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2024-12-12 04:21本頁面
【正文】 nts in order to extract answers to nonrecurring and unplanned questionsthat aren痶 available in regular reports. These questions might initially be vagueand/or poorly defined but people can “browse” through the database until they havethe needed information. In short the DBMS will “manage” the stored data items andassemble the needed items from the mon database in response to the queries ofthose who aren’t programmers. A database management system DBMS is posed of three major parts:1astorage subsystem that stores and retrieves data in files2 a modeling andmanipulation subsystem that provides the means with which to anize the data andto add delete maintain and update the data3and an interface between the DBMSand its users. Several major trends are emerging that enhance the value and usefulnessof database management systems Managers: who require more uptodata information to make effective decision Customers: who demand increasingly sophisticated information services and more current information about the status of their orders invoices and accounts. Users: who find that they can develop custom applications with database systems in a fraction of the time it takes to use traditional programming languages. Organizations : that discover information has a strategic value they utilize their database systems to gain an edge over their Database Model A data model describes a way to structure and manipulate the data in a structural part of the model specifies how data should be representedsuch as treetables and so on .The manipulative part of the model specifies the operation withwhich to add delete display maintain print search select sort and update the Model The first database management systems used a hierarchical modelthat istheyarranged records into a tree structure. Some records are root records and all othershave unique parent records. The structure of the tree is designed to reflect the order inwhich the data will be used that is the record at the root of a tree will be accessed firstthen records one level below the root and so on. The hierarchical model was developed because hierarchical relationships aremonly found in business applications. As you have known an anization charoften describes a hierarchical relationship: top management is at the highest levelmiddle management at lower levels and operational employees at the lowest that within a strict hierarchy each level of management may have manyemployees or levels of employees beneath it but each employee has only onemanager. Hierarchical data are characterized by this onetomany relationship amongdata. In the hierarchical approach each relationship must be explicitly defined whenthe data
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