In relational databases, which statement about keys is correct?

• A. A foreign key uniquely identifies a row in its own table.
• B. A primary key references a non-key column from another table.
• C. A foreign key can be null without any constraint.
• D. A primary key uniquely identifies a row in a table, and a foreign key references the primary key in another table to enforce referential integrity.

Answer: (D)

Understanding how keys enforce relationships in relational databases helps you see why this statement is correct. A primary key uniquely identifies each row in its own table, meaning no two rows share the same primary key value and it can’t be NULL. A foreign key is a column (or set of columns) in another table that references the primary key (or a candidate key) in the first table. This creates a link between the two tables and enforces referential integrity: non-null foreign key values must match an existing primary key value in the referenced table, so related records stay consistent across tables. The described arrangement—primary key for unique identification within a table, plus a foreign key that references the primary key in another table to enforce referential integrity—is exactly how relational databases maintain valid relationships. The other statements miss this interplay: a foreign key is not the unique identifier of its own row; primary keys don’t reference non-key columns in another table; and while foreign keys can be NULL in optional relationships, the constraint still exists to enforce integrity for non-null values.