數(shù)據(jù)庫(kù)管理系統(tǒng)外文翻譯.doc

外文翻譯教 學(xué) 部 專(zhuān) 業(yè)計(jì)算機(jī)科學(xué)與技術(shù)班 級(jí)學(xué) 號(hào)姓 名指導(dǎo)教師負(fù)責(zé)教師沈陽(yáng)航空航天大學(xué)北方科技學(xué)院2010年6月沈陽(yáng)航空航天大學(xué)北方科技學(xué)院畢業(yè)設(shè)計(jì)(外文翻譯)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 storage, retrieval , modification, and deletion of data in conjunction with various data-processing 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 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 attribute 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 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 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 needs, 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 reports. 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 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 data;(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 services 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 strategic 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 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 model-that is-they 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 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 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 one-to-many 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 organized 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 persons 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 English-like description that specifies the operation and the described record or combination of records. A query optimizer translates the description into a procedure to perform the database manipulation.Network ModelThe network model creates relationships among data through a linked-list structure in which subordinate records can be linked to more than one parent record. This approach combines records with links, which are called pointers. The pointers are addresses that indicate the location of a record. With the network approach, a subordinate record can be linked to a key record and at the same time itself be a key record linked to other sets of subordinate records. The network mode historically has had a performance advantage over other database models. Today , such performance characteristics are only important in high-volume ,high-speed transaction processing such as automatic teller machine networks or airline reservation system.Both hierarchical and network databases are application specific. If a new application is developed ,maintaining the consistency of databases in different applications can be very difficult. For example, suppose a new pension application is developed .The data are the same, but a new database must be created.Object Model The newest approach to database management uses an object model , in which records are represented by entities called objects that can both store data and provide methods or procedures to perform specific tasks.The query language used for the object model is the same object-oriented programming language used to develop the database application .This can create problems because there is no simple , uniform query language such as SQL . The object model is relatively new, and only a few examples of object-oriented database exist. It has attracted attention because developers who choose an object-oriented programming language want a database based on an object-oriented model.Distributed DatabaseSimilarly , a distributed database is one in which different parts of the database reside on physically separated computers . One goal of distributed databases is the access of information without regard to where the data might be stored. Keeping in mind that once the users and their data are separated , the communication and networking concepts come into play .Distributed databases require software that resides partially in the larger computer. This software bridges the gap between personal and large computers and resolves the problems of incompatible data formats. Ideally, it would make the mainframe databases appear to be large libraries of information, with most of the processing accomplished on the personal computer.A drawback to some distributed systems is that they are often based on what is called a mainframe-entire model , in which the larger host computer is seen as the master and the terminal or personal computer is seen as a slave. There are some advantages to this approach . With databases under centralized control , many of the problems of data integrity that we mentioned earlier are solved . But todays personal computers, departmental computers, and distributed processing require computers and their applications to communicate with each other on a more equal or peer-to-peer basis. In a database, the client/server model provides the framework for distributing databases.One way to take advantage of many connected computers running database applications is to distribute the application