|
Answer 2:
Data independence, is usually considered from two
points of view Physical
data independence and Logical data independence. The physical data
independence allows changes in the physical storage devices or
organization of the files to be made without requiring changes in
the conceptual view or any of the external views and hence in the
application programs using the database. Thus the files may migrate
from one type of physical media to another of the file structure
may change without any need for changes in the application programs.
Logical data independence implies that application programs need not
be changed if fields are added to an existing record nor do they have
to be changed if fields not used by application programs are deleted.
Logical data independence indicates that the conceptual schema can be
changed without affecting the existing external schemas. Data
independence is advantageous in the database environment since
it allows for changes at one level of the database without affecting
other levels.
Logical data
independence is more difficult to achieve than physical
independence. Since application programs are heavily dependent on the
logical structure of the data they access .
The concept of data
independence is similar in many respect to the concept of abstract
data type in modern programming language like C++. Both hide
implementation details from the users. This allow users to concentrate
on the general structure rather than low-level implementation
details.
|