In a relational database, a primary key is:

Correct option is A

A primary key in a relational database is a field or a combination of fields that uniquely identify each record in a table. It ensures that no duplicate values exist in the column(s) defined as the primary key and that every record is uniquely identifiable.

Important Key Points:

1. A primary key must contain unique values and cannot contain NULL values.
2. Each table in a relational database can have only one primary key.
3. Primary keys establish relationships between tables when combined with foreign keys.

Knowledge Booster:

• Field that stores numeric data: Incorrect, as primary keys can also store non-numeric data like strings or UUIDs.
• A foreign key from another table: Incorrect; a foreign key is a field in one table that references the primary key in another.
• An index used for faster data retrieval: While primary keys are automatically indexed, indexing is a separate feature.
• A data type constraint for character fields: Incorrect, as primary keys are not limited to character fields and are not a data type constraint.