into cooperating parts that are independent of one anther. A client is an end user or computer program that requests resources across a network. A server is a computer running software that fulfills those requests across a network . When the resources are data in a database ,the client/server model provides the framework for distributing database.A file serve is software that provides access to files across a network. A dedicated file server is a single computer dedicated to being a file server. This is useful ,for example ,if the files are large and require fast access .In such cases, a minicomputer or mainframe would be used as a file server. A distributed file server spreads the files around on individual computers instead of placing them on one dedicated computer.Advantages of the latter server include the ability to store and retrieve files on other computers and the elimination of duplicate files on each computer. A major disadvantage , however, is that individual read/write requests are being moved across the network and problems can arise when updating files. Suppose a user requests a record from a file and changes it while another user requests the same record and changes it too. The solution to this problems called record locking, which means that the first request makes others requests wait until the first request is satisfied . Other users may be able to read the record, but they will not be able to change it .A database server is software that services requests to a database across a network. For example, suppose a user types in a query for data on his or her personal computer . If the application is designed with the client/server model in mind ,the query language part on the personal computer simple sends the query across the network to the database server and requests to be notified when the data are found.Examples of distributed database systems can be found in the engineering world. Suns Network Filing System(NFS),for example, is used in computer-aided engineering applications to distribute data among the hard disks in a network of Sun workstation.Distributing databases is an evolutionary step because it is logical that data should exist at the location where they are being used . Departmental computers within a large corporation ,for example, should have data reside locally , yet those data should be accessible by authorized corporate management when they want to consolidate departmental data . DBMS software will protect the security and integrity of the database , and the distributed database will appear to its users as no different from the non-distributed database .In this information age, the data server has become the heart of a company. This one piece of software controls the rhythm of most organizations and is used to pump information lifeblood through the arteries of the network. Because of the critical nature of this application, the data server is also the one of the most popular targets for hackers. If a hacker owns this application, he can cause the companys heart to suffer a fatal arrest.Ironically, although most users are now aware of hackers, they still do not realize how susceptible their database servers are to hack attacks. Thus, this article presents a description of the primary methods of attacking database servers (also known as SQL servers) and shows you how to protect yourself from these attacks. You should note this information is not new. Many technical white papers go into great detail about how to perform SQL attacks, and numerous vulnerabilities have been posted to security lists that describe exactly how certain database applications can be exploited. This article was written for the curious non-SQL experts who do not care to know the details, and as a review to those who do use SQL regularly. What Is a SQL Server?A database application is a program that provides clients with access to data. There are many variations of this type of application, ranging from the expensive enterprise-level Microsoft SQL Server to the free and open source mySQL. Regardless of the flavor, most database server applications have several things in common. First, database applications use the same general programming language known as SQL, or Structured Query Language. This language, also known as a fourth-level language due to its simplistic syntax, is at the core of how a client communicates its requests to the server. Using SQL in its simplest form, a programmer can select, add, update, and delete information in a database. However, SQL can also be used to create and design entire databases, perform various functions on the returned information, and even execute other programs.To illustrate how SQL can be used, the following is an example of a simple standard SQL query and a more powerful SQL query: Simple: Select * from dbFurniture.tblChairThis returns all information in the table tblChair from the database dbFurniture.Complex: EXEC master.xp_cmdshell dir c:This short SQL command returns to the client the list of files and folders under the c: directory of the SQL server. Note that this example uses an extended stored procedure that is exclusive to MS SQL Server. The second function that database server applications share is that they all require some form of authenticated connection between client and host. Although the SQL language is fairly easy to use, at least in its basic form, any client that wants to perform queries must first provide some form of credentials that will authorize the client; the client also must define the format of the request and response.This connection is defined by several attributes, depending on the relative location of the client and what operating systems are in use. We could spend a whole article discussing various technologies such as DSN connections, DSN-less connections, RDO, ADO, and more, but these subjects are outside the scope of this article. If you want to learn more about them, a little Googleing will provide you with more than enough information. However, the following is a list of the more common items included in a connection request.Database source Request type Database User ID PasswordBefore any connection can be made, the client must define what type of database server it is connecting to. This is handled by a software component that provides the client with the instructions needed to create the request in the correct format. In addition to the type of database, the request type can be used to further define how the clients request will be handled by the server. Next comes the database name and finally the authentication information.All the connection information is important, but by far the weakest link is the authentication informationor lack thereof. In a properly managed server, each database has its own users with specifically designated permissions that control what type of activity they can perform. For example, a user account would be set up as read only for applications that need to only access information. Another account should be used for inserts or updates, and maybe even a third account would be used for deletes. This type of account control ensures that any compromised account is limited in functionality. Unfortunately, many database programs are set up with null or easy passwords, which leads to successful hack attacks.14數(shù)據(jù)庫(kù)管理系統(tǒng)介紹數(shù)據(jù)庫(kù)（database,有時(shí)拼作data base）又稱(chēng)為電子數(shù)據(jù)庫(kù)，是專(zhuān)門(mén)組織起來(lái)的一組數(shù)據(jù)或信息，其目的是為了便于計(jì)算機(jī)快速查詢及檢索。數(shù)據(jù)庫(kù)的結(jié)構(gòu)是專(zhuān)門(mén)設(shè)計(jì)的，在各種數(shù)據(jù)處理操作命令的支持下，可以簡(jiǎn)化數(shù)據(jù)的存儲(chǔ)，檢索，修改和刪除。數(shù)據(jù)庫(kù)可以存儲(chǔ)在磁盤(pán)，磁帶，光盤(pán)或其他輔助存儲(chǔ)設(shè)備上。數(shù)據(jù)庫(kù)由一個(gè)或一套文件組成，其中的信息可以分解為記錄，每一記錄又包含一個(gè)或多個(gè)字段（或稱(chēng)為域）。字段是數(shù)據(jù)存取的基本單位。數(shù)據(jù)庫(kù)用于描述實(shí)體，其中的一個(gè)字段通常表示與實(shí)體的某一屬性相關(guān)的信息。通過(guò)關(guān)鍵字以及各種分類(lèi)（排序）命令，用戶可以對(duì)多條記錄的字段進(jìn)行查詢，重新整理，分組或選擇，以實(shí)體對(duì)某一類(lèi)數(shù)據(jù)的檢索，也可以生成報(bào)表。所有數(shù)據(jù)庫(kù)（最簡(jiǎn)單的除外）中都有復(fù)雜的數(shù)據(jù)關(guān)系及其鏈接。處理與創(chuàng)建，訪問(wèn)以及維護(hù)數(shù)據(jù)庫(kù)記錄有關(guān)的復(fù)雜任務(wù)的系統(tǒng)軟件包叫做數(shù)據(jù)庫(kù)管理系統(tǒng)（DBMS）。DBMS軟件包中的程序在數(shù)據(jù)庫(kù)與其用戶間建立接口。（這些用戶可以是應(yīng)用程序員，管理員及其他需要信息的人員和各種操作系統(tǒng)程序）。DBMS可組織，處理和表示從數(shù)據(jù)庫(kù)中選出的數(shù)據(jù)元。該功能使決策者能搜索，探查和查詢數(shù)據(jù)庫(kù)的內(nèi)容，從而對(duì)在正規(guī)報(bào)告中沒(méi)有的，不再出現(xiàn)的且無(wú)法預(yù)料的問(wèn)題做出回答。這些問(wèn)題最初可能是模糊的并且（或者）是定義不恰當(dāng)?shù)?，但是人們可以瀏覽數(shù)據(jù)庫(kù)直到獲得所需的信息。簡(jiǎn)言之，DBMS將“管理”存儲(chǔ)的數(shù)據(jù)項(xiàng)，并從公共數(shù)據(jù)庫(kù)中匯集所需的數(shù)據(jù)項(xiàng)以回答非程序員的詢問(wèn)。DBMS由3個(gè)主要部分組成：（1）存儲(chǔ)子系統(tǒng)，用來(lái)存儲(chǔ)和檢索文件中的數(shù)據(jù)；（2）建模和操作子系統(tǒng)，提供組織數(shù)據(jù)以及添加，刪除，維護(hù)，更新數(shù)據(jù)的方法；（3）用戶和DBMS之間的接口。在提高數(shù)據(jù)庫(kù)管理系統(tǒng)的價(jià)值和有效性方面正在展現(xiàn)以下一些重要發(fā)展趨勢(shì)；1.管理人員需要最新的信息以做出有效的決策。2.客戶需要越來(lái)越復(fù)雜的信息服務(wù)以及更多的有關(guān)其訂單，發(fā)票和賬號(hào)的當(dāng)前信息。3.用戶發(fā)現(xiàn)他們可以使用傳統(tǒng)的程序設(shè)計(jì)語(yǔ)言，在很短的一段時(shí)間內(nèi)用數(shù)據(jù)庫(kù)系統(tǒng)開(kāi)發(fā)客戶應(yīng)用程序。4.商業(yè)公司發(fā)現(xiàn)了信息的戰(zhàn)略價(jià)值，他們利用數(shù)據(jù)庫(kù)系統(tǒng)領(lǐng)先于競(jìng)爭(zhēng)對(duì)手。數(shù)據(jù)庫(kù)模型數(shù)據(jù)庫(kù)模型描述了在數(shù)據(jù)庫(kù)中結(jié)構(gòu)化和操縱數(shù)據(jù)的方法，模型的結(jié)構(gòu)部分規(guī)定了數(shù)據(jù)如何被描述（例如樹(shù)，表等）：模型的操縱部分規(guī)定了數(shù)據(jù)添加，刪除，顯示，維護(hù)，打印，查找，選擇，排序和更新等操作。分層模型第一個(gè)數(shù)據(jù)庫(kù)管理系統(tǒng)使用的是分層模型，也就是說(shuō)，將數(shù)據(jù)記錄排列成樹(shù)形結(jié)構(gòu)。一些記錄時(shí)根目錄，在其他所有記錄都有獨(dú)立的父記錄。樹(shù)形結(jié)構(gòu)的設(shè)計(jì)反映了數(shù)據(jù)被使用的順序，也就是首先訪問(wèn)處于樹(shù)根位置的記錄，接下來(lái)是跟下面的記錄，等等。分層模型的開(kāi)發(fā)是因?yàn)榉謱雨P(guān)系在商業(yè)應(yīng)用中普遍存在，眾所周知，一個(gè)組織結(jié)構(gòu)圖表就描述了一種分層關(guān)系：高層管理人員在最高層，中層管理人員在較低的層次，負(fù)責(zé)具體事務(wù)的雇員在最底層。值得注意的是，在一個(gè)嚴(yán)格的分層結(jié)構(gòu)體系中，在每個(gè)管理層下可能有多個(gè)雇員或多個(gè)層次的雇員，但每個(gè)雇員只有一個(gè)管理者。分層結(jié)構(gòu)數(shù)據(jù)的典型特征是數(shù)據(jù)之間的一對(duì)多關(guān)系