In a database, what uniquely identifies each record?

In a database, what uniquely identifies each record?

• A. Foreign key
• B. Index
• C. Unique key
• D. Primary key

Answer: (D)

In relational databases, each row must have a single, reliable way to be distinguished from every other row, and this role is filled by the primary key. The primary key is a column (or set of columns) that uniquely identifies every record and cannot contain null values, so there is exactly one primary key per table. It also serves as the anchor for relationships because other tables reference it via foreign keys. Other options play different roles: a foreign key points to the primary key in another table and is about relationships, not guaranteeing uniqueness in the local table; an index speeds up lookups but does not by itself ensure uniqueness; a unique key ensures values are unique but can allow nulls in some databases and is not the designated identifier for records across the dataset.

In relational databases, each row must have a single, reliable way to be distinguished from every other row, and this role is filled by the primary key. The primary key is a column (or set of columns) that uniquely identifies every record and cannot contain null values, so there is exactly one primary key per table. It also serves as the anchor for relationships because other tables reference it via foreign keys.

Other options play different roles: a foreign key points to the primary key in another table and is about relationships, not guaranteeing uniqueness in the local table; an index speeds up lookups but does not by itself ensure uniqueness; a unique key ensures values are unique but can allow nulls in some databases and is not the designated identifier for records across the dataset.