计算机外文翻译外文文献英文文献数据库系统

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1、外文资料原文 Database Systems1. Introduction to Database SystemToday, more than at any previous time, the success of an organization depends on its ability to acquire accurate and timely data about its operation, to manage this data effectively, and to use it to analyze and guide its activities. Phrases s

2、uch as the information superhighway have become ubiquitous, and information processing is a rapidly growing multibillion dollar industry .The amount of information available to us is literally exploding, and the value of data as an organizational asset is being widely recognized. This paradox drives

3、 the need for increasingly powerful and flexible data management systems . A database is a collection of data , typically describing the activities of one or more related organizations . For example , a university database might contain information about the following . Entities such as students , f

4、aculty , courses , and classrooms . Relationships between entities , such as students enrollment in courses , faculty teaching courses , and the use of rooms for courses .A database management system , or DBMS , is software designed to assist in maintaining and utilizing large collections of data ,

5、and the need for such systems , as well as their use , is growing rapidly . The alternative to using a DBMS is to use ad hoc approaches that do not carry over from one application to another , for example , to store the data in files and write application-specific code to manage it . The area of dat

6、abase management systems is a microcosm of computer science in general . The issues addressed and the techniques used span a wide spectrum , including languages , object-orientation and other programming paradigms , compilation , operating systems concurrent programming , data structures , algorithm

7、s ,theory , parallel and distributed systems , user interfaces , expert systems and artificial intelligence , statistical techniques , and dynamic programming . Database management continues to gain importance as more and more data is brought on-line, and made ever more accessible through computer n

8、etworking. Today the field is being driven by exciting visions such as multimedia databases, interactive video, digital libraries, a host of scientific projects such as the human genome mapping effort and NASAs Earth Observation System project, and the desire of companies to consolidate their decisi

9、on-making processes and mine their data repositories for useful information about their business . Commercially , database management systems represent one of the largest and most vigorous market segments . Thus the study of database systems could prove to be richly rewarding in more ways than one .

10、2. Database consistsA database consists of a file or a set of files. The information in these files may be 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 att

11、ribute of the entity described by the database. Using keywords and various sorting commands, users can rapidly search, rearrange, group, and select the fields in many records to retrieve or create reports on particular aggregates of data.Database records and files must be organized to allow retrieva

12、l of the information. Early systems were arranged sequentially (i.e., alphabetically, numerically, or chronologically); the development of direct-access storage devices made possible random access to data via indexes. Queries are the main way users retrieve database information. Typically, the user

13、provides a string of characters, and the computer searches the database for a corresponding sequence and provides the source materials in which those characters appear. A user can request, for example, all records in which the content of the field for a persons last name is the word Smith.In flat da

14、tabases , records are organized according to a simple list of entities; many simple databases for personal computers are flat in structure. The records in hierarchical databases are organized in a treelike structure, with each level of records branching off into a set of smaller categories. Unlike h

15、ierarchical databases, which provide single links between sets of records at different levels, network databases create multiple linkages between sets by placing links, or pointers, to one set of records in another; the speed and versatility of network databases have led to their wide use in busines

16、s.Relational databases are used where associations among files or records cannot be expressed by links; a simple flat list becomes one table, or “relation, and multiple relations can be mathematically associated to yield desired information. Object-oriented databases store and manipulate more comple

17、x data structures, called “objects, which are organized into hierarchical classes that may inherit properties from classes higher in the chain; this database structure is the most flexible and adaptable.3. Structure of the Relational databaseThe relational model is the basis for any relational datab

18、ase management system (RDBMS).A relational model has three core components: a collection of objects or relations, operators that act on the objects or relations, and data integrity methods. In other words, it has a place to store the data, a way to create and retrieve the data, and a way to make sur

19、e that the data is logically consistent.A relational database uses relations, or two-dimensional tables, to store the information needed to support a business.3.1.Tables, Row, and ColumnsA table in a relational database, alternatively known as a relation, is a two-dimensional structure used to hold

20、related information. A database consists of one or more related tables.Note: Dont confuse a relation with relationships. A relation is essentially a table, and a relationship is a way to correlate, join, or associate two tables.A row in a table is a collection or instance of one thing, such as one e

21、mployee or one line item on an invoice. A column contains all the information of a single type, and the piece of data at the intersection of a row and a column, a field, is the smallest piece of information that can be retrieved with the databases query language. For example, a table with informatio

22、n about employees might have a column called LAST_NAME that contains all of the employees last names. Data is retrieved from a table by filtering on both the row and the column.3.2.Primary Keys, Data types, and Foreign KeysRelation: A two-dimensional structure used to hold related information, also

23、known as a table.Row: A group of one or more data elements in a database table that describes a person, place, or thing.Column: The component of a database table that contains all of the data of the same name and type across all rows.Primary Key: A column (or columns) in a table that makes the row i

