The present invention provides a database system capable of dealing with dynamic changes in an event that vary continuously with time. A database system outputs a retrieval output from data in response to a user's retrieval request on the basis of a retrieval logic. The retrieval logic is allowed to evolve on the basis of a logical model. This logical model establishes a general relationship which leads results from conditions relating to a group of events that are dealt by a data group. Moreover, a database system optimizes a data structure on the basis of the logical model. The logical model can be transferred in a separated manner from the specific data contents of the data group. Therefore, the logical model can be exported to another database system of the same kind, and also imported from another data base system of the same kind.