Entity–Relationship (E–R) modeling: which of the following is NOT a basic element present in all versions of the E–R model?

Introduction / Context:
The Entity–Relationship (E–R) model captures the conceptual structure of data. It focuses on what data means, not on implementation details. Knowing its core elements is foundational for translating requirements into a robust schema.

Given Data / Assumptions:

• E–R diagrams consistently include entities, attributes, and relationships.
• Primary keys are a relational implementation device used when mapping to tables.
• The question asks which item is not a universal E–R element across variants of the model.

Concept / Approach:
Entities represent things of interest, attributes describe those things, and relationships show associations. Primary keys are part of relational schema design (logical/physical), though many notations show identifiers; the concept of a “primary key” specifically is relational rather than intrinsic to all E–R notations.

Step-by-Step Solution:

Verification / Alternative check:
Review Chen E–R and extended EER variations: identifiers may be shown, but the relational term “primary key” arises during mapping to relations.

Why Other Options Are Wrong:

• Entities: fundamental conceptual objects.
• Attributes: properties of entities and relationships.
• Relationships: associations between entities.

Common Pitfalls:
Equating identifiers shown in E–R with relational primary keys; the former are conceptual identifiers and may map to different key choices during implementation.

Final Answer:
Primary keys