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1、希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ 软考计算机英语数据库DBMS Introduction The 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 example,the department motor vehicles has a data

2、 base of all the informal：on drivers licenses in all counties and cities in state.Some of the information in the files(e.g.a past criminal record,convictions，and the like)maybe made available to police departments,other information(e.g.a balance due on a speeding ticket)maybe made available to licen

3、sing 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 retrieval of data(felids,records and files)in a Database.It als

4、o controls the security and integrity of the databases The 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 manually are more practical the aid of the computer.In principle

5、,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 this is known as a database management system,or DBMS for short.Th

6、ere 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 requirements change.New categories of data can be added to the database witho

7、ut disruption to the existing system.http:/ 翻译：DBMS 简介数据库这个词经常用来描述一堆相关文件，这些文件被编成一个完整的复杂结构，这个结构对于相同的数据可针对不同的人提供不同的渠道。例如，在一个特定的州的所有县和城市，汽车部门有关所有司机执照信息的一个数据库。文件中的一些信息例如一个过去犯罪记录，证明有罪记录等等警察部门可以得到，其它信息（例如，对于超速行驶所支付的余额报酬），许可证局可以得到。还有其它信息（例如犯罪的数量和类型）保险公司会有记录。数据库管理系统是一个一套极其复杂的软件程序。在数据库中，它控制着数据（字段、希赛 IT 英语频道h

8、ttp:/ IT 英语频道最强大综合的IT 英语资源网http:/ 记录和文件）的组织、存储和检索。它也控制着数据库的安全性和整体性。数据库管理系统从应用程序处接受数据的要求并指示操作系统转移相应的数据。用计算机来协助完成那些用手工完成很费时的工作是很实际的一种方法。原则上讲，存于计算机牛的数据库与记录在纸上存于档案粗中的数据库没什么两样。但用计算机完成数据库的维护和信息的获取不但省事且速度很快。能够完成所有这些工作的计算机化的数据库即称作数据库管理系统，简称DBMS。将信息储存在计算机里有若干种方式，但这些方式并非全部都是真正的数据库管理系统。当使用数据库时，随着组织的信息要求的变化，信息系

9、统通常容易地被改变。数据的新的分类能够加入到数据库中而不破坏现存的系统。http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ Version System Management of Database Despite a large number of proposals on version support in the context of computer aided design and software engineering,the absence of a consensus on version semantics has be

10、en a key impediment to version support in database systems Because of the differences between files and databases,it is intuitively clear that the model of versions in database systems cannot be as simple as that adopted in file systems to support software engineeringFor data-bases,it may be necessa

11、ry to manage not only versions of single objects(e.g.a software module,document,but also versions of a collection of objects(e.g.a compound document,a user manual，etcand perhaps even versions of the schema of database(c.g.a table or a class,a collection of tables or classes)Broadly,there are three d

12、irections of research and development in versioning First is the notion of a parameterized versioning，that is,designing and implementing a versioning system whose behavior may be tailored by adjusting system parameters This may be the only viable approach,in view of the fact that there are various p

13、lausible choices for virtually every single aspect of versioning The second is to revisit these plausible choices for every aspect of versioning,with the view to discarding some of themes either impractical or flawed.The third is the investigation into the semantics and implementation of versioning

14、collections of objects and of versioning the database schema http:/ 翻译：数据库的文本管理在计算机辅助设计和软件工程的环境影响下，尽管对本文支持有许多建议，但对本文语义的意见不一致一直大大妨碍了数据库系统的文本支持。由于文件和数据库之间的差别，从直观上很清楚数据库系统的文本模式不可能与文件系统所采用的支持软件工程的文本模式同样简单。对于数据库来说，有必要不仅管理单个事务的文本（例如，一个软件模块，一份文件等等）而且也要管理许多事务（例如一个复合文件，一个用户手册等等）甚至还要管理数据库构图的文本。（例如一个表格和或一个类别，许

15、多表格，许多类别）。从广义上说，在文本的研究和发展方面有三个方向。第一，是参数文本的概念，即设计和任一种文本系统，其行为可能通过调节系统参数而改制。鉴于对文本的每一个单方而有许多似乎可行的选择这一事实，这可能是唯一可行的方法。第二个方向是重访文本每一个方面的可一行的选择，目的在于将其中或者是不实用的或者有毛病的选择丢掉。第三个方向是对语义进行研究并执行描述一大批物体和数据库的构图。http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ Transaction Management of Database The objective of long

