Logical data independence

Logical data independence provides the independence in a way that changes in conceptual model do not affect the external views. Or simply it can be stated at the Immunity of external level from changes at conceptual level.

Although we have data independence at different levels, but we should be careful before making a change to anything in database because not all changes are accepted transparently at different levels. There may be some changes which may cause damage or inconsistency in the database levels. The changes which can be done transparently may include the following:

o     Adding a file to the database

o     Adding a new field in a file

o     Changing the type of a specific field

But a change which may look similar to that of the changes stated above could cause problems in the database; for example: Deleting an attribute from the database structure,

This could be serious because any application which is using this attribute may not be able to run any more. So having data independence available to us we still get problem after a certain change, it means that before making a certain change its impact should also be kept in mind and the changes should be made while remaining in the limits of the data independence.

Fig:3. The levels where the Conceptual and Physical data independence are effective

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