专业英语李白萍版(wust期末考)第11章要点

1、Unit 11 DataBaseUnit 11 DataBasePassage A Database: A Better Way to Manage and Organize Data Passage B DBMS and Data Independence, Integrity, Security Passage C Database System and Three Data Models Unit 11 DataBasePassage A Database: A Better Way to Manage and Organize Data They recognize that the

2、information accumulated on sales trends, competitors products and services, employee skills, and production processes is a valuable resource that would be difficult if not impossible to replace. Unfortunately, in many cases this resource is located in different files in different departments through

3、out the organization, often known only to the individuals who work with their specific portion of the total information.Unit 11 DataBase There are problems with traditional data management. Many (of these problems) result from viewing applications independently. Most organizations prepare their payr

4、olls by computer because using a machine instead of a small army of clerks saves money. Since the responsibility for accessing the physical data rests with the database management system, the programmer can ignore the physical data structureUnit 11 DataBase Most database management systems allow the

5、 user to establish different levels of security over information in the database.Unit 11 DataBase(c) Integrated files. A key advantage of a database management system is its capability to integrate, or join together, data from more than one file for inquiry or reporting purposes.(d) Improved data se

6、curity. Most database management systems allow the user to establish different levels of security over information in the database. For example, a department manager may have read only privileges on certain payroll data; the manager could inquire about the data but not change it. The payroll supervi

7、sor would have full update privileges: the supervisor could not only inquire about the data but could also make changes. A non-management employee would probably have no access privileges to the payroll data and could neither inquire about nor change the data.3Unit 11 DataBase(e) Reduced development

8、 time. Because data is better organized in a database, development of programs that use this data is more efficient and takes less time. The need to create new files is reduced. Instead, new attributes are added to existing files.Unit 11 DataBaseNOTES1 By contrast, a file management system is softwa

9、re that allows the user to create, maintain, and access one file at a time.相反, 文件管理系统则是允许用户在某一时刻生成、 维护和访问一个文件的软件。 by contrast相反的。 at a time . 在某一时刻。Unit 11 DataBase2 Since the responsibility for accessing the physical data rests with the database management system, the programmer can ignore the phys

10、ical data structure.由于存取实际数据的责任由数据库系统承担，因此程序员就可以不考虑实际的数据结构。 rest with意思为“取决于，属于的权限”，它在由since引导的原因状语从句中作谓语。Unit 11 DataBase3 A non-management employee would probably have no access privileges to the payroll data and could neither inquire about nor change the data.而非管理型员工则可能没有访问工资数据的权利, 他们既不能查询也不能对数据进

11、行修改。 neither . nor . 既不也不。 have no access privileges to 对没有访问权利。Unit 11 DataBaseExercises 1. Fill in the blanks with the best choice.(1) A file is collection of related that is usually stored on a auxiliary storage device.a. records b. facts c. data d. word(2) A record is a collection of related .a.

12、 facts b. fields c. words d. bytes(3) A field, also called a data item or data element, is a .a. word b. record c. byte d. fact(4) file organization means that records are stored one after the other, normally in ascending or descending order, based on a value in each record called key.a. Direct b. S

13、equential c. Indexed d. RelativeUnit 11 DataBase(5) file organization stores the records in an indexed file in an ascending or descending sequence based on the value in the key field of the record.a. Indexed b. Sequential c. Direct d. Relative(6) When fast access to a particular record is required,

14、we should not use file organization. a. indexed b. direct c. sequential d. relative(7) Compared with the traditional data management, a database system can offer us .a. reduced data redundancy b. improved data integrityc. reduced development time d. all of aboveUnit 11 DataBase(8) Data redundancy an

15、d data dependency can be avoided by using .a. sequential files b. random files c. files d. a database(9) With access, records can be accessed in any order.a. direct b. record c. file d. sequential(10) Data occurs when the same data are stored in two or more files.a. redundancy b. dependency c. acces

16、s d. lossUnit 11 DataBase2. Answer the questions.(1) Describe sequential file organization.(2) What is an indexed file? Describe how the index is used to retrieve records from an indexed file.(3) Write a definition for a direct file.(4) Write a definition for the term database.(5) What are the advan

17、tages of a database management system over a file-oriented system?Unit 11 DataBasePassage B DBMS and Data Independence, Integrity, SecurityYou know that a database is a collection of logically related data elements that may be structured in various ways to meet the multiple processing and retrieval

18、needs of organizations and individuals. Theres nothing new about databasesearly ones were chiseled in stone, penned on scrolls, and written on index cards. But now databases are commonly recorded on magnetize-able media, and computer programs are required to perform the necessary storage and retriev

