Entity integrity rule in the relational model: Which statement correctly expresses the entity integrity constraint for primary keys?

Introduction / Context:
Entity integrity is a foundational rule in relational databases that ensures each row represents a real, uniquely identifiable entity instance. It specifies how primary keys must behave to guarantee unambiguous identification.

Given Data / Assumptions:

• Every table has a designated primary key.
• Primary keys are used as targets for foreign keys from other tables.
• Null represents the absence of a value and cannot identify a row.

Concept / Approach:

The entity integrity rule states that a primary key must be unique and contain no null values. Nulls would imply “unknown” or “not applicable,” which violates the identity requirement of the key. Composite primary keys are allowed; uniqueness is essential, not single-column status.

Step-by-Step Solution:

Verification / Alternative check:

Attempt to insert a row with a NULL in any PK component; the DBMS should reject it. Check that duplicate PK values are also rejected.

Why Other Options Are Wrong:

“No composite PK”: false—composite keys are valid.

“No PK may be unique”: false—the PK must be unique.

“PK may not equal FK values”: FKs commonly equal PK values; that is the point of referencing.

Common Pitfalls:

Misusing nullable surrogate keys; allowing NULLs in parts of composite keys; failing to index PKs for performance.

Final Answer:

no primary key attribute may be null.