In relational databases, what is the difference between a primary key and a foreign key?

• A. A primary key can be NULL; a foreign key does not reference another table.
• B. A primary key is only for indexing; a foreign key has no relation to other tables.
• C. A primary key uniquely identifies a row in its own table, while a foreign key references a primary key in another table to enforce relationships.
• D. A primary key is used to enforce referential integrity; a foreign key uniquely identifies a row in its table.

Answer: (C)

The main idea is that a primary key uniquely identifies each row in its own table, while a foreign key creates a link to a row in another table by referencing that table’s primary key. This pairing is what establishes and enforces the relationships between tables.

Think of a Customers table with customer_id as its primary key. Each customer has a unique id. Now, an Orders table can include a column called customer_id that serves as a foreign key referencing Customers(customer_id). This means every order points to a valid customer, preserving referential integrity. The primary key guarantees each row in its table is unique, and the foreign key ensures relationships across tables are consistent.

Why the other ideas don’t fit: a primary key is not allowed to be NULL and its job isn’t just indexing; it uniquely identifies rows. A foreign key does reference a row in another table to maintain the relationship. And the primary key isn’t what enforces referential integrity—that role belongs to the foreign key, while the primary key ensures uniqueness within its own table.