數(shù)據(jù)庫 外文翻譯 外文文獻 英文文獻 數(shù)據(jù)庫管理系統(tǒng)的介紹.doc

a introduction to database management system raghu ramakrishnana database sometimes spelled data base is also called an electronic database referring to any collection of data or information that is specially organized for rapidsearch and retrieval by a computer. databases are structured to facilitate the storageretrieval modification and deletion of data in conjunction with various data-processingoperations .databases can be stored on magnetic disk or tape optical disk or some othersecondary storage device.a database consists of a file or a set of files. the information in these files may be brokendown into records each of which consists of one or more fields. fields are the basic unitsof data storage and each field typically contains information pertaining to one aspector attribute of the entity described by the database . using keywords and various sortingcommands users can rapidly search rearrange group and select the fields in manyrecords to retrieve or create reports on particular aggregate of data.complex data relationships and linkages may be found in all but the simplestdatabases .the system software package that handles the difficult tasks associated withcreating accessing and maintaining database records is called a database managementsystemdbms.the programs in a dbms package establish an interface between thedatabase itself and the users of the database. these users may be applicationsprogrammers 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 inorder to extract answers to nonrecurring and unplanned questions that arent availablein regular reports. these questions might initially be vague and/or poorly defined butpeople can “browse” through the database until they have the needed information. inshort the dbms will “manage” the stored data items and assemble the needed itemsfrom the common database in response to the queries of those who arentprogrammers.a database management system dbms is composed of three major parts:1a storagesubsystem that stores and retrieves data in files2 a modeling and manipulationsubsystem that provides the means with which to organize the data and to add deletemaintain and update the data3and an interface between the dbms and its users.several major trends are emerging that enhance the value and usefulness of databasemanagement systemsmanagers: who require more up-to-data information to make effective decisioncustomers: who demand increasingly sophisticated information services and morecurrent information about the status of their orders invoices and accounts.users: who find that they can develop custom applications with database systems in afraction of the time it takes to use traditional programming languages.organizations : that discover information has a strategic value they utilize their databasesystems to gain an edge over their competitors.the database modela data model describes a way to structure and manipulate the data in a database. thestructural part of the model specifies how data should be representedsuch as tree tablesand so on .the manipulative part of the model specifies the operation with which to adddelete display maintain print search select sort and update the data.hierarchical modelthe first database management systems used a hierarchical model-that is-they arrangedrecords into a tree structure. some records are root records and all others have uniqueparent records. the structure of the tree is designed to reflect the order in which the datawill be used that is the record at the root of a tree will be accessed first then records onelevel below the root and so on.the hierarchical model was developed because hierarchical relationships are commonlyfound in business applications. as you have known an organization char often describesa hierarchical relationship: top management is at the highest level middle managementat lower levels and operational employees at the lowest levels. note that within a stricthierarchy each level of management may have many employees or levels of employeesbeneath it but each employee has only one manager. hierarchical data are characterizedby this one-to-many relationship among data.in the hierarchical approach each relationship must be explicitly defined when thedatabase is created. each record in a hierarchical database can contain only one key fieldand only one relationship is allowed between any two fields. this can create a problembecause data do not always conform to such a strict hierarchy.relational modela major breakthrough in database research occurred in 1970 when e. f. codd proposeda fundamentally different approach to database management called relationalmodel which uses a table as its data structure.the relational database is the most widely used database structure. data is organized intorelated tables. each table is made up of rows called and columns called fields. eachrecord contains fields of data about some specific item. for example in a table containinginformation on employees a record would contain fields of data such as a persons lastname first name and street address.structured query languagesqlis a query language for manipulating data in a relationaldatabase .it is nonprocedural or declarative in which the user need only specify anenglish-like description that specifies the operation and the described record orcombination of records. a query optimizer translates the description into a procedure toperform the database manipulation.network modelthe network model creates relationships among data through a linked-list structure inwhich subordinate records can be linked to more than one parent record. this approachcombines records with links which are called pointers. the pointers are addresses thatindicate the location of a record. with the network approach a subordinate record can belinked to a key record and at the same time itself be