24、n the table distinguishable from every other row in the same table.Data types: numeric values, character or alphabetic values, and date values.A foreign key enforces the concept of referential integrity in a relational database.Foreign Key: A column (or columns) in a table that draws its values from

25、 a primary or unique key column in another table. A foreign key assists in ensuring the data integrity of a table. Referential Integrity A method employed by a relational database system that enforces one-to-many relationships between tables.3.3.Data ModelingIn this process, the developer conceptual

26、izes and documents all the tables for the database. One of the common methods for modeling a database is called ERA, which stands for entities, relationships, and attributes. The database designer uses an application that can maintain entities, their attributes, and their relationships. In general,

27、an entity corresponds to a table in the database, and the attributes of the entity correspond to columns of the table.Data Modeling: A process of defining the entities, attributes, and relationships between the entities in preparation for creating the physical database.The data-modeling process invo

28、lves defining the entities, defining the relationships between those entities, and then defining the attributes for each of the entities. Once a cycle is complete, it is repeated as many times as necessary to ensure that the designer is capturing what is important enough to go into the database. Let

29、s take a closer look at each step in the data-modeling process.3.4. Defining the EntitiesFirst, the designer identifies all of the entities within the scope of the database application.The entities are the persons, places, or things that are important to the organization and need to be tracked in th

30、e database. Entities will most likely translate neatly to database tables.3.5. Defining the Relationships Between EntitiesOnce the entities are defined, the designer can proceed with defining how each of the entities is related. Often, the designer will pair each entity with every other entity and a

31、sk, Is there a relationship between these two entities? Some relationships are obvious; some are not.In the widget company database, there is most likely a relationship between EMP and DEPT, but depending on the business rules, it is unlikely that the DEPT and SALGRADE entities are related. If the b

32、usiness rules were to restrict certain salary grades to certain departments, there would most likely be a new entity that defines the relationship between salary grades and departments. This entity would be known as an associative or intersection table and would contain the valid combinations of sal

33、ary grades and departments.Associative Table: A database table that stores the valid combinations of rows from two other tables and usually enforces a business rule. An associative table resolves a many-to-many relationship.In general, there are three types of relationships in a relational database:

34、One-to-many the most common type of relationship is one-to-many. This means that for each occurrence in a given entity, the parent entity, there may be one or more occurrences in a second entity, the child entity, to which it is related. For example, in the widget company database, the DEPT entity i

35、s a parent entity, and for each department, there could be one or more employees associated with that department. The relationship between DEPT and EMP is one-to-many.One-to-one In a one-to-one relationship, a row in a table is related to only one or none of the rows in a second table. This relation

36、ship type is often used for sub typing . For example, an EMPLOYEE table may hold the information common to all employees, while the FULLTIME, PARTTIME, and CONTRACTOR tables hold information unique to full-time employees, part-time employees, and contractors, respectively. These entities would be co

37、nsidered subtypes of an EMPLOYEE and maintain a one-to-one relationship with the EMPLOYEE table. These relationships are not as common as one-to-many relationships, because if one entity has an occurrence for a corresponding row in another entity, in most cases, the attributes from both entities sho

38、uld be in a single entity.Many-to-many in a many-to-many relationship, one row of a table may be related to many rows of another table, and vice versa. Usually, when this relationship is implemented in the database, a third entity is defined as an intersection table to contain the associations betwe

39、en the two entities in the relationship. For example, in a database used for school class enrollment, the STUDENT table has a many-to-many relationship with the CLASS tableone student may take one or more classes, and a given class may have one or more students. The intersection table STUDENT_CLASS

40、would contain the combinations of STUDENT and CLASS to track which students are in which classes.4.DBMS IntroductionThe term database is often to describe a collect: on of related files that is organized into an inte-.7ated structure that provides different people variances to the same data. For exa

41、mple, the department motor vehicles has a data base of all the informal：on drivers licenses in all counties and cities in state. Some of the information in the files (e.g. past criminal record, convictions，and the like) may be made available to police departments, other information (e.g. a balance d

42、ue on a speeding ticket) maybe made available to licensing bureaus, and still other information (e.g. number and type of convictions)may go to insurance companies A Data Base Management System (DBMS) is an extremely complicated set of software programs that controls the organization, storage and ret

43、rieval of data(felids, records and files)in a Database. It also controls the security and integrity of the databasesThe DBMS accepts requests for data from the application program and instructs the operating system to transfer the appropriate data. Tasks that would be time-consuming to accomplish ma

44、nually are more practical the aid of the computer. In principle, a database in a computer is not different from a database recorded on paper and filed in cabinetsBut the computer does the tedious work of maintaining and accessing a database, and does it fast. A computerized database that can do all

45、this is known as a database management system, or DBMS for short. There are a number of ways to store information in a computer, but not all of these are true database management systems. When a DBMS is used, information systems can be changed much more easily as the organizations information requir

