Entity integrity rule in the relational model: Which statement correctly expresses the entity integrity constraint for primary keys?
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Ano primary key attribute may be null.
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Bno primary key can be composite.
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Cno primary key may be unique.
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Dno primary key may be equal to a value in a foreign key.
Answer
Correct Answer: no primary key attribute may be null.
Explanation
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:
Define PK attributes as NOT NULL.Enforce uniqueness on the PK (single or composite).Use FK constraints in child tables to reference the PK.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.