Uniqueness constraints in object models.\n\n"Specifying a key ensures that no two objects of a class share the same value for the key attribute(s)."

Introduction / Context:
Whether in relational databases or object databases, a key identifies records or objects uniquely. This question evaluates your understanding that declaring a key enforces uniqueness on the specified attributes for all instances of a class (or rows in a table).

Given Data / Assumptions:

• A class has one or more attributes designated as a key (candidate key, primary key, or unique constraint).
• The DBMS or object database enforces the uniqueness constraint.
• Objects/rows violating the constraint are rejected.

Concept / Approach:
A key is a minimal set of attributes whose values uniquely identify an entity. In relational systems, PRIMARY KEY or UNIQUE constraints enforce this. In object models, a declared key similarly prevents two distinct objects from sharing the same key value combination.

Step-by-Step Solution:

Verification / Alternative check:
Attempt an INSERT/CREATE with a duplicate key. The system raises a unique constraint violation, demonstrating enforcement.

Why Other Options Are Wrong:

• “Invalid” contradicts the definition of a key.
• Limiting to relational systems or numeric keys is incorrect; uniqueness is model-agnostic.
• Physical storage order has nothing to do with logical uniqueness.

Common Pitfalls:
Confusing keys with indexes (indexes speed lookups; keys define uniqueness rules); using surrogate keys and forgetting to enforce business keys when required.

Final Answer:
Valid (keys enforce uniqueness across objects)