19、al operations.Unit 11 DataBaseYoull see in the following pages that 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 creating, accessing, and maintaining database records is called a d

20、atabase management system (DBMS). The programs in a DBMS package establish an interface between the database itself and users of the database.(These users may be applications programmers, managers and others with information needs, and various OS program.)Unit 11 DataBaseA DBMS can organize, process

21、, and present selected data elements from 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 reports.1 These questions might initially be vague and/or

22、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 who arent programmers. In a file-oriented system, use

23、rs 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 information. 2 The availability of a DBMS, however, offers users a much faster alternative communications path.Unit 11 DataBaseData

24、IndependenceAn important point about database systems is that the database should exist independently of any of the specific applications. Traditional data processing applications are data dependent. COBOL programs contain file descriptions and record descriptions that carefully describe the format

25、and characteristics of the data.Unit 11 DataBaseUsers should be able to change the structure of the database without affecting the applications that use it. For example, suppose that the requirements of your applications change. A simple example would be expanding ZIP codes from five digits to nine

26、digits. In a traditional approach using COBOL programs, each individual COBOL application program that used that particular field would have to be changed, recompiled, and retested. The programs would be unable to recognize or access a file that had been changed and contained a new data description;

27、 this, in turn, might cause disruption in processing unless the change were carefully planned.Unit 11 DataBaseMost database programs provide the ability to change the database structure by simply changing the ZIP code field and the data-entry form. In this case, data independence allows for minimal

28、disruption of current and existing applications. Users can continue to work and can even ignore the nine-digit code if they choose. Eventually, the file will be converted to the new nine-digit ZIP code, but the ease with which the changeover takes place emphasizes the importance of data independence

29、.3Unit 11 DataBaseData IntegrityData integrity refers to the accuracy, correctness, or validity of the data in the database. In a database system, data integrity means safeguarding the data against invalid alteration or destruction. In large on-line database system, data integrity becomes a more sev

30、ere problem and two additional complications arise. The first has to do with many users accessing the database concurrently. For example, if thousands of travel agents book the same seat on the same flight, the first agents booking will be lost. In such cases the technique of locking the record or f

31、ield provides the means for preventing one user from accessing a record while another user is updating the same record.Unit 11 DataBaseThe second complication relates to hardware, software, or human error during the course of processing and involves database transactions to keep the database in a co

32、nsistent state of integrity. A database transaction is a group of database modifications treated as a single unit. For example, an agent booking an airline reservation involves several database updates (i.e., adding the passengers name and address and updating the seats-available field), which compr

33、ise a single transaction. The database transaction is not considered to be completed until all updates have been completed; otherwise, none of the updates will be allowed to take place.Unit 11 DataBaseData SecurityData security refers to the protection of a database against unauthorized or illegal a

34、ccess or modification. This usually involves one or more levels of password protection that are specified in the data dictionary. For example, a high-level password might allow a user to read from, write to. and modify the database structure, whereas a low-level password might only allow a user to r

35、ead from the database.Often an audit trailthe recorded history of the modifications to a databasecan be used to identify where and when a database was tampered with and it can also be used to restore the file to its original condition.Unit 11 DataBaseNOTES1 This capability enables decision makers to

36、 search, probe, and query database contents in order to extract answers to nonrecurring and unplanned questions that arent available in regular reports.该功能使决策者可以搜索、试探和查询数据库中的内容，从而对在正式报告中没有的、不再出现的且无计划的问题作出回答。 这是一个复合句，基本结构为“enables decision makers to search, in order to”，到句末是目的状语，其中that引导的定语从句修饰questi

37、ons。Unit 11 DataBase2 In 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 information.在面向文件的系统中，需要特定信息的用户可以将他们的要求传送给程序员。该程序员在时间允许时，将编写一个或多个程序以提取数据和准备信息。 n

38、eeding special information是现在分词短语作定语，修饰users；从who到句末是定语从句，修饰programmer，其中when time permits是时间状语从句。Unit 11 DataBase3 Eventually, the file will be converted to the new nine-digit ZIP code, but the ease with which the changeover takes place emphasizes the importance of data independence.该文件最终将转换成新的9位ZI

39、P代码，如此容易地完成了修改更突出了数据独立性的重要性。 ease此处是对用户来讲，59位ZIP编码修改很容易，with which是带前置词的定语从句。Unit 11 DataBaseExercises 1. True/False.(1) A pointer is a data item in one record that identifies the storage location of another logically related record.( )(2) A data base is a collection of logically related data elemen

40、ts that are structured in various ways to meet the multiple processing and retrieval needs of organizations and individuals.( )(3) The programs in a DBMS package establish an interface between the database itself and the users of the database.( )Unit 11 DataBase(4) DBMS cannot structure and tie toge

41、ther the logically related data from several large files.( )(5) In a computer system, these records that are logically contiguous in one application must be physically stored together.( )(6) The physical location of the stored records in many computer-based information systems is invisible to users.

