JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD MCA 2023 DATABASE MANAGEMENT SYSTEMS Previous Question Paper (March - 2023 - R20)

Data models are conceptual tools used to describe data, data relationships, data semantics, and consistency constraints. They are broadly classified into the following categories:

• Conceptual Data Models: Provide a high-level view of data requirements, such as the Entity-Relationship (ER) model, which focuses on entities and their relationships.
• Logical Data Models: Describe how data is structured for implementation in a database. Examples include the Relational model, Hierarchical model, and Network model.
• Physical Data Models: Describe how data is stored in the secondary storage, focusing on access methods and file organizations.
• Object-Oriented Data Models: Represent data as objects, incorporating concepts like encapsulation, inheritance, and polymorphism.

A nested query (or subquery) is a SQL query embedded within another SQL query. The inner query is executed first, and its result is used by the outer query to filter or process data. Nested queries are typically used in the WHERE, FROM, or HAVING clauses.

For example, to find employees earning more than the average salary:

They can be classified as non-correlated subqueries, which execute independently, or correlated subqueries, which depend on the outer query for their values.