单选题 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 (请作答此空) of a relation does not generally change.