What does a primary key in a database ensure?

A primary key in a database is a critical component in the design of relational databases. It is a unique identifier for each record in a table, ensuring that no two records can have the same value for that specific field. This uniqueness requirement is essential for maintaining the integrity of the data and allows for reliable retrieval and reference of records.

When a primary key is defined, it enforces a constraint that prevents duplicate entries in that column, thereby supporting the concept of entity integrity. This means that each record can be uniquely identified, which is crucial for operations such as updates, deletions, and relationships with other tables.

In this context, the other options do not accurately describe the function of a primary key:

• The option regarding indexing does not specifically capture the uniqueness aspect of the primary key, as indexing may occur with or without a primary key.

• Compatibility of data types is more related to the definition of columns in a table and their data types rather than the role of a primary key.

• Encryption of data pertains to data security rather than the structural functions of a primary key.

Thus, the correct understanding of a primary key revolves around its role in ensuring the uniqueness of records within a database table.