計(jì)算機(jī)外文翻譯---數(shù)據(jù)庫管理系統(tǒng)的介紹

1、<p>　　數(shù)據(jù)庫管理系統(tǒng)的介紹</p><p>　　Raghu Ramakrishnan</p><p>　　數(shù)據(jù)庫（database,有時(shí)拼作data base）又稱為電子數(shù)據(jù)庫，是專門組織起來的一組數(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)庫的結(jié)構(gòu)是專門設(shè)計(jì)的，在各種數(shù)據(jù)處理操作命令的支持下，可以簡化數(shù)據(jù)的存儲(chǔ)，檢索，修改和刪除。數(shù)據(jù)庫可以存儲(chǔ)在磁盤，磁帶，光盤

2、或其他輔助存儲(chǔ)設(shè)備上。</p><p>　　數(shù)據(jù)庫由一個(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（或稱為域）。字段是數(shù)據(jù)存取的基本單位。數(shù)據(jù)庫用于描述實(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。通過關(guān)鍵字以及各種分類（排序）命令，用戶可以對多條記錄的字段進(jìn)行查詢，重新整理，分組或選擇，以實(shí)體對某一類數(shù)據(jù)的檢索，也可以生成報(bào)表。</p><p>　　

3、所有數(shù)據(jù)庫（最簡單的除外）中都有復(fù)雜的數(shù)據(jù)關(guān)系及其鏈接。處理與創(chuàng)建，訪問以及維護(hù)數(shù)據(jù)庫記錄有關(guān)的復(fù)雜任務(wù)的系統(tǒng)軟件包叫做數(shù)據(jù)庫管理系統(tǒng)（DBMS）。DBMS軟件包中的程序在數(shù)據(jù)庫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）。</p><p>　　DBMS可組織，處理和表示從數(shù)據(jù)庫中選出的數(shù)據(jù)元。該功能使決策者能搜索，探查和查詢數(shù)據(jù)庫的內(nèi)容，從而對在正規(guī)報(bào)告中沒有的

4、，不再出現(xiàn)的且無法預(yù)料的問題做出回答。這些問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)庫直到獲得所需的信息。簡言之，DBMS將“管理”存儲(chǔ)的數(shù)據(jù)項(xiàng)，并從公共數(shù)據(jù)庫中匯集所需的數(shù)據(jù)項(xiàng)以回答非程序員的詢問。</p><p>　　DBMS由3個(gè)主要部分組成：（1）存儲(chǔ)子系統(tǒng)，用來存儲(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

5、之間的接口。在提高數(shù)據(jù)庫管理系統(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；</p><p>　　1.管理人員需要最新的信息以做出有效的決策。</p><p>　　2.客戶需要越來越復(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。</p><p>　　3.用戶發(fā)現(xiàn)他們可以使用傳統(tǒng)的程序設(shè)計(jì)語言，在很短的一段時(shí)間內(nèi)用數(shù)據(jù)庫系統(tǒng)開發(fā)客戶應(yīng)用程序。</

6、p><p>　　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)庫系統(tǒng)領(lǐng)先于競爭對手。</p><p><b>　　數(shù)據(jù)庫模型</b></p><p>　　數(shù)據(jù)庫模型描述了在數(shù)據(jù)庫中結(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，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。</

7、p><p><b>　　分層模型</b></p><p>　　第一個(gè)數(shù)據(jù)庫管理系統(tǒng)使用的是分層模型，也就是說，將數(shù)據(jù)記錄排列成樹形結(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，也就是首先訪問處于樹根位置的記錄，接下來是跟下面的記錄，等等。</p><p>　　分層模型的開發(fā)是因?yàn)榉謱雨P(guān)系在商業(yè)應(yīng)用中普

8、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)之間的一對多關(guān)系。</p><p>　　在分層方法中，當(dāng)數(shù)據(jù)庫建立時(shí)，每一關(guān)系即被明確地定義。在分層數(shù)據(jù)庫中的每一記錄只能包含一個(gè)關(guān)鍵字段，任

9、意兩個(gè)字段之間只能有一種關(guān)系。由于數(shù)據(jù)并不總是遵循這種嚴(yán)格的分層關(guān)系，所以這樣可能會(huì)出現(xiàn)一些問題。</p><p><b>　　關(guān)系模型</b></p><p>　　在1970年，數(shù)據(jù)庫研究取得了重大突破。E.F.Codd提出了一種截然不同的數(shù)據(jù)庫管理方法，使用表作為數(shù)據(jù)結(jié)構(gòu),稱之為關(guān)系模型.</p><p>　　關(guān)系數(shù)據(jù)庫是使用最廣的數(shù)據(jù)結(jié)構(gòu)

10、,數(shù)據(jù)被組織成關(guān)系表,每個(gè)表由稱作記錄的行和稱作字段的列組成。每個(gè)記錄包含了專用項(xiàng)目的字段值。例如，在一個(gè)包含雇員信息的表中，一個(gè)記錄包含了像一個(gè)人姓名和地址這樣的字段的值。</p><p>　　結(jié)構(gòu)化查詢語言（SQL）是一種在關(guān)系型數(shù)據(jù)庫中用于處理數(shù)據(jù)的查詢語言。它是非過程化語言或者說是描述性的，用戶只須指定一種類似于英語的描述，用來確定操作，記錄或描述記錄組合。查詢優(yōu)化器將這種描述翻譯為過程執(zhí)行數(shù)據(jù)庫操作。&

11、lt;/p><p><b>　　網(wǎng)狀模型</b></p><p>　　網(wǎng)狀模型在數(shù)據(jù)之間通過鏈接表結(jié)構(gòu)創(chuàng)建關(guān)系，子記錄可以鏈接到多個(gè)父記錄。這種將記錄和鏈接捆綁到一起的方法叫做指針，他是指向一個(gè)記錄存儲(chǔ)位置的存儲(chǔ)地址。使用網(wǎng)狀方法，一個(gè)子記錄可以鏈接到一個(gè)關(guān)鍵記錄，同時(shí)，它本身也可以作為一個(gè)關(guān)鍵記錄。鏈接到其他一系列子記錄。在早期，網(wǎng)狀模型比其他模型更有性能優(yōu)勢；但是在今

12、天，這種優(yōu)勢的特點(diǎn)只有在自動(dòng)柜員機(jī)網(wǎng)絡(luò)，航空預(yù)定系統(tǒng)等大容量和高速處理過程中才是最重要的。</p><p>　　分層和網(wǎng)狀數(shù)據(jù)庫都是專用程序，如果開發(fā)一個(gè)新的應(yīng)用程序，那么在不同的應(yīng)用程序中保持?jǐn)?shù)據(jù)庫的一致性是非常困難的。例如開發(fā)一個(gè)退休金程序，需要訪問雇員數(shù)據(jù)，這一數(shù)據(jù)同時(shí)也被工資單程序訪問。雖然數(shù)據(jù)是相同的，但是也必須建立新的數(shù)據(jù)庫。</p><p><b>　　對象模型&l

13、t;/b></p><p>　　最新的數(shù)據(jù)庫管理方法是使用對象模型，記錄由被稱作對象的實(shí)體來描述，可以在對象中存儲(chǔ)數(shù)據(jù)，同時(shí)提供方法或程序執(zhí)行特定的任務(wù)。</p><p>　　對象模型使用的查詢語言與開發(fā)數(shù)據(jù)庫程序所使用的面向?qū)ο蟮某绦蛟O(shè)計(jì)語言是相同的，因?yàn)闆]有像SQL這樣簡單統(tǒng)一的查詢語言，所以會(huì)產(chǎn)生一些問題。對象模型相對較新，僅有少數(shù)幾個(gè)面向?qū)ο蟮臄?shù)據(jù)庫實(shí)例。它引起了人們的關(guān)注，

14、因?yàn)檫x擇面向?qū)ο蟪绦蛟O(shè)計(jì)語言的開發(fā)人員希望有一個(gè)基于在對象模型基礎(chǔ)上的數(shù)據(jù)庫。</p><p><b>　　分布式數(shù)據(jù)庫</b></p><p>　　類似的，分布式數(shù)據(jù)庫指的是數(shù)據(jù)庫的各個(gè)部分分別存儲(chǔ)在物理上相互分開的計(jì)算機(jī)上。分布式數(shù)據(jù)庫的一個(gè)目的是訪問數(shù)據(jù)信息時(shí)不必考慮其他位置。注意，一旦用戶和數(shù)據(jù)分開，通信和網(wǎng)絡(luò)則開始扮演重要角色。</p><

15、;p>　　分布式數(shù)據(jù)庫需要部分常駐于大型主機(jī)上的軟件，這些軟件在大型機(jī)和個(gè)人計(jì)算機(jī)之間建立橋梁，并解決數(shù)據(jù)格式不兼容的問題。在理想情況下，大型主機(jī)上的數(shù)據(jù)庫看起來像是一個(gè)大的信息倉庫，而大部分處理則在個(gè)人計(jì)算機(jī)上完成。</p><p>　　分布式數(shù)據(jù)庫系統(tǒng)的一個(gè)缺點(diǎn)是它們常以主機(jī)中心模型為基礎(chǔ)，在這種模型中，大型主機(jī)看起來好像是雇主，而終端和個(gè)人計(jì)算機(jī)看起來好像是奴隸。但是這種方法也有許多優(yōu)點(diǎn)：由于數(shù)據(jù)庫的

