Jawaharlal Nehru Technological University Hyderabad MCA 2021 Database Management Systems Previous Question Paper (February - 2021 - R13)

Data abstraction in a DBMS hides complex storage details from users to simplify interaction. It is organized into three levels:

1. Physical Level: The lowest level, describing how data is actually stored on storage devices (e.g., blocks, indices, compression).
2. Logical Level: Describes what data is stored and the relationships among them. It defines the database schema, including entities, attributes, and constraints, without revealing physical implementation.
3. View Level: The highest level, where only part of the database is visible to specific users. It provides security and simplifies data access by hiding irrelevant details or sensitive information.

Class hierarchies, also known as Specialization and Generalization, represent an 'IS-A' relationship between a superclass and its subclasses. This allows attributes and relationships to be inherited from the superclass to the subclasses.

For example, consider an entity Employee. Employees can be categorized into subclasses such as Engineer, Manager, and Salesperson. The Employee entity acts as the superclass containing common attributes like EmployeeID, Name, and DateOfJoining. The subclasses inherit these attributes but also possess their own specific attributes:

• Engineer: Includes specific attributes like TechnicalSkillSet and ProjectID.
• Manager: Includes specific attributes like DepartmentManaged and BonusPercentage.

This hierarchy reduces redundancy and organizes the data model by grouping common properties at the highest possible level while maintaining unique characteristics at the lower levels.