计算机专业英语6.docx

计算机专业英语6.docx

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计算机专业英语6

Unit6Database

SectionA

DatabaseOverview

I.Introduction

Datastoragetraditionallyusedindividual,unrelatedfiles,sometimescalledHatfiles.Inthepast,eachapplicationprograminanorganizationuseditsownfile.Inauniversity,forexample,eachdepartmentmighthaveitsownsetoffiles:

therecordofficekeptafileaboutthestudentinformationandtheirgrades,thefinancialaidofficekeptitsownfileaboutstudentsthatneededfinancialaidtocontinuetheireducation,theschedulingofficekeptthenamesoftheprofessorsandthecoursestheywereteaching,thepayrolldepartmentkeptitsownfileaboutthewholestaff（includingprofessors）,andsoon.Today,however,alloftheseflatfilescanbecombinedinasingleentity,thedatabaseforthewholeuniversity.

Althoughitisdifficulttogiveauniversallyagreeddefinitionofadatabase,weusethefollowