What is a primary key in a database?

A primary key in a database serves as a unique identifier for each record within a table. This means that no two records can have the same value for the primary key field, ensuring that each entry can be distinctly identified. This uniqueness is crucial for data integrity, ensuring that operations like updates and deletions can accurately target singular records without ambiguity.

The primary key also plays a significant role in establishing relationships between different tables in a relational database. For example, it can be referenced by foreign keys in other tables, facilitating the connection and organization of data across the database. By enforcing uniqueness and providing a way to reference rows, the primary key is foundational to maintaining a structured and reliable database environment.

In contrast, the other options do not accurately define a primary key: unlocking a database, improving performance with an index, or describing a type of data entry do not capture the essential role of a primary key in ensuring each record is uniquely identifiable within a database.