Question

Why do we need different levels of abstraction in DBMS, explain with examples?

Answer 1

Data Abstraction is a process of hiding irrelevant details from the end user. The data abstraction provides us with a different view and help in achieving data independence and to enhance the security of data. Database systems consist of complicated data structures and relations. To make it easier for users to access the data, these complications are kept hidden, and only the relevant part of the database is made accessible to the users through data abstraction. There are three levels of data abstraction.

1. View Level/External level :  This is the highest level of abstraction. At this level, user just interact with system with the help of GUI and enter the details at the screen, they are not aware of how the data is stored and what data is stored .This level tells the application about how the data should be shown to the user. For Example: if we have a login-id and password in a university system, a student can view his/her marks, attendance, fee structure, semester details etc. But the faculty of the university will have a different view. He/she can have options like salary, edit marks of a student, enter attendance of the students, etc. So, both the student and the faculty have a different view. By doing so, the security of the system also increases.
2. Logical Level :   This level is also known as the conceptual level. At this level, we can see what data is stored in the database without knowing the implementation details such as the data structures and tree implementations. This level also tells us about the relations between the different fields and database tables. This level also stores the relationship among the data entities in relatively simple structures. The database administrators have full access to this level of data. For Example: in an university database system, the relationship between an entity ‘Professor’ and another entity ‘Student’, can be one-to-many. The fields describing the entity student would be the same as that describing a general entity ‘Person’ in along with fields such as subjects, marks, rank, etc.Any changes to this level should not affect the external or the physical level.
3. Physical Level/Internal Level : This is the lowest level of data abstraction. It tells us how the data is actually stored in a DB. The Database Administrators(DBA) decide that which data should be kept at which particular disk drive, how the data has to be fragmented, where it has to be stored etc. They decide if the data has to be centralized or distributed. Implementation details such as indexing methods like B+ trees or hashing and access methods such as sequential or random access which we are used those are described in this level.