46、ements change. New categories of data can be added to the database without disruption to the existing system.4.1. DBMS(Database Management System)DBMS is able to access and retrieve data from nonkey record fields. That is, the DBMS is able to structure and tie together the logically related data fro

47、m several large files.4.2. Logical Structures Identifying these logical relationships is a job of the data administrator. A data definition language is used for this purpose. The DBMS may then employ one of the following logical structuring techniques during storage access, and retrieval operations:

48、1.List structures. In this logical approach, records are linked together by the use of pointers. A pointer is a data item in one record that identifies the storage location of another logically related record. Records in a customer master file, for example, will contain the name and address of each

49、customer and each record in this file is identified by an account number. During an accounting period, a customer may buy a number of items on different days. Thus, the company may maintain an invoice file to reflect these transactions.A list structure could be used in this situation to show the unp

50、aid invoices at any given time. Each record in the customer file would contain a field that would point to the record location of the first invoice for that customer in the invoice file. This invoice record, in turn, would be linked to later invoices for the customer. The last invoice in the chain w

51、ould be identified by the use of a special character as a pointer.2.Hierarchical (tree) structures. In this logical approach, data units are structured in multiple levels that graphically resemble an upside down tree with the root at the top and the branches formed below. Theres a superior-subordina

52、te relationship in a hierarchical (tree) structure. Below the single-root data component are subordinate elements or nodes, each of which, in turn, own one or more other elements (or none). Each element or branch in this structure below the root has only a single owner. Thus, a customer owns an invo

53、ice, and the invoice has subordinate items. The branches in a tree structure are not connected.3.Network Structures. Unlike the tree approach, which does not permit the connection of branches, the network structure permits the connection of the nodes in a multidirectional manner. Thus, each node may

54、 have several owners and may, in turn, own any number of other data units. Data management software permits the extraction of the needed information from such a structure by beginning with any record in a file. Database Management SystemsDBMSAnd Management Information SystemsMISYou know that a datab

55、ase is a collection of logically related data elements that may be structured in various ways to meet the multiple processing and retrieval needs of organizations and individualsTheres nothing new about databasesearly ones were chiseled in stone，penned on scrolls，and written on index cardsBut now da

56、tabases are commonly recorded on magnetizable media，and computer programs are required to perform the necessary storage and retrieval operationsYoull see in the following pages that complex data relationships and linkages may be found in all but the simplest databasesThe system software package that

57、 handles the difficult tasks associated with creating，accessing，and maintaining database records is called a database management systemDBMSThe programs in a DBMS package establish an interface between the database itself and the users of the databaseThese users may be applications programmers，manage

58、rs and others with information needs，and various OS programsA DBMS can organize，process，and present selected data elements from the databaseThis capability enables decision makers to search，probe，and query database contents in order to extract answers to nonrecurring and unplanned questions that are

59、nt available in regular reportsThese questions might initially be vague and / or poorly defined，but people can“browsethrough the database until they have the needed informationIn short，the DBMS will“managethe stored data items and assemble the needed items from the common database in response to the

60、 queries of those who arent programmersIn a file-oriented system，users needing special information may communicate their needs to a programmer，who，when time permits，will write one or more programs to extract the data and prepare the information4The availability of a DBMS，however，offers users a much

61、faster alternative communications path.The management information systemMISconcept has been defined in dozens of waysSince one organizations model of an MIS is likely to differ from that of another，its not surprising that their MIS definitions would also vary in scope and breadthFor our purposes，an

62、MIS can be defined as a network of computer-based data processing procedures developed in an organization and integrated as necessary with manual and other procedures for the purpose of providing timely and effective information to support decision making and other necessary management functionsMIS

63、has different models ， In addition to what might be termed the horizontal management structure ，an organization is also divided vertically into different specialties and functions which require separate information flowsCombining the horizontal managerial levels with the vertical specialties produce

64、s the complex organizational structure Underlying this structure is a database consisting，ideally，of internally and externally produced data relating to past，present，and predicted future events。The formidable task of the MIS designer is to develop the information flow needed to support decision maki

65、ngGenerally speaking，much of the information needed by managers who occupy different levels and who have different responsibilities is obtained from a collection of existing information systemsor subsystemsThese systems may be tied together very closely in an MISMore often，however，they are more loosely coupled外文翻译中文数据库系统1. 数据库系统引论今天，也就是比以往任何时候都更加明显，一个组织的成功取决于它准确而及时地获取关于它运营数据的能力及有效管理这些数据的能力，并用它来分析和指导其活动。一些短语诸如“超级公路信息已随处可见，信息处理成为一个迅速增长为每年几十亿美元的产业。我们今天能得到的信息实在是爆炸性的，而数据作为一个组织资产的价值正得到广泛的认可。这种议论推动了对强大及灵活的数据库管理系统的不断增长的需要。数据库是一组数据的集合，它可典型地描绘一个或多个相关组织的活动。举例来说，一个大