Tuesday, 2 October 2012

DIFFERENCE BETWEEN PHYSICAL AND LOGICAL DATA INDEPENDENCE


One of the biggest advantages of database is data independence. It means we can change the conceptual schema at one level without affecting the data at other level. It means we can change the structure of a database without affecting the data required by users and program. This feature was not available in file oriented approach. There are two types of data independence and they are:

      1. Physical data independence

      2. Logical data independence

      Data Independence The ability to modify schema definition in on level without affecting schema definition in the next higher level is called data independence. There are two levels of data independence:

      1. Physical data independence is the ability to modify the physical schema without causing application programs to be rewritten. Modifications at the physical level are occasionally necessary to improve performance. It means we change the physical storage/level without affecting the conceptual or external view of the data. The new changes are absorbed by mapping techniques.

      2. Logical data independence in the ability to modify the logical schema without causing application program to be rewritten. Modifications at the logical level are necessary whenever the logical structure of the database is altered (for example, when money-market accounts are added to banking system).

      Logical Data independence means if we add some new columns or remove some columns from table then the user view and programs should not changes. It is called the logical independence. For example: consider two users A & B. Both are selecting the empno and ename. If user B add a new column salary in his view/table then it will not effect the external view user; user A, but internal view of database has been changed for both users A & B. Now user A can also print the salary.

      User A’s External View

           

      (View before adding a new column)

   

      User B’s external view

         

      (View after adding a new column salary)

      It means if we change in view then program which use this view need not to be changed.

      Logical data independence is more difficult to achieve than is physical data independence, since application programs are heavily dependent on the logical structure of the data that they access.

      Logical data independence means we change the physical storage/level without effecting the conceptual or external view of the data. Mapping techniques absorbs the new changes.

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