1. A system for duplicating an update database in a computer during online processing, comprising: a first unit to create at least a copy database by logically copying contents in the update database, in sequence on a block-by-block basis in accordance with a specification of a desired extraction range, during online processing, regardless of whether the physical data structure of the update database is not the same as that of the copy database; a second unit to create selected journal data by recording an update data journal of the update database caused by transactions while copying the contents of the update database in a journal file, and by selecting data relating to parts of the copy database in which the copy has finished from the update data journal; and a third unit to create a duplicate database, from the copy database created by the first unit and the selected journal data created by the second unit.