What is Data Independence ?

Answer 1

Ability to modify a schema definition in one level without affecting a schema definition in the other levels.
The interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others.

Two levels of data independence
Physical data independence
Logical data independence

Logical data independence:
The capacity to change the conceptual schema without having to change external schema or application prgms ex: Employee (E#, Name, Address, Salary)
A view including only E# and Name is not affected by changes in any other attributes.

Physical data independence:
The capacity to change the internal schema without having to change the conceptual (or external) schema internal schema may change to improve the performance (e.g., creating additional access structure) easier to achieve logical data independence, because application programs are dependent on logical structures.

Answer 2

Data independence means that "the application is independent of the storage structure and access strategy of data". In other words, The ability to modify the schema definition in one level should not affect the schema definition in the next higher level.
Two types of Data Independence:
1. Physical Data Independence: Modification in physical level should not affect the logical level.
2. Logical Data Independence: Modification in logical level should affect the view level.
NOTE: Logical Data Independence is more difficult to achieve