A database instance (or extension) describes the content stored in a database at a given point in time. If the database has a fixed schema, every database instance must conform to it — that is, every tuple in the instance must respect the domains, key constraints, and functional dependencies defined by the schema.

Properties

• The instance changes as data is inserted, updated, or deleted. The schema typically remains fixed.
• At any point in time, the instance must satisfy all integrity constraints declared in the schema (e.g. every super key must uniquely identify tuples, no primary key value may be null).