What characterizes a PRIMARY KEY constraint?

• A. It allows NULL values
• B. It uniquely identifies each record in a database
• C. It can have duplicate values
• D. It is optional for each table

Answer: (B)

A PRIMARY KEY constraint is a fundamental aspect of relational database design that is used to uniquely identify each record within a table. The primary key serves as a unique identifier for rows in a database table, ensuring that no two records can have the same value for the specified key column(s). This uniqueness is crucial for maintaining the integrity and reliability of the data, allowing for efficient data retrieval and manipulation.

By enforcing uniqueness, the primary key enables relationships between tables, particularly in scenarios where foreign keys reference these identifiers. This helps to establish data integrity across the relational database system by preventing duplicate entries and ensuring that each record can be distinctly identified.

In contrast to the other options, a primary key cannot allow NULL values since that would contradict its purpose of uniquely identifying each record. Additionally, it is essential that the values in a primary key must be unique, ruling out the possibility of duplicate values. Lastly, while some constraints in a database can be optional, the primary key is a required constraint for each table if it is to maintain a proper relational structure. Each table should ideally have one primary key to uphold these characteristics and functionalities.