Question

Can anybody explain how data independence in ensured in a relational database? What says that nothing will change for the user if the database structure changes?

For example, I have relation R (and have created an application which uses this relation) and the database admin decides to make a decomposition of R to R1 and R2. How is application inalterability ensured for the end user?

Answer 1

I asked myself exactly the same question during my Database class.

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.