Understanding transitive dependency: Which option best describes a transitive dependency within a relation?

Difficulty: Easy

Correct Answer: A functional dependency between two or more nonkey attributes.

Explanation:


Introduction / Context:
Transitive dependencies create hidden redundancy by allowing a non-key attribute to determine another non-key attribute indirectly through the primary key. Recognizing and removing them is key to achieving Third Normal Form (3NF) or Boyce–Codd Normal Form (BCNF).



Given Data / Assumptions:

  • The relation is at least in 1NF.
  • We focus on dependencies among attributes.
  • Key attributes determine non-key attributes directly; the issue arises when non-key attributes determine other non-key attributes.


Concept / Approach:

A transitive dependency occurs when key → A and A → B, where A and B are non-key attributes. Thus, key → B holds transitively via A. This creates redundancy and potential anomalies, because changing A may require updating B everywhere A appears.



Step-by-Step Solution:

Identify key → non-key dependencies (normal and expected).Look for non-key → non-key dependencies (A → B).If key → A and A → B with A, B non-key, then B is transitively dependent on the key.


Verification / Alternative check:

Group by A and check whether B varies within those groups; if not, A determines B. Combined with key → A, that implies a transitive path key → B.



Why Other Options Are Wrong:

Dependencies among key attributes are not the target; keys are determinants by definition.

“In 1NF/2NF” describes normal forms, not the dependency type itself.



Common Pitfalls:

Confusing derived/calculated values with dependencies; mixing up partial and transitive dependencies in composite-key situations.



Final Answer:

A functional dependency between two or more nonkey attributes.

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