When we talk about a database, we must differentiate between the database （  ）,which is the logical design of the database, and the database （  ）, which is a snapshot of the data in the database at a given instant in time. The concept of a relation corresponds to the programming-language notion of a variable, while the concept of a relation schema corresponds to the programming-language notion of type definition. In general, a relation schema consists of a list of（  ）and their corresponding domains. The concept of a relation instance corresponds to the programming-language notion of a value of a （  ） . The value of a given variable may change with time; similarly the contents of a relation instance may change with time as the relation is updated. In contrast, the （  ） оf a relation does not generally change.