What is a primary key?

• A. A field (or group of fields) that uniquely identifies an entity in a table
• B. A field that contains the most data
• C. The same as a foreign key
• D. A field used to store last names

Answer: (A)

A primary key is a field (or a group of fields) that uniquely identifies each row in a table. It must have a unique value for every record and cannot be null, which lets you reliably locate and reference specific records. A primary key can be a single column or a combination of columns (a composite key), and databases often auto-generate it (like an auto-incrementing number) to guarantee uniqueness. This differs from a foreign key, which links a row in one table to a row in another table and can have duplicates or nulls in many designs. It also isn’t simply a field used for storing data like last names, which may repeat and thus wouldn’t guarantee uniqueness.