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1、扬州大学计算机专业毕业设计(外文翻译)外文资料Database Management SystemsA database (sometimes spelled data base） is also called an electronic database , referring to any collection of data， or information, that is specially organized for rapid search and retrieval by a computer。 Databases are structured to facilitate the
2、 storage, retrieval , modification， and deletion of data in conjunction with various dataprocessing operations 。Databases can be stored on magnetic disk or tape， optical disk， or some other secondary storage device。A database consists of a file or a set of files. The information in these files may b
3、e broken down into records， each of which consists of one or more fields。 Fields are the basic units of data storage , and each field typically contains information pertaining to one aspect or attribute of the entity described by the database 。 Using keywords and various sorting commands， users can
4、rapidly search ， rearrange， group， and select the fields in many records to retrieve or create reports on particular aggregate of data。Complex data relationships and linkages may be found in all but the simplest databases 。The system software package that handles the difficult tasks associated with
5、creating ,accessing, and maintaining database records is called a database management system(DBMS）.The programs in a DBMS package establish an interface between the database itself and the users of the database.。 （These users may be applications programmers， managers and others with information need
6、s, and various OS programs。）A DBMS can organize， process, and present selected data elements form the database. This capability enables decision makers to search, probe, and query database contents in order to extract answers to nonrecurring and unplanned questions that arent available in regular re
7、ports。 These questions might initially be vague and/or poorly defined ,but people can “browse through the database until they have the needed information. In short， the DBMS will “manage” the stored data items and assemble the needed items from the common database in response to the queries of those
8、 who arent programmers。A database management system （DBMS) is composed of three major parts:(1)a storage subsystem that stores and retrieves data in files;(2) a modeling and manipulation subsystem that provides the means with which to organize the data and to add ， delete， maintain， and update the d
9、ata;（3)and an interface between the DBMS and its users. Several major trends are emerging that enhance the value and usefulness of database management systems；Managers: who require more up-to-data information to make effective decisionCustomers: who demand increasingly sophisticated information serv
10、ices 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 strat
11、egic value； they utilize their database systems to gain an edge over their competitors.The Database ModelA 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 ma
12、nipulative 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 model-that isthey arranged records into a tree structure。 Some records are r
13、oot 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 bec
14、ause hierarchical relationships are commonly found in business applications。 As you have known， an organization 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 withi
15、n 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 one-tomany relationship among data。In the hierarchical approach， each relationship must be explicitly defined
16、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 ta