42、( )Unit 11 DataBase2. Match the following terms to the appropriate definition.(1) database(2) database management system (DBMS)(3) hierarchical approach(4) network approach(5) Pointer(6) relational approach(7) data integrity(8) data securityUnit 11 DataBasea. Storage of data in which records are rel

43、ated by a linked list of pointers.b. A collection of one or more files treated as a whole unit.c. Storage of data in which records are divided into an inverted tree based on master/subordinate field relationships.d. Software that organizes, manipulates, and retrieves data stored in a database.e. Sto

44、rage of data in which horizontal rows designate records and vertical columns designate fields.f. An identifier that indicates the location of a record.g. Protection of a database against access or modification without authorization.h. Protection of a database against invalid alteration or destructio

45、n.Unit 11 DataBasePassage C Database System and Three Data ModelsThe conception used for describing files and databases has varied substantially in the same organization.A database may be defined as a collection of interrelated data stored together with as little redundancy as possible to serve one

46、or more applications in an optimal fashion; the data are stored so that they are independent of programs which use the data; a common and controlled approach is used in adding new data and in modifying and retrieving existing data within the database. One system is said to contain a collection of da

47、tabases if they are entirely separate in structure.Unit 11 DataBaseA database may be designed for batch processing, real-time processing, or in-line processing. A database system involve application program, DBMS, and database.One of the most important characteristics of most databases is that they

48、will constantly need to change and grow. Easy restructuring of the database must be possible as new data types and new applications are added. The restructuring should be possible without having to rewrite the application program and in general should cause as little upheaval as possible. The ease w

49、ith which a database can be changed will have a major effect on the rate at which data-processing application can be developed in a corporation.Unit 11 DataBaseThe term data independence is often quoted as being one of the main attributes of a database. It implies that the data and the application p

50、rograms which use them are independent so that either may be changed without changing the other. When a single set of data items serves a variety of applications, different application programs perceive different relationships between the data items. To a large extent, database organization is conce

51、rned with the representation of relationship between data items and records as well as how and where the data are stored. A database used for many applications can have multiple interconnections between the data item about which we store information referred to as entities. Unit 11 DataBaseAn entity

52、 may be a tangible object or non-tangible. It has various properties which we may wish to record. It can describes the real world. The data item represents an attribute, and the attribute must be associated with the relevant entity. We design values to the attributes, one attribute has a special sig

53、nificance in that it identifies the entity.Unit 11 DataBaseAn attribute or set of attributes which the computer uses to identify a record or tuple is referred to as a key. The primary key is defined as that key used to uniquely identify one record or tuple. The entity identifier consisting of one or

54、 more attributes. The primary key is of great importance because it is used by the computer in locating the record or tuple by means of an index or addressing algorithm.Unit 11 DataBaseIf the function of a database were merely to store data, its organization would be simple. Most of the complexities

55、 arise from the fact that is must also show the relationships between the various items of data that are stored. It is different to describe the data in logical or physical.The logical database description is referred to as a schema.A schema is a chart of the types of data that one used. It gives th

56、e names of the entities and attributes, and specifics the relations between them. It is framwork into which the values of the data-items can be fitted.Unit 11 DataBaseWe must distinguish between a record type and a instance of the record. When we talk about a “personnel record”, this is really a rec

57、ord type. There are no data values associated with it.The term schema is used to mean an overall chart of all of the data-item types and record types stored in a database. The term subschema refers to an application programmers view of the data he uses. Many different sub-schemas can be derived from

58、 one schema.The schema and the subschema are both used by the database management system, the primary function of which is to serve the application programs by executing their data operations.Unit 11 DataBaseA DBMS will usually be handling multiple data calls concurrently. It must organize its syste

59、m buffers so that different data operations can be in process together. It provides a data definition language to specify the conceptual schema and most likely, some of the details regarding the implementation of the conceptual schema by the physical schema. The data definition language is a high-le

60、vel language, enabling one to describe the conceptual schema in terms of a “data model”.Unit 11 DataBaseThe choice of a data model is a difficult one, since it must be rich enough in structure to describe significant aspects of the real world, yet it must be possible to determine fairly automaticall