16、-duration transactions is to model long-duration,interactive Database access sessions in application environmentsThe fundamental assumption about short-duration of transactions that underlies the traditional model of transactions is inappropriate for long-duration transactionsThe implementation of t

17、he traditional model of transactions may cause intolerably long waits when transactions aleph to acquire locks before accessing data,and may also cause a large amount of work to be lost when transactions are backed out in response to user-initiated aborts or system failure situations The objective o

18、f a transaction model is to pro-vide a rigorous basis for automatically enforcing criterion for database consistency for a set of multiple concurrent read and write accesses to the database in the presence of potential system failure situationsThe consistency criterion adopted for traditional transa

19、ctions is the notion of scrializability Scrializa-bility is enforced in conventional database systems through the use of locking for automatic concurrency control,and logging for automatic recovery from system failure situations A transaction”that doesn t provide a basis for automatically enforcing

20、data-base consistency is not really a transaction.To be sure,a long-duration transaction need not adopt seri-alizability as its consistency criterion However.there must be some consistcricv criterion http:/ 翻译：数据库的业务管理长期业务的目的是：在应用环境中模拟长期的、相互作用的数据库存取对话传统业务模式中关于短期业务的基本假设对于长期业务是不适用的。在存取数据前，当交易试图获得锁定时，传

21、统模式业务的执行可能会引起长期等待。也可能引起大量工作丢失，当交易随着用户产生的障碍或系统失灵被放弃时。业务模式的目的是提供严格的基础，在潜系统失灵的情况下，为了保持一套多个同时读写存取及数据库一致性，业务模式自动实行一个标准。在传统业务中采取的一致性标准是一种串行概念。在传统数据库系统中为自动并行控制，通过使用锁定而实行串行，并为了从系统失灵中自动恢复而联机。一个不能为实施自动数据库一致性而提供基础的业务不是一个真业务。确切地说，一个长期业务不必非采用串行作为它的一致标准，但是，必须有一些一致性的标准。http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网

22、http:/ The Single-database Model The single-database model has received considerably more attention than the multiple-database modelThe focus of research into single-database model has been on addressing the twin problem of long waits and loss of work that the long duration of transactions brings ab

23、out.The single-database model requires the intro-diction of a notion of database consistency,and protocols for concurrency control and recovery,that are different from those supported in traditional database systems If a reasonable notion of database consistency is to be supported(Ic.the database sy

24、stem is to enforce it automatically),there are bound to be”conflict situations where one transaction comes into an access conflict against some other transaction.If await is to be avoided，some means of a negotiated settlement of the conflict must be provided,thereby dragging the users into the detai

25、ls of concurrency controlThe single-database model is more appropriate-ate than the multiple-database model in an environment where it is difficult to determine in advance logical partitions of the database that correspond to work to be performed,and the users closely cooperate.http:/ 翻译：单一数据库模式单一数据

26、库模式比多个数据库模式受到了更多的重视。研究单个数据库模式的焦点集中在长时间寻址等待和长期业务带来的工作损失该两个问题。单一数据库模式要求引进数据库一致的观念和并行控制及恢复的协议，此协议不同于传统数据库系统中所支持的那些协议。如果一个数据库一致的合理观念要得到支持（即数据库系统要自动实行这个观念）、一定会有矛盾的情形产生、在这种情形中一个业务与其它业务产生存取矛盾如果要避免等待，就必须提供某种协商解决矛盾的方法，从而迫使用户详细了解并行控制单一数据库模式在某一种环境中比多个数据库模式更恰当，在这个环境中，要事先确定与要执行的工作相适应的数据库的逻辑分割是很难的，同时用户们又紧密地合作在一起。

27、http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ Multiple-database Model The multiple-database model is represented by proposals for shared and private database architect rues,checkout and checking of data to and from shared and private databases Each user may populates/her private database with

28、data checked out of the shared database,perform updates against the data.and check them back into the shared Database.The multiple-database model can be used to work around the conflict situations inherent in long lunation database sessionsSince each user from the shared database and work against.is

