nonkey

Nonkey is a term sometimes used in database theory and data modeling to refer to attributes that are not keys in a relation. Because terminology varies across authors, nonkey is not a formal standard and its precise meaning can differ by context. In many discussions it is used informally to describe attributes that are not candidate keys or not prime attributes.

In relational theory, a candidate key is a minimal set of attributes that uniquely identifies each tuple in a relation. Attributes that participate in at least one candidate key are called prime attributes. Attributes that are not part of any candidate key are non-prime attributes. Some writers use nonkey to describe non-prime attributes, while others use nonkey more loosely to mean any attribute that is not a primary key, which can lead to ambiguity.

Example: consider a relation R(A, B, C, D) with candidate keys {A, B} and {C}. In this case, A, B, and C are prime attributes, since each appears in a candidate key. D is a non-prime (non-key) attribute. If one adopts a looser usage where nonkey simply means “not a candidate key,” then D would be considered non-key, while A, B, and C remain prime. The exact interpretation depends on the source and the context of the discussion.

Uses and nuances: the distinction between key and nonkey attributes matters in normalization, where non-key attributes should depend on keys and transitive dependencies among non-key attributes are addressed to achieve normal forms such as 3NF or BCNF. In practice, many designers prefer terms like prime and non-prime attributes to avoid the ambiguity surrounding nonkey.