Which statement about natural keys is true?

• A. Natural keys are always stable and never change.
• B. Natural keys are ideal when they are numeric.
• C. Natural keys can change or be composite, causing issues; surrogate keys are often preferred.
• D. Natural keys are always single-column numeric identifiers.

Answer: (C)

In database design, a natural key is a key formed from real-world data that has business meaning. These keys can be unstable because the underlying data can change or be updated, and they can also be composite when no single attribute uniquely identifies a row. That instability or complexity makes relying on natural keys as primary keys problematic, since changes would require updates across many related records and multi-column keys can complicate relationships and indexing. Because of this, surrogate keys—artificial, stable identifiers such as auto-increment numbers or GUIDs—are often preferred for primary keys, while the natural attributes are kept with unique constraints to preserve business meaning without tying them to the primary key.

Natural keys are not guaranteed to be stable; data like codes, emails, or other identifiers can change. They’re not inherently ideal when numeric, since numerics don’t address the stability or uniqueness issues and can still pose maintenance challenges. And natural keys aren’t restricted to a single numeric column—they can be non-numeric or require multiple columns to ensure uniqueness, which is why the statement emphasizing stability or single numeric identity isn’t accurate.