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2025-10-31 04:21 本頁面
【正文】 on 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 database is created. Each record in a hierarchical database can contain only onekey field and only one relationship is allowed between any two fields. This can createa problem because data do not always conform to such a strict Model A major breakthrough in database research occurred in 1970 when E. F. Coddproposed a fundamentally different approach to database management calledrelational model which uses a table as its data structure. The relational database is the most widely used database structure. Data isanized into related tables. Each table is made up of rows called and columns calledfields. Each record contains fields of data about some specific item. For example in atable containing information on employees a record would contain fields of data suchas a person’s last name first name and street address. Structured query languageSQLis a query language for manipulating data in arelational database .It is nonprocedural or declarative in which the user need onlyspecify an Englishlike descrip
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