外文資料翻譯-----數(shù)據(jù)庫管理系統(tǒng)的介紹(參考版)
2024-11-17 11:56本頁面
【正文】 they utilize their database systems to gain an edge over their petitors. The Database Model A data model describes a way to structure and manipulate the data in a database. The structural part of the model specifies how data should be represented(such as tree, tables, and so on ).The manipulative part of the model specifies the operation with which to add, delete, display, maintain, print, search, select, sort and update the data. Hierarchical Model The first database management systems used a hierarchical modelthat isthey arranged records into a tree structure. Some records are root records and all others have unique parent records. The structure of the tree is designed to reflect the order in which the data will be used that is ,the record at the root of a tree will be accessed first, then records one level below the root ,and so on. The hierarchical model was developed because hierarchical relationships are monly found in business applications. As you have known, an anization char often describes a hierarchical relationship: top management is at the highest level, middle management at lower levels, and operational employees at the lowest levels. Note that within a strict hierarchy, each level of management may have many employees or levels of employees beneath it, but each employee has only one manager. Hierarchical data are characterized by this onetomany relationship among data. In the hierarchical approach, each relationship must be explicitly defined when the database is created. Each record in a hierarchical database can contain only one key field and only one relationship is allowed between any two fields. This can create a problem because data do not always conform to such a strict hierarchy. Relational Model A major breakthrough in database research occurred in 1970 when E. F. Codd proposed a fundamentally different approach to database management called relational model ,which uses a table as its data structure. The relational database is the most widely used database structure. Data is anized into related tables. Each table is made up of rows called and columns called fields. Each record contains fields of data about some specific item. For example, in a table containing information on employees, a record would contain fields of data such as a person’s last name ,first name ,and street address. Structured query language(SQL)is a query language for manipulating data in a relational database .It is nonprocedural or declarative, in which the user need only specify an Englishlike description that specifies the operation and the described record or bination of records. A query optimizer translates the description into a procedure to perform the database manipulation. Network Model The work model creates relationships among data through a linkedlist structure in which subordinate records can be linked to more than one parent record. This approach bines records with links, which are called pointers. The pointers are addresses that indicate the location of a record. With the work approach, a subordinate record can be linked to a key record and at the same ti
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