Question

Explain the difference between physical and logical data independence.

Answer 1

Data Independence - The ability to change the schema at lower level (in ANSI/SPARC three level architecture) without affecting the schema at the next higher level is called data independence. It is about liberty.

Logical Data Independence - The ability to change the conceptual schema, i.e, the overall database view without affecting the next higher level external schema, i.e. end users’ view of the database is logical data independence.

For example, let us suppose that you would like to add an attribute to a table, to add a new table, to establish a relationship for a new table etc. For adding one or all of these there is no compulsion in modifying the existing end user views or the programs that give those views. Such liberty is logical data independence.

Physical Data Independence - The ability to modify the way database stored (data structures, file organization, etc. - internal view of a database) without affecting the next higher level conceptual schema is the physical data independence.

For example, one need not modify the structure of a database table to use different storage devices, or different file organization etc.