外文翻譯《數(shù)據(jù)庫管理系統(tǒng)》(共10頁)

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1、精選優(yōu)質(zhì)文檔-----傾情為你奉上 A introduction to Database Management System Raghu Ramakrishnan 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. Data

2、bases 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 fi

3、les. 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 keyword

4、s 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

5、 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 programme

6、rs, 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 unplan

7、ned 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, the DBMS will “manage” the stored data items and assemble the needed items from the

8、common database in response to the queries of those who aren’t 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 t

9、he 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 decision Custom

10、ers: 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 la

11、nguages. Organizations : that discover information has a strategic value; they utilize their database systems to gain an edge over their competitors. The Database Model A data model describes a way to structure and manipulate the data in a database. The structural part of the model specifies how

12、 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 mode

13、l-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

14、 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 lo

15、wer 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

16、. 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

17、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 widel

18、y 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

19、 person’s 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 r

20、ecord or combination of records. A query optimizer translates the description into a procedure to perform the database manipulation. Network Model The network model creates relationships among data through a linked-list structure in which subordinate records can be linked to more than one parent r

21、ecord. 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 subordinat

22、e 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 hierarchi

23、cal 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. O

24、bject 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

25、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 c

26、hoose an object-oriented programming language want a database based on an object-oriented model. Distributed Database Similarly , 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 in

27、formation 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

28、 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

29、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 da

30、ta integrity that we mentioned earlier are solved . But today’s 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 fr

31、amework 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.

32、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 serv

33、er 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 plac

34、ing 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

35、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 r

36、equest 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

37、 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. S

38、un’s 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 u

39、sed . 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

40、 the distributed database will appear to its users as no different from the non-distributed database . 數(shù)據(jù)庫管理系統(tǒng)的介紹 Raghu Ramakrishnan 數(shù)據(jù)庫（database,有時拼作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，檢索，修改和刪除。數(shù)

41、據(jù)庫可以存儲在磁盤，磁帶，光盤或其他輔助存儲設(shè)備上。 數(shù)據(jù)庫由一個或一套文件組成，其中的信息可以分解為記錄，每一記錄又包含一個或多個字段（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，也可以生成報表。 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

42、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)報告中沒有的，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。 DBMS由3個主要部分組成：（1）存儲子系統(tǒng)，用來存儲和檢索文件中的數(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

43、數(shù)據(jù)庫管理系統(tǒng)的價值和有效性方面正在展現(xiàn)以下一些重要發(fā)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(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)計語言，在很短的一段時間內(nèi)用數(shù)據(jù)庫系統(tǒng)開發(fā)客戶應(yīng)用程序。 4.商業(yè)公司發(fā)現(xiàn)了信息的戰(zhàn)略價值，他們利用數(shù)據(jù)庫系統(tǒng)領(lǐng)先于競爭對手。 數(shù)據(jù)庫模型 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。 分層模型 第一個數(shù)據(jù)庫管理系統(tǒng)使

44、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。 分層模型的開發(fā)是因?yàn)榉謱雨P(guān)系在商業(yè)應(yīng)用中普遍存在，眾所周知，一個組織結(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，在一個嚴(yán)格的分層結(jié)構(gòu)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(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。 在分層方法中，當(dāng)數(shù)據(jù)庫建立時，每一關(guān)系即被明確地定義。在

45、分層數(shù)據(jù)庫中的每一記錄只能包含一個關(guān)鍵字段，任意兩個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，所以這樣可能會出現(xiàn)一些問題。 關(guān)系模型 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. 關(guān)系數(shù)據(jù)庫是使用最廣的數(shù)據(jù)結(jié)構(gòu),數(shù)據(jù)被組織成關(guān)系表,每個表由稱作記錄的行和稱作字段的列組成。每個記錄包含了專用項(xiàng)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(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

