Friday, 12 May 2017

Logical data independence in DBMS

Logical data independence in database management system, Define logical data independence, logical data independence example


Logical Data Independence


The ability to change the logical schema (conceptual schema) without affecting the next higher level external schema (view level) is called the logical data independence.
That is, if one would like to add an attribute, or to add constraints on the data, he/she does not need to change the program that already exists which accesses the database.
Example:

  • Logical level deals with the overall structure of a database with constraints, relationships etc.
  • External schema (view level) is the end user view of a database. It usually refers to the user interfaces or application programs.
Let us assume a table STUDENT with the following schema;
STUDENT(Reg_no, SName, DOB, Phone)
This is conceptual (logical) schema.
Also, assume that there are two views defined on STUDENT. The application program shows only students’ register number and name to the user as per the definition of view 1. Another program shows only register number of students and their phones to the end user as per the definition of view 2. Now, if you would like to add an attribute Address with STUDENT table, then your table’s conceptual schema looks like as follows;
STUDENT(Reg_no, SName, DOB, Phone, Address)
Do I need to change my views’ definition as I have added new attribute? Certainly not. This immunity of the view level from the changes of conceptual level is called logical data independence.

Conceptual level
View level
View 1
View 2
Before change of logical schema
STUDENT(Reg_no, SName, DOB, Phone)
Reg_no, SName
Reg_no, Phone
After change of the logical schema
STUDENT(Reg_no, SName, DOB, Phone, Address)
Reg_no, SName
Reg_no, Phone

In this table, views defined on table student have not changed even after the changes have occurred in the base table.

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