a key record linked to other sets ofsubordinate records. the network mode historically has had a performance advantageover other database models. today such performance characteristics are only importantin high-volume high-speed transaction processing such as automatic teller machinenetworks or airline reservation system.both hierarchical and network databases are application specific. if a new application isdeveloped maintaining the consistency of databases in different applications can bevery difficult. for example suppose a new pension application is developed .the data arethe same but a new database must be created.object modelthe newest approach to database management uses an object model in which recordsare represented by entities called objects that can both store data and provide methods orprocedures to perform specific tasks.the query language used for the object model is the same object-oriented programminglanguage used to develop the database application .this can create problems becausethere is no simple uniform query language such as sql . the object model is relativelynew and only a few examples of object-oriented database exist. it has attracted attentionbecause developers who choose an object-oriented programming language want adatabase based on an object-oriented model.distributed databasesimilarly a distributed database is one in which different parts of the database reside onphysically separated computers . one goal of distributed databases is the access ofinformation without regard to where the data might be stored. keeping in mind that oncethe users and their data are separated the communication and networking conceptscome into play .distributed databases require software that resides partially in the larger computer. thissoftware bridges the gap between personal and large computers and resolves theproblems of incompatible data formats. ideally it would make the mainframe databasesappear to be large libraries of information with most of the processing accomplished onthe personal computer.a drawback to some distributed systems is that they are often based on what is called amainframe-entire model in which the larger host computer is seen as the master and theterminal or personal computer is seen as a slave. there are some advantages to thisapproach . with databases under centralized control many of the problems of dataintegrity that we mentioned earlier are solved . but todays personal computersdepartmental computers and distributed processing require computers and theirapplications to communicate with each other on a more equal or peer-to-peer basis. in adatabase the client/server model provides the framework for distributing databases.one way to take advantage of many connected computers running databaseapplications is to distribute the application into cooperating parts that are independent ofone anther. a client is an end user or computer program that requests resources across anetwork. a server is a computer running software that fulfills those requests across anetwork . when the resources are data in a database the client/server model provides theframework for distributing database.a file serve is software that provides access to files across a network. a dedicated fileserver is a single computer dedicated to being a file server. this is useful for example ifthe files are large and require fast access .in such cases a minicomputer or mainframewould be used as a file server. a distributed file server spreads the files around onindividual computers instead of placing them on one dedicated computer.advantages of the latter server include the ability to store and retrieve files on othercomputers and the elimination of duplicate files on each computer. a majordisadvantage however is that individual read/write requests are being moved acrossthe network and problems can arise when updating files. suppose a user requests arecord from a file and changes it while another user requests the same record andchanges it too. the solution to this problems called record locking which means that thefirst request makes others requests wait until the first request is satisfied . other usersmay 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. forexample suppose a user types in a query for data on his or her personal computer . if theapplication is designed with the client/server model in mind the query language part onthe personal computer simple sends the query across the network to the database serverand requests to be notified when the data are found.examples of distributed database systems can be found in the engineering world. sunsnetwork filing systemnfsfor example is used in computer-aided engineeringapplications 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 existat the location where they are being used . departmental computers within a largecorporation for example should have data reside locally yet those data should beaccessible by authorized corporate management when they want to consolidatedepartmental 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 thenon-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)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)庫用于描述實體，其中的一個字段通常表示與實體的某一屬性相關(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行查詢，重新整理，分組或選擇，以實體對某一類數(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）。 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?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，并從公共數(shù)據(jù)庫中匯集所需的數(shù)據(jù)項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，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)庫中數(shù)據(jù)的性質(zhì) 1.數(shù)據(jù)整體性：數(shù)據(jù)庫是一個單位或是一個應(yīng)用領(lǐng)域的通用數(shù)據(jù)處理系統(tǒng)，他存儲的是屬于企業(yè)和事業(yè)部門、團體和個人的有關(guān)數(shù)據(jù)的集合。數(shù)據(jù)庫中的數(shù)據(jù)是從全局觀點出發(fā)建立的，他按一定的數(shù)據(jù)模型進行組織、描述和存儲。其結(jié)構(gòu)基于數(shù)據(jù)間的自然聯(lián)系，從而可提供一切必要的存取路徑，且數(shù)據(jù)不再針對某一應(yīng)用，而