What is a primary key in a relational database?

A primary key serves as a unique identifier for each record within a table in a relational database. This uniqueness ensures that no two rows in a table can have the same primary key value, allowing for precise identification and retrieval of records. The primary key constraint is fundamental for maintaining data integrity and is essential in establishing relationships between different tables in the database.

Primary keys can be composed of a single column or a combination of columns (composite key) that together uniquely identify a record. When a primary key is properly implemented, it enables efficient data retrieval operations and helps prevent duplicate entries, thus supporting the overall structure and consistency of the database.

In contrast, the other choices describe different concepts that do not define the primary key. For instance, methods for data encryption pertain to data security and confidentiality, not record identification. References to foreign tables relate to foreign keys, which are used to establish relationships between tables rather than uniquely identifying records. Lastly, data types that are used in database columns define the kind of data that can be stored in those columns but do not relate to the role of defining unique identifiers for records.