16、集中控制，前面提到的數(shù)據(jù)完整性和安全性的問題就迎刃而解了。當(dāng)今的個(gè)人計(jì)算機(jī)，部門級(jí)計(jì)算機(jī)和分布式處理都需要計(jì)算機(jī)之間以及應(yīng)用程序之間在相等或?qū)Φ鹊幕A(chǔ)上相互通信，在數(shù)據(jù)庫中客戶機(jī)/服務(wù)器模型為分布式數(shù)據(jù)庫提供了框架結(jié)構(gòu)。</p><p>　　利用相互連接的計(jì)算機(jī)上運(yùn)行的數(shù)據(jù)庫應(yīng)用程序的一種方法是將程序分解為相互獨(dú)立的部分。客戶端是一個(gè)最終用戶或通過網(wǎng)絡(luò)申請資源的計(jì)算機(jī)程序，服務(wù)器是一個(gè)運(yùn)行著的計(jì)算機(jī)軟件，存儲(chǔ)著那

17、些通過網(wǎng)絡(luò)傳輸?shù)纳暾?。?dāng)申請的資源是數(shù)據(jù)庫中的數(shù)據(jù)時(shí)，客戶機(jī)/服務(wù)器模型則為分布式數(shù)據(jù)庫提供了框架結(jié)構(gòu)。</p><p>　　文件服務(wù)器指的是一個(gè)通過網(wǎng)絡(luò)提供文件訪問的軟件，專門的文件服務(wù)器是一臺(tái)被指定為文件服務(wù)器的計(jì)算機(jī)。這是非常有用的，例如，如果文件比較大而且需要快速訪問，在這種情況下，一臺(tái)微型計(jì)算機(jī)或大型主機(jī)將被用作文件服務(wù)器。分布式文件服務(wù)器將文件分散到不同的計(jì)算機(jī)上，而不是將它們集中存放到專門的文件服務(wù)

18、器上。</p><p>　　后一種文件服務(wù)器的優(yōu)點(diǎn)包括在其他計(jì)算機(jī)上存儲(chǔ)和檢索文件的能力，并可以在每一臺(tái)計(jì)算機(jī)上消除重復(fù)文件。然而，一個(gè)重要的缺點(diǎn)是每個(gè)讀寫請求需要在網(wǎng)絡(luò)上傳播，在刷新文件時(shí)可能出現(xiàn)問題。假設(shè)一個(gè)用戶申請文件中的一個(gè)數(shù)據(jù)并修改它，同時(shí)另外一個(gè)用戶也申請這個(gè)數(shù)據(jù)并修改它，解決這種問題的方法叫做數(shù)據(jù)鎖定，即第一個(gè)申請使其他申請?zhí)幱诘却隣顟B(tài)，直到完成第一個(gè)申請，其他用戶可以讀取這個(gè)數(shù)據(jù)，但不能修改。&l

19、t;/p><p>　　數(shù)據(jù)庫服務(wù)器是一個(gè)通過網(wǎng)絡(luò)為數(shù)據(jù)庫申請?zhí)峁┓?wù)的軟件，例如，假設(shè)某個(gè)用戶在他的個(gè)人計(jì)算機(jī)上輸入了一個(gè)數(shù)據(jù)查詢命令，如果應(yīng)用程序按照客戶機(jī)/服務(wù)器模型設(shè)計(jì)，那么個(gè)人計(jì)算機(jī)上的查詢語言通過網(wǎng)絡(luò)傳送數(shù)據(jù)庫服務(wù)器上，當(dāng)發(fā)現(xiàn)數(shù)據(jù)時(shí)發(fā)出通知。</p><p>　　在工程界也有許多分布式數(shù)據(jù)庫的例子，如SUN公司的網(wǎng)絡(luò)文件系統(tǒng)（NFS）被應(yīng)用到計(jì)算機(jī)輔助工程應(yīng)用程序中，將數(shù)據(jù)分散到由S

20、UN工作站組成的網(wǎng)絡(luò)上的不同硬盤之間。</p><p>　　分布式數(shù)據(jù)庫是革命性的進(jìn)步，因?yàn)榘褦?shù)據(jù)存放在被使用位置上是很合乎常理的。例如一個(gè)大公司不同部門之間的計(jì)算機(jī)，應(yīng)該將數(shù)據(jù)存儲(chǔ)在本地，然而，當(dāng)被授權(quán)的管理人員需要整理部門數(shù)據(jù)時(shí)，數(shù)據(jù)應(yīng)該能夠被訪問。數(shù)據(jù)庫信息系統(tǒng)軟件將保護(hù)數(shù)據(jù)庫的安全性和完整性，對用戶而言，分布式數(shù)據(jù)庫和非分布式數(shù)據(jù)庫看起來沒有什么差別。</p><p><b&

21、gt;　　原文</b></p><p>　　Database Management Systems( 3th Edition ),Wiley ,2004, 5-12</p><p>　　A introduction to Database Management System</p><p>　　Raghu Ramakrishnan</p>

22、<p>　　A 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.

