Correct option is D
In the Relational Data Model, data is queried and manipulated through a well-defined set of operations (relational algebra/SQL).
These include selection, projection, union, difference, Cartesian product, joins, aggregation, etc.
Such operations take relations as input and produce relations as output, preserving closure—key to composable queries.
Objects, entities, and constraints describe what data is or the rules it must follow, not how we query/manipulate it.
Therefore, the mechanism for querying/manipulating data is the set of operations.
Such operations take relations as input and produce relations as output, preserving closure—key to composable queries.
Objects, entities, and constraints describe what data is or the rules it must follow, not how we query/manipulate it.
Therefore, the mechanism for querying/manipulating data is the set of operations.
Important Key Points
- Relational Algebra: Core operations—σ (select), π (project), ⋈ (join), ∪, −, ×.
- Closure Property: Output is a relation, enabling chaining of operations.
- SQL Mapping: SELECT-FROM-WHERE, JOIN, GROUP BY, HAVING, UNION, etc., implement these operations.
- Constraints vs Operations: Constraints (keys, FKs, checks) ensure integrity; they don’t query data.
- Entities/Objects: Conceptual/OO notions; in relational terms, data lives in relations (tables).
- Optimization: DBMS rewrites operation trees for efficient execution (cost-based optimizer).
Knowledge Booster
- Why not (a) Objects? Object orientation is not the RDM’s manipulation model; relations and operations are.
- Why not (b) Constraints? They enforce rules (e.g., uniqueness) but don’t perform queries.
- Why not (c) Entities? Entities belong to conceptual modeling (ER); in RDM, tables/relations are queried via operations.
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