What is the difference between a candidate key and a primary key?

The distinction between a candidate key and a primary key is foundational in database design. A candidate key serves as a potential unique identifier for records within a table. This means that it can uniquely identify each record based on its values, but it has not necessarily been chosen as the primary key.

In contrast, the primary key is specifically selected from the candidate keys and is used to uniquely identify records within a table. The primary key cannot contain NULL values, must have a unique value for every record, and serves as the main point of reference for relationships in a database.

Thus, while a candidate key can exist among several potential identifiers, the primary key is the one selected for actual usage in ensuring the uniqueness of records and maintaining the integrity of the database. This fundamental understanding reinforces why the specified answer accurately captures the relationship between the two.