23、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, o

24、r some other secondary storage device.</p><p>　　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.

25、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, use

26、rs can rapidly search , rearrange, group, and select the fields in many records to retrieve or create reports on particu</p><p>　　Complex data relationships and linkages may be found in all but the simplest

27、 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 e

28、stablish 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.)</p><p>　　

29、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 u

30、nplanned questions that aren’t 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,

31、 the DBMS will “manage” the stored data items and assemble t</p><p>　　A database management system (DBMS) is composed of three major parts:(1)a storage subsystem that stores and retrieves data in files;(2) a

32、 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

33、 emerging that enhance the value and usefulness of database management systems;</p><p>　　Managers: who require more up-to-data information to make effective decision</p><p>　　Customers: who dema

34、nd increasingly sophisticated information services and more current information about the status of their orders, invoices, and accounts.</p><p>　　Users: who find that they can develop custom applications wi

35、th database systems in a fraction of the time it takes to use traditional programming languages.</p><p>　　Organizations : that discover information has a strategic value; they utilize their database systems

36、to gain an edge over their competitors.</p><p>　　The Database Model</p><p>　　A data model describes a way to structure and manipulate the data in a database. The structural part of the model sp

37、ecifies 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

38、 data.</p><p>　　Hierarchical Model </p><p>　　The first database management systems used a hierarchical model-that is-they arranged records into a tree structure. Some records are root records an

39、d 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

40、below the root ,and so on.</p><p>　　The hierarchical model was developed because hierarchical relationships are commonly found in business applications. As you have known, an organization char often describe

41、s 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

42、have many employees or levels of employees beneath it, but each employee has only one manager. Hierarchical d</p><p>　　In the hierarchical approach, each relationship must be explicitly defined when the data

43、base 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 s

44、trict hierarchy.</p><p>　　Relational Model </p><p>　　A major breakthrough in database research occurred in 1970 when E. F. Codd proposed a fundamentally different approach to database managemen

45、t called relational model ,which uses a table as its data structure.</p><p>　　The relational database is the most widely used database structure. Data is organized into related tables. Each table is made up

46、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 person’

47、s last name ,first name ,and street address.</p><p>　　Structured query language(SQL)is a query language for manipulating data in a relational database .It is nonprocedural or declarative, in which the user n

48、eed 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 manipula

49、tion.</p><p>　　Network Model</p><p>　　The network model creates relationships among data through a linked-list structure in which subordinate records can be linked to more than one parent record

50、. 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

51、 at the same time itself be a key record linked to other sets of subordinate records. The network mode historically has had </p><p>　　Both hierarchical and network databases are application specific. If a ne

52、w 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 databas

53、e must be created.</p><p>　　Object Model </p><p>　　The newest approach to database management uses an object model , in which records are represented by entities called objects that can both sto

54、re data and provide methods or procedures to perform specific tasks.</p><p>　　The query language used for the object model is the same object-oriented programming language used to develop the database applic

55、ation .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

56、because developers who choose an object-oriented programming language want a database based on an object-oriented model.</p><p>　　Distributed Database</p><p>　　Similarly , a distributed database

57、 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 min

58、d that once the users and their data are separated , the communication and networking concepts come into play .</p><p>　　Distributed databases require software that resides partially in the larger computer

59、. 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, wit

60、h most of the processing accomplished on the personal computer.</p><p>　　A drawback to some distributed systems is that they are often based on what is called a mainframe-entire model , in which the larger h

61、ost 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

62、 that we mentioned earlier are solved . But today’s personal computers, departmental computers, and distributed processing require computers and t</p><p>　　One way to take advantage of many connected comput

63、ers 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

64、 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.</p><p>　　A

65、 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

66、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.</p>

67、<p>　　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 re

68、ad/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

69、. The solution to this problems called record locking, which means that</p><p>　　A database server is software that services requests to a database across a network. For example, suppose a user types in a qu

70、ery 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 datab

71、ase server and requests to be notified when the data are found.</p><p>　　Examples of distributed database systems can be found in the engineering world. Sun’s Network Filing System(NFS),for example, is used

72、in computer-aided engineering applications to distribute data among the hard disks in a network of Sun workstation.</p><p>　　Distributing databases is an evolutionary step because it is logical that data sho

73、uld 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 manageme