46、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。 網(wǎng)狀模型 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，他是指向一個記錄存儲位置的存儲地址。使用網(wǎng)狀方法，一個子記錄可以鏈接到一個關(guān)鍵記錄，同時，它本身也可以作為一個關(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；但是在今天，這種優(yōu)勢的特點(diǎn)只有在自動柜員機(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。 分層和網(wǎng)狀數(shù)據(jù)庫都是專用程序，如果開發(fā)一個新的應(yīng)用程序，那么在不同的

47、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)一個退休金程序，需要訪問雇員數(shù)據(jù)，這一數(shù)據(jù)同時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。 對象模型 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，可以在對象中存儲數(shù)據(jù)，同時提供方法或程序執(zhí)行特定的任務(wù)。 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，所以會產(chǎn)生一些問題。對象模型相對較新，僅有少數(shù)幾個面向?qū)ο蟮臄?shù)據(jù)庫實(shí)例。它引起了人們的關(guān)注，因?yàn)檫x擇面向?qū)ο蟪绦蛟O(shè)計語言的開發(fā)人員希望有一個基于在對象模型基礎(chǔ)上的數(shù)據(jù)庫。 分布式

48、數(shù)據(jù)庫 類似的，分布式數(shù)據(jù)庫指的是數(shù)據(jù)庫的各個部分分別存儲在物理上相互分開的計算機(jī)上。分布式數(shù)據(jù)庫的一個目的是訪問數(shù)據(jù)信息時不必考慮其他位置。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。 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)和個人計算機(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)庫看起來像是一個大的信息倉庫，而大部分處理則在個人計算機(jī)上完成。 分布式數(shù)據(jù)庫系統(tǒng)的一個缺點(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，而終端和個人計算機(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)庫的集中控制，前面

49、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)今的個人計算機(jī)，部門級計算機(jī)和分布式處理都需要計算機(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)。 利用相互連接的計算機(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?？蛻舳耸且粋€最終用戶或通過網(wǎng)絡(luò)申請資源的計算機(jī)程序，服務(wù)器是一個運(yùn)行著的計算機(jī)軟件，存儲著那些通過網(wǎng)絡(luò)傳輸?shù)纳暾垺．?dāng)申請的資源是數(shù)據(jù)庫中的數(shù)據(jù)時，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)。 文件服務(wù)器指的是一個通過網(wǎng)絡(luò)提供文件訪問的軟件，專門的文件服務(wù)器是一臺被指定為文件服務(wù)器的計算機(jī)。這是非常有

50、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(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算機(jī)上，而不是將它們集中存放到專門的文件服務(wù)器上。 后一種文件服務(wù)器的優(yōu)點(diǎn)包括在其他計算機(jī)上存儲和檢索文件的能力，并可以在每一臺計算機(jī)上消除重復(fù)文件。然而，一個重要的缺點(diǎn)是每個讀寫請求需要在網(wǎng)絡(luò)上傳播，在刷新文件時可能出現(xiàn)問題。假設(shè)一個用戶申請文件中的一個數(shù)據(jù)并修改它，同時另外一個用戶也申請這個數(shù)據(jù)并修改它，解決這種問題的方法叫做數(shù)據(jù)鎖定，即第一個申請使其他申請?zhí)幱诘却隣顟B(tài)，直到完成第一個申請，其他用戶可以讀取這個數(shù)據(jù)，但不能修改。 數(shù)據(jù)庫服務(wù)器是一個通

51、過網(wǎng)絡(luò)為數(shù)據(jù)庫申請?zhí)峁┓?wù)的軟件，例如，假設(shè)某個用戶在他的個人計算機(jī)上輸入了一個數(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，那么個人計算機(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)時發(fā)出通知。 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)用到計算機(jī)輔助工程應(yīng)用程序中，將數(shù)據(jù)分散到由SUN工作站組成的網(wǎng)絡(luò)上的不同硬盤之間。 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。例如一個大公司不同部門之間的計算機(jī)，應(yīng)該將數(shù)據(jù)存儲在本地，然而，當(dāng)被授權(quán)的管理人員需要整理部門數(shù)據(jù)時，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。 專心---專注---專業(yè)