DEPARTMENT
-> CONCERPTUAL SCHEMA OF DATABASE
EMPLOYEE.Soc_Sec_No:integer primary key;
-> INTERNAL SCHEMA OF THE DATABASE
type EMPLOYEE=record length 120
EMPLOYEE.name:string length 125 offset 0;
EMPLOYEE.soc_sec_no:integer positive 9 dec digits offset 25 unique use for index;
EMPLOYEE.department:string length 6 offset 34;
EMPLOYEE.address:string length 50 offset 40;