Tuesday, February 24, 2015

Codd's Twelve Rules - Rule 9 - Logical Data Independence

Codd's Twelve Rules - Rule 9 - Logical Data Independence

Rule 9
Logical Data Independence
“The ability to change the conceptual (logic) schema without having to change the next higher level external schema or application programs”.
“Insulation of application programs and terminal users from the negative effects of information-preserving changes of the logical database schema”.
1. The addition or removal of new entities, attributes, or relationships to the conceptual schema should be possible without having to change existing external schemas or having to rewrite existing application programs.
2. Logical Data independence means if we add some new columns or remove some columns from table then the user view and programs should not change.
3. The term "Logical Data Independence" refer to the ability to present the stored information in different ways to different users. The way you store the data and the way you present it to a particular user is independent. Different users perceive the same data differently.
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. It means if we change in view then program which use this view need not to be changed.
For a table with the schema Employee(Eno, Ename, Street, City, Salary), a view with the attributes (Eno, Ename) will not be affected if any other attributes of Employee is altered.
More examples/illustrations can be tried in the following links;

Logical Data Independence is more difficult to achieve when compared to Physical Data Independence since application programs are heavily dependent on the logical structure of the data that they access.
Some DBMS that fulfills this property
Almost all Relational DBMSs support this property with certain limits (say for example if columns or tables removed).
Together, Rule 8 Physical Data Independence and Rule 9 Logical Data Independence, specify that specific access or storage techniques used by the RDBMS—and even changes to the structure of the tables in the database—shouldn’t affect the user’s ability to work with the data.

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