In relational databases, what is a foreign key best described as?

Introduction / Context:
Foreign keys are the backbone of referential integrity. They link child tables to parent tables, ensuring relationships remain consistent over time.

Given Data / Assumptions:

• We need the definition that characterizes a foreign key.
• Focus is on purpose and relational meaning.

Concept / Approach:
A foreign key is one or more columns in a child table whose values must match a candidate key (usually the primary key) in the parent table. This guarantees only existing parent rows can be referenced by the child.

Step-by-Step Solution:

Verification / Alternative check:
Any relational textbook or DBMS manual defines foreign keys this way; DDL syntax (FOREIGN KEY ... REFERENCES parent(col)) confirms it.

Why Other Options Are Wrong:
Data types / null status: These are column properties, not what a foreign key is.
All of the above: Incorrect since only one statement accurately defines a foreign key.

Common Pitfalls:
Confusing logical FK concept with physical enforcement; some systems allow foreign keys to be deferred or not declared, but the concept remains the same.

Final Answer:
a column that contains the primary key value of another (parent) table