A primary key may be null.

More Questions from Logical Database Design

• The primary key of the new relation in a many-to-many relationship is a composite key comprised of the primary keys of each of the binary entities.
• An enterprise key is a primary key whose value is unique for a given relation.
• Normalization is a formal process for deciding which attributes should be grouped together in a relation.
• A relation is a two-dimensional table.
• A null value is assigned when no other value applies.
• A candidate key must uniquely identify each row.
• A composite attribute does not get mapped into a relation.
• A synonym is two or more attributes that have different names but the same meaning.
• If a relation is in third normal form, it does not need to be in second normal form.
• Well-structured relations encourage anomalies of data.

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