Data Independence

Data Independence

The ability to modify schema definition in one level without affecting schema definition in the next higher level is called data independence.

In other words ,we can say that each higher level of the data architecture is immune to changes of its lower level of the architecture.

For example:-Metadata ,Metadata itself follows a layered architecture, so that when we change data at one layer, it does not affect the data at another level. We can say that data is mapped to each other but the is data independent.

Data Independence is classified into two categories:

1) Physical Data Independence : - Physical data independence is the ability to change the physical data without impacting the schema or logical level. 

In other words,we can that it is power to modify the physical schema without causing application programs to be rewritten.

Modification are necessary at physical level to improve the performance.

2) Logical Data Independence : - Logical data independence is the ability to modify the conceptual schema or logical level without causing application programs to be rewritten.

Modification is necessary to be done at the logical level whenever the logical structure of the database is altered.

For example: consider two users X & Y. Both are selecting the fields "EmployeeID" and "EmployeeName". If user Y adds a new column (e.g. salary) to his table, it will not effect the external view for user X, though the internal schema of the database has been changed for both users X & Y.