1、英文翻譯數據庫管理系統的介紹RaghuRamakrishnan數據庫（ database,有時被拼作 data base）又稱為電子數據庫，是專門組織起來的一組數據或信息，其目的是為了便于計算機快速查詢及檢索。數據庫的結構是專門設計的，在各種數據處理操作命令的支持下，可以簡化數據的存儲、檢索、修改和刪除。數據庫可以存儲在磁盤、磁帶、光盤或其他輔助存儲設備上。數據庫由一個或一套文件組成，其中的信息可以分解為記錄，每一條記錄又包含一個或多個字段（或稱為域）。字段是數據存取的基本單位。數據庫用于描述實體，其中的一個字段通常表示與實體的某一屬性相關的信息。通過關鍵字以及各種分類（排序）命令，用戶可以對

2、多條記錄的字段進行查詢，重新整理，分組或選擇，以實體對某一類數據的檢索，也可以生成報表。所有數據庫（除最簡單的）中都有復雜的數據關系及其鏈接。處理與創建，訪問以及維護數據庫記錄有關的復雜任務的系統軟件包叫做數據庫管理系統（ DBMS）。DBMS 軟件包中的程序在數據庫與其用戶間建立接口。 （這些用戶可以是應用程序員，管理員及其他需要信息的人員和各種操作系統程序）DBMS可組織、處理和表示從數據庫中選出的數據元。該功能使決策者能搜索、探查和查詢數據庫的內容，從而對正規報告中沒有的，不再出現的且無法預料的問題做出回答。這些問題最初可能是模糊的并且（或者）是定義不恰當的，但是人們可以瀏覽數據庫直到獲

3、得所需的信息。簡言之， DBMS將“管理”存儲的數據項和從公共數據庫中匯集所需的數據項用以回答非程序員的詢問。DBMS 由 3 個主要部分組成：（1）存儲子系統，用來存儲和檢索文件中的數據 （2）建模和操作子系統，提供組織數據以及添加、刪除、維護、更新數據的方法 （3）用戶和 DBMS之間的接口。在提高數據庫管理系統的價值和有效性方面正在展現以下一些重要發展趨勢：1.管理人員需要最新的信息以做出有效的決策。2.客戶需要越來越復雜的信息服務以及更多的有關其訂單，發票和賬號的當前信息。3.用戶發現他們可以使用傳統的程序設計語言，在很短的一段時間內用數據庫系統開發客戶應用程序。4.商業公司發現了信息

4、的戰略價值，他們利用數據庫系統領先于競爭對手。數據庫模型數據庫模型描述了在數據庫中結構化和操縱數據的方法，模型的結構部分規定了數據如何被描述（例如樹，表等）：模型的操縱部分規定了數據添加、刪除、顯示、維護、打印、查找、選擇、排序和更新等操作。分層模型第一個數據庫管理系統使用的是分層模型，也就是說，將數據記錄排列成樹形結構。一些記錄根目錄，在其他所有記錄中都有獨立的父記錄。樹形結構的設計反映了數據被使用的順序，也就是首先訪問處于樹根位置的記錄，接下來是根下面的記錄等。分層模型的開發是因為分層關系在商業應用中普遍存在。眾所周知，一個組數據庫管理方法，使用表作為數據結構關系數據庫是使用最廣的數據結構

5、織結構圖表就描述了一種分層關系：高層管理人員在高層，中層管理人員在較低的層次，負責具體事務的雇員在底層。值得注意的是，在一個嚴格的分層結構體系中，在每個管理層下可能有多個雇員或多個層次的雇員，但每個雇員只有一個管理者。分層結構數據的典型特征是數據之間的一對多關系。 在分層方法中，當數據庫建立時，每一關系即被明確地定義。在分層數據庫中的每一記錄只能包含一個關鍵字段，任意兩個字段之間只能有一種關系。由于數據并不總是遵循這種嚴格的分層關系，所以這樣可能會出現一些問題。關系模型在 1970 年，數據庫研究取得了重大突破。提出了一種截然不同的,稱之為關系模型 .,數據被組織成關系表 ,每個表由稱作記錄的

6、行和稱作字段的列組成。每個記錄包含了專用項目的字段值。例如，在一個包含雇員信息的表中，一個記錄包含了像一個人姓名和地址這樣的字段的值。結構化查詢語言（）是一種在關系型數據庫中用于處理數據的查詢語言。SQL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。查詢優化器將這種描述翻譯為過程執行數據庫操作。網狀模型網狀模型在數據之間通過鏈接表結構創建關系，子記錄可以鏈接到多個父記錄。這種將記錄和鏈接捆綁到一起的方法叫做指針，它是指向一個記錄存儲位置的存儲地址。使用網狀方法，一個子記錄可以鏈接到一個關鍵記錄，同時，它本身也可以作為一個關鍵記錄鏈接到其他

7、一系列子記錄。在早期，網狀模型比其他模型更有性能優勢，但是在今天，這種優勢的特點只有在自動柜員機網絡，航空預定系統等大容量和高速處理過程中才是最重要的。分層和網狀數據庫都是專用程序，如果開發一個新的應用程序，那么在不同的應用程序中保持數據庫的一致性是非常困難的。例如開發一個退休金程序，需要訪問雇員數據，這一數據同時也被工資單程序訪問。雖然數據是相同的，但是也必須建立新的數據庫。對象模型最新的數據庫管理方法是使用對象模型，記錄由被稱作對象的實體來描述，可以在對象中存儲數據，同時提供方法或程序執行特定的任務。對象模型使用的查詢語言與開發數據庫程序所使用的面向對象的程序設計語言是相同的，因為沒有像

