The table enforces that each primary key value is unique so no two rows share the same key.

A primary key is a table constraint that uniquely identifies each row and it prevents duplicate values for the key columns. Database systems enforce this by creating a unique constraint or index and they typically also require primary key columns to be non nullable. The uniqueness requirement is what makes the primary key suitable for referencing rows from other tables and for maintaining referential integrity.

Every table is required to declare exactly one primary key is incorrect because SQL does not force every table to have a primary key. Many tables are created without a primary key and it is valid to design tables without one when appropriate.

A relational table can hold multiple primary keys is incorrect because a table may have only a single primary key constraint. You can define multiple unique constraints but those are not primary keys. A primary key can span multiple columns when composite keys are used but it is still a single primary key.

Defining a primary key forces all rows in a partition to share the same key value is incorrect because a primary key enforces uniqueness not sameness. Partitioning and primary keys are separate concepts. The partition key or a clustering column may determine how rows are grouped but they do not change the uniqueness requirement of the primary key.

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