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DBMS Normalization

A concise guide to the normal forms, why they matter, and how to reason about them in a viva or interview.

Key concepts

  • Normalization — organizing columns and tables to reduce redundancy and avoid update, insert, and delete anomalies.
  • Functional dependency — X → Y means X uniquely determines Y.
  • Candidate key — a minimal set of attributes that identifies a row.

Important questions

  1. What anomalies does normalization prevent?
  2. State the requirement for 1NF, 2NF, 3NF, and BCNF.
  3. When is denormalization justified?

Short answers

Normal form Requirement
1NF Atomic values; no repeating groups
2NF 1NF and no partial dependency on part of a composite key
3NF 2NF and no transitive dependency on the key
BCNF For every dependency X → Y, X is a superkey

Revision mnemonic

The key, the whole key, and nothing but the key — 2NF removes partial dependencies, 3NF removes transitive ones.

Examples

Consider Enrollment(student_id, course_id, student_name, instructor).

  • student_id → student_name is a partial dependency (violates 2NF) because the key is (student_id, course_id).
  • Split into Student(student_id, student_name) and Enrollment(student_id, course_id, instructor).

Revision checklist

  • Identify candidate keys for a given relation
  • Spot a partial vs transitive dependency
  • Explain a trade-off where denormalization wins

References