8、SQL這樣簡單統一的查詢語言，所以會產生一些問題。對象模型相對較新，僅有少數幾個面向對象的數據庫實例。它引起了人們的關注，因為選擇面向對象程序設計語言的開發人員希望有一個基于在對象模型基礎上的數據庫。分布式數據庫類似的，分布式數據庫指的是數據庫的各個部分分別存儲在物理上相互分開的計算機上。分布式數據庫的一個目的是訪問數據信息時不必考慮其他位置。注意，一旦用戶和數據分開，通信和網絡則開始扮演重要角色。分布式數據庫需要部分常駐于大型主機上的軟件，這些軟件在大型機和個人計算機之間建立橋梁，并解決數據格式不兼容的問題。在理想情況下，大型主機上的數據庫看起來像是一個大的信息倉庫，而大部分處理則在個人計算

9、機上完成。分布式數據庫系統的一個缺點是它們常以主機中心模型為基礎，在這種模型中，大型主機看起來好像是雇主，而終端和個人計算機看起來好像是奴隸。但是這種方法也有許多優點：由于數據庫的集中控制，前面提到的數據完整性和安全性的問題就迎刃而解。當今的個人計算機，部門級計算機和分布式處理都需要計算機之間以及應用程序之間在相等或對等的基礎上相互通信，在數據庫中客戶機/ 服務器模型為分布式數據庫提供了框架結構。利用相互連接的在計算機上運行的數據庫應用程序的一種方法是將程序分解為相互獨立的部分。客戶端是一個最終用戶或通過網絡申請資源的計算機程序，服務器是一個運行著的計算機軟件，存儲著那些通過網絡傳輸的申請。當

10、申請的資源是數據庫中的數據時，客戶機/ 服務器模型則為分布式數據庫提供了框架結構。文件服務器指的是一個通過網絡提供文件訪問的軟件，專門的文件服務器是一臺被指定為文件服務器的計算機，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，一臺微型計算機或大型主機將被用作文件服務器。分布式文件服務器將文件分散到不同的計算機上，而不是將它們集中存放到專門的文件服務器上。后一種文件服務器擁有在其他計算機上存儲和檢索文件的能力，并可以在每一臺計算機上消除重復文件。然而，一個重要的缺點是每個讀寫請求需要在網絡上傳播，在刷新文件時可能出現問題。假設一個用戶申請文件中的一個數據并修改它，同時另外一

11、個用戶也申請這個數據并修改它，解決這種問題的方法叫做數據鎖定，即第一個申請使其他申請處于等待狀態，直到完成第一個申請，其他用戶可以讀取這個數據，但不能修改。數據庫服務器是一個通過網絡為數據庫申請提供服務的軟件。例如，假設某個用戶在他的個人計算機上輸入了一個數據查詢命令，如果應用程序按照客戶機/ 服務器模型設計，那么個人計算機上的查詢語言通過網絡傳送到數據庫服務器上，當發現數據時發出通知。在工程界也有許多分布式數據庫的例子，如 SUN公司的網絡文件系統（NFS）被應用到計算機輔助工程應用程序中，將數據分散到由 SUN 工作站組成的網絡上的不同硬盤之間。英文原文DatabaseManagement

12、Systems(3th Edition),Wiley,2004,5-12AnIntroductionto DatabaseManagementSystemRaghuRamakrishnanA database (sometimes spelled data base) is also called an electronic database, referring to any collection of data or information, and that is specially organized for rapid search and retrieval by a comput

13、er. 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

14、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 keywo

15、rds 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 ha

16、ndles 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, m

17、anagers 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 quest

18、ions that aren tavailable in regularreports. 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 databas

19、e in response to the queries of those who aren tprogrammers.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

20、, 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:1.Managers: who require more up-to-data information to make effective decision.2.Customers: who demand in

21、creasingly sophisticated information services and more current information about the status of their orders, invoices, and accounts.3.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.4.Organiza

22、tions: that discover information has a strategic value; they utilize their databasesystems to gain an edge over their competitors.TheDatabaseModelA 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

23、(such as tree, tables, andso 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.HierarchicalModelThe first database management systems used a hierarchical model-that is-they arranged records int

24、o 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

25、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 a

26、t 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 rel

27、ationship 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.RelationalMode

28、lA 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 r

29、elated 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 informationon employees, a record would contain fields of data such as a person slast name, first name,and street address.St

30、ructured query language(SQL)isa 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 transla

31、tes the description into a procedure to perform the database manipulation.NetworkModelThe 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 cal

32、led 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 networkmode historically hashad a performance adv

33、antage 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

34、 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.ObjectModelThe newest approach to database management uses an object model, in

35、 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 proble

36、ms 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-orient

37、ed model.DistributedDatabaseSimilarly, 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

38、 and their data being 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. I

39、deally, 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 amainframe-entiremodel, in which the larger host computer

40、is seen as the master and theterminal or personal computer is seen as a slave. There are some advantages to this approach.With databases undercentralized control, many of the problems of data integritythatwementioned earlier are solved. But today spersonal computers, departmentalcomputers, anddistri

41、buted processing require computers and their applications to communicate with each otheron a more equal or peer-to-peerbasis. In a database, the client/servermodel provides theframework for distributing databases.One way to take advantage of many connected computers running database applications is

42、to distribute the application into cooperating parts that are independent of one anther. A clientis 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 databa

43、se,the client/server model provides the framework for distributing database.A file server 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.