29、/her private database,disconnected from the、hared database(at least on the surface)，the usersan avoid the conflict situations In particular,multile users may be simultaneously updating the same thought having to wait for other users to comate late their updatesHowever,when updated is to benecked int

30、o the shared database,it may have to be checked in as a new version,necessitating version management.Further,when data in a private data-base references data in the shared data，or vice versa,a private database is not really disconnected from the shared database.For example,the evaluation of query in

31、 general will require the database system to access both a private database and the shared data-base,even if the query may have been formulated against a private database The multiple-database model is more appropriate than the single-database model in an environment where it is easy to determine in

32、 advance logical partitions of the database that correspond to work to be performed.http:/ 翻译：多个数据库模式多个数据库模式是通过共享的和专用的数据库结构来回检查数据的输入结果和输出纤果而体现出来。每一个用户都能给他她的专用数据库提供从共用数据库那儿检查定义的数据，实行修改数据，检查数招，然后送回共用数据库。多个数据库模式可用来围绕长期数据库对话所固有的矛盾形势而工作，因为每一个用户可从共用数据库拷贝数据这不同于他她的专用数据库，该数据库与共用数据库至少在表面上分开，这样用户就能避免矛盾形势。特别是多个

33、用户可能同时修改同一数据，而不必等待其他用户来完成他们的更改。但是，当更改的数据要进入共用数据库检查时，作为新的版本该数据以版本管理为条件可能必须受到检查。进一步说，当专用数据库的数据参考共用数据库的数据时，或反过来说共用数据库的数据参考专用数据库的数据时，一个专用数据库不是真的与共用数据库分沃。例如，评估一般的询问要求数据库系统既存取一个专用数据库也存取一个共用教据库，即使询问可能逆着专用数据库而被明确地阐述。多个数据库模式在一种环境中比单个教据库模式更恰当。在这种环境中容易事先确定与要执行的工作相适应的数据库的分割。http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的

34、IT 英语资源网http:/ Data Security and Independence Data security prevents unauthorized users from viewing or updating the database Using passwords,users are allowed access to the entire database or subsets of the database,called subschema(pronounced sub-scheme)，For example,an employee database can contai

35、n all the data about an individual employee,but one group of users may be authorized to view only payroll data,while others are allowed access to only work history and medical data.The DBMS can maintain the integrity of the database by not allowing more than one user to up-date the same record at th

36、e same time,The DBMS can keep duplicate records out of the database,for example,no two customers with the same customer numbers(Key fields)can be entered into the Data-Base.When a DBMS is used,the detailed knowledge of the physical organization of the data does not have to be built into every applic

37、ation program.The application program asks the DBMS for data by field pine,for example,I coded representation of give customer name and balance due would be sent to he DBMSWithout a DBMS,the programmer must secrecy space for the full structure of the in the program,Any change in data structure requi

38、res hangs in all the applications programs http:/ 翻译：数据的安全性和独立性数据安全防止未经批准的用户阅览和修改数据库。通过使用密码，用户才允许接近整个数据库或叫子结构的数据库子集合。例如，一个雇员数据库装有关于雇员个人的全部数据，但是只允许一组用户看一个工资单数据，而其他用户只允许看工作历史和医疗方面的数据。数据库管理系统通过阻止一六以上用户在相同时间更改同一记录的方法来保持数据库的完整。数具库管理系统能够防止副本记录进入数据库内。例如，不允许具有同一关键字段的两个客户进入数据具内。当一个数据库管理系统被使用时，有关数据管理组织的详细情形不必

39、被建立在每一个应用程序中：应用程序通过文件名向数据库管理系统要数据。例如，一个有代码鸭表达式“将客户名称和应付余额给我”会被送进数据库管理系统中。如果没有数据管理系统，程序员必须给程序中记录的全部结构留下空间数据结构的任何变化要求所有的应用程序作相应的变化。http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ DataBase Query Entry and Updating If the DBMS provides a way to interactively and update the database,as well as interr

40、ogate it capability allows for managing personal data-Aces however,it does not automatically leave an audit trail of actions and does not provide the kinds of controla necessary in a multiuser organization.These-controls are only available when a set of application programs are customized for each d

41、ata entry and updating function.Software for personal computers which perform me of the DBMS functions have been very popular arsenal computers were intended for use by individuals for personal information storage and process-These machines have also been used extensively small enterprises,professio

42、nals like doctors,acrylics,engineers,lawyers and so on By the nature of.intended usage,database systems on these machines except from several of the requirements of full doge database systems Since data sharing is not：Tended,concurrent operations even less so,the)fewer can be less complex.Security a

43、nd integrity7aintenance are de-emphasized or absent.As data-)limes will be small,performance efficiency Is also important.In fact,the only aspect of a database system that is Important is data Independence.Data-.dependence,as stated earlier.Means that applicant programs and user queries need not rec

44、ognizant physical organization of data on secondary storage.The importance of this aspect,particularly for the personal computer user,is that this greatly simplifies database usage.The user can store,access and manipulate data at a high level(close to the application)and be totally shielded from the

45、 low level(close to the machine)details of data organization.We will not discuss details of specific PC DBMS software packages hereLet us summarize in the following the strengths and weaknesses of personal computer data-base software systems：The most obvious positive factor is the user friendliness

46、of the software.A user with no prior computer background would be able to use the system to store personal and professional data,retrieve and perform relayed processing.The user should,of course,satiety himself about the quality of software and the freedom from errors(bugs)so that invest-merits in d

47、ata arc protected.For the programmer implementing applications with them,the advantage lies in the support for applications development in terms of input screen generations,output report generation etcoffered by theses stemsThe main negative point concerns absence of data protection features Unless

48、encrypted,data cane accessed by whoever has access to the machine Data can be destroyed through mistakes or maliciousintent.The second weakness of manv of the PC-basedsystems is that of performance.If data volumes growup to a few thousands of records，performance couldbe a bottleneck For organization

49、 where growth in data volumesis expected,availability of,the same or compatiblesoftware on large machines should be considered.This is one of the most common misconceptionsabout database management systems that are used inpersonal computersThoroughly comprehensive andsophisticated business systems c

50、an be 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ developed indBASh,Paradox and other DBMSs However,theyare created by experienced programmers using theDBMSs own programming language.That is not thesame as users who create and manage personal filesthat are not part of the mainstream company system

51、http:/ 翻译：数据库的查询登记和修改如果数据库管理系统提供一种方法来相互作用地进入和修改数据库，同时查询数据库，这种方法就为管理个人数据库创造了条件。但是，它不能自动留下一串查账行动，也不能在多用户组织中提供那种必要的控制。只有当一套应用程序为每一个数据进入和修改功能设定客户规格时，才一有可能得到这些控制。当前，执行数据库系的一些功能的个人计算机软件一直很流行。个人计算机是供个人信息存储和处理而用。这些机器也广泛地被一些小企业、职业家如建筑师，医师、工程师、律师等等所使用。从使用意图的性质看，这些机器上的数据库系统没有正式数据库系统的一些要求。由于不需要数据分享，也不需要同时操作。软件也

52、就不那么复杂。安全和整体维护也不那么重要，甚至没有随着数据容量变性能效率也不那么重要。事实上，数据库系统唯一重要的方面是数据独立。正如以前所述，数据独立是指在辅助存储器上，应用程序和用户询问不必知道数排的物理组织。这个方面的重要性，尤其对个人计算机用户来说，大大简化了数据库的使用。用户能够高层次地存储，存取和操纵数据，并且能够完全不受数据组织低层次细节的影响。在这里我们将不讨论一个特定个人计算机数据库管理系统软件包细节。我们将在下面总结一下个人计算机数据库软件系统的优缺点：最明显的积极方面是软件的用户友好型。一个事先不了解计算机知识的用户能够使用该系统来储存个人和职业数据，检索和执行有关处理。

53、当然，用户应该使自己对软件质量感到满足，高质量的软件仗数据的出错率减少从而使数据投资得到保护。对于完成应用的程序员而言，这个优点存在于支持这些系统所提供的在输入屏幕产生，输出报告产生方面的应用发展。主要消极点是缺少数据保护特征。除非被译成密码，数据不能被任何接触机器的人所存取。数据也能由于错误或被怀有恶意的人毁掉。以个人计算机为基础的许多系统的第二个缺点是效率方面的缺点。如果数据发展到几千个记录，效率问题可能是一个难关。由于数据容量的增长是一种发展趋势，所以应该考虑在较大的权器上获得相同或相容的软件。这是对用个人计算机数据库管理系统的最常见的错误概念之一：高综合和复杂的商业系统可以在dBASE

54、，Paradox 和其它数据库誉理系统中得以发展。但是，这些商业系统是由富有经验的程序设计员通过使用数据库管理系统专用的程序语言而建立的，这与建立和管理不是公司主要系统部分的个人文件的用户有所不同。http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ DBMS Structuring T echniques Spatial Management of DataBase Spatial data management has been an activearea of research in the database field for two

55、decades，with much of the research being focused on develop-ing data structures for storing and indexing spatialdata.However,no commercial database system pro-vides facilities for directly defining and storing spa-tial data,and forniulating queries based on researchconditions on spatial data.We belie

56、ve the followingare the relevant issues on which near-term researchshould be focused(in the order of decreasing impor-tance and urgency)First,relational query optimization techniquesneed to be extended to deal with spatial queries，thatis，queries that contain search conditions on spatialpredicates to

57、 be developed.Second,more work needs to be done on experi-mental validation of the relative performance of someof the more promising data structures and indexingstructures proposed thus far,with consideration of amuch broader set of operations than just a few opera-tions that have typically been use

58、d in the limited per-formance studies conducted thus far.Third,it is difficult to build into a single data-base system multiple data structures for spatial index-ing,and all spatial operators that are useful for awide variety of spatial applications，as such,it isdesirable to build a database system

59、so that it will beas easy as possible to extend the system with addi-tional data structures and spatial operators http:/ 翻译：数据库的空间组织二十年来空间数据管理一直是数据库研究领域的活跃区，研究的焦点集中在发展存储和索引空间数据的数据结构上。但是，没有商业数据库系统直接为确定和存储空间数据提供设备，明确地表达以空间数据为研究条件的询问。我们相信下列条件是一些相关问题，在这些问题上，应把焦点集中在近期研究，（按照重要程度和迫切程度排列）第一，关系询问最恰当技术，需要扩展以应

60、付空间询问，也就是说，对包含查询条件的询问，需要得以发展。第二对于迄今提出的一些更有希望的数据结构和索引结构有关效率的实验有一效性，需要做更多的工作，还要考虑涉及面很广的一套操作，而不仅仅是迄今进行的有限的效率研究方面被作典型使用的七个操作。希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT 英语资源网http:/ 第三，很难在单个数据库系统内加入空间索引的多个数据结构和对十各种各样空间应用有的所有空间操作员。因此建立一个数据库是很理想的，这样将会尽可能容易地用额外的数据结构和空间操作员来扩大此系统。http:/ 希赛 IT 英语频道http:/ IT 英语频道最强大综合的IT

61、英语资源网http:/ Tcrnporal Managcment of DataBase There are two components to temporal datamanagement:history data management and versionmanagement.Both have been the subjects of researchfor over a decade.The troublesome aspect of tempo-ral data management is that the boundary betweenapplications and dat

62、abase systems has not beenclearly drawn Specifically,it is not clear how muchof the typical semantics and facilities of temporaldata management can and should be directly incorpo-rated in a database systqm，and how much should belcft to applications and users In this section,we willprovide a list of

63、short-term research issues thatshould be examined to shed light on this fundamentalquestion.The focus of research into history data manage-ment has been on defining the semantics of time andtime interval,and issues related to understanding thesemantics of queries and updates against histo 叮 datastor

64、ed in an attribute of a record.Typically,in thecontext of relational databases，a temporal attribute isdefined to hold a sequence of history data for theattribute.Ahistory data consists of a data item and atime interval for which the data item is valid.A quenmav-then be issued to retrieve historv dat

65、a for aspecified time intei-val for the temporal attribute.Themechanism for supporting temporal attributes is simi-lar to that for supporting set-valued attributes in adatabase system,such as UnISQL In the absence of a support for temporal attrib-utes,application developers who need to model andmana

66、ge history data have simply simulated temporalattributes by creating attributc(s)for the time inter-val,along with the temporal attribute.This ofcourse may result in duplication of records in a table,and more complicated search predicates in queries The one necessary topic of research in history data.nanagement is to quantitatively establish the per-.ormance(and even productivity)differences betweenasing a database system that directly supports attributes and using a conventional DataBascs,syste