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2024-10-02 17:12本頁(yè)面
【正文】 describe the conceptual schema in terms for a “data model”.The choice of a data model is a difficult one, since it must be such enough in structure to describe significant aspects of the real world, yet it must be possible to determine fairly automatically an efficient implementation of the conceptual conceptual schema by a physical schema it should be emphasized that while a dims might be used to build small databases many databases involve millions of bytes and an inefficient implementation can be disastrous.Logical schemas are defined as data, pedals with the underlying structure of particular database management systems superimposed on them at the present time there are three main underlying structures for database management systems these are:.relational .hierarchical .networkThe hierarchical and network structures have been used for dbms since the 1960’s . the relational structure was introduced in the early 1970’s.In the relational model twodimensional tables represent the entities and their relationships every table represents an entities are represented by mon columns containing values from a domain or range of possible values.The end user is presented with a simple data model his and her request and don not reflect any plexities due to systemoriented aspects a relational data model is what the user sees , but it is mot necessarily what will be implemented physically.The relational data model removes the details of storage structure and access strategy from the user interface the model providers a relatively higher degree of datato make use of this property of the relational data model however, the design of the relations must be plete and accurate.Although some dbms based on the relational data model are mercially available
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