When we talk about a database,we must differentiate between the database（71）,which is the logical design of the database,and the database（72）,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（73）and their corresponding domains.The concept of a relation instance corresponds to the programming-language notion of a value of a（74）.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（75）оf a relation does not generally change.
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