What is a primary key in a relational database?

• A. Defines a table's schema
• B. Links to a primary key in another table
• C. Uniquely identifies a record in a table
• D. Stores large binary data

Answer: (C)

A primary key uniquely identifies a record in a table. In a relational database, every row must be distinct, and the primary key enforces this by ensuring its values are unique and not null. This lets you retrieve, update, or reference a specific row unambiguously. It can be a single column, like student_id, or a combination of columns forming a composite key. The primary key also enables relationships with other tables via foreign keys that point to it, linking related data. The option about defining a table’s schema describes the overall structure, not just the unique identity of rows; the idea of linking to a primary key in another table describes a foreign key, not the primary key itself. Storing large binary data relates to data storage types, not the key’s role. So the primary key is best described as the feature that uniquely identifies a record in a table.