What is a primary key in a database?

A primary key is indeed a unique identifier for a record in a database. Its primary function is to ensure that each entry in a database table can be uniquely identified, which is crucial for maintaining data integrity and enabling efficient data retrieval. By designating a column (or a set of columns) as the primary key, the database ensures that no two records can have the same value for that key. This feature prevents duplicate entries and helps in establishing relationships between different tables when used alongside foreign keys.

For example, in a table of customer records, a primary key such as CustomerID ensures that each customer can be uniquely identified, making it possible to reference that customer from other tables in the database, such as orders or transactions.

The other options do not accurately define a primary key. A foreign key is related to primary keys in that it references a primary key in another table, but it is not a unique identifier for a record within its own table. Additionally, not every field used in indexing qualifies as a primary key; indexes can be created on non-unique fields as well. Lastly, a temporary identifier does not serve the purpose of uniquely identifying records over time, as primary keys are meant to be consistent and permanent identifiers for records within a database.