Data Abstraction:

• Hiding certain details of how the data are stored and maintained. A major purpose of database system is to provide users with an “Abstract View” of the data. In DBMS there are 3 levels of data abstraction. The goal of the abstraction in the DBMS is to separate the users request and the physical storage of data in the database.

Levels of Abstraction:

1. Physical Level: The lowest Level of Abstraction describes “How” the data are actually stored.

2. Logical Level: This data Abstraction describes “What” data are to be stored in the database and what relationships exist among those data.

3. View Level: It is the highest level of data Abstracts that describes only part of entire database. Different users require different types of data elements from each database. For one physical and logical levels of the database there may be more than one view level.

The system may provide many views for the some database. 

Data Independence:

1. A very important advantage of using DBMS is that it offers Data Independence. The capacity to change the schema at one level without having change the other.

2. Logical Data Independence: The capacity to change the conceptual schema. The database can be expanded (adding records) or reduced (deleting records).

3. Physical Data Independence: The capacity to change the physical or internal Schema. Changes to the internal schema may be needed because some physical files had to be recognized.
   Ex: Creating additional access structures to improve the performance (Indexing).