data independence in relational model

• According Codd's 12 rules, there are two kinds of data independence:

Physical Data Independence requires that changes at the physical level (like data structures) have no impact in the applications that consume the database. For example, let's say you decide to stop using a Hash Index in your table and decide to use a B-Tree Index instead: Your application that executes queries against this table doesn't have to change at all.

Logical Data Independence states that changes at the logical level (tables, columns, rows) will have no impact in the applications that access the database. As you already noticed, this feature is harder to implement that Physical Data Independence but there are still cases when this feature works. For example, if you add Tables, Columns or Rows to your current scheme the already working queries aren't affected at all.
http://stackoverflow.com/questions/10861501/data-independence-in-relational-database

• Logical Data Independence

Logical data independence is a kind of mechanism, which liberalizes itself from actual data stored on the disk. If we do some changes on table format it should not change the data residing on disk.
Physical Data Independence
All schemas are logical and actual data is stored in bit format on the disk. Physical data independence is the power to change the physical data without impacting the schema or logical data.
For example, in case we want to change or upgrade the storage system itself, that is, using SSD instead of Hard-disks should not have any impact on logical data or schemas.
http://www.tutorialspoint.com/dbms/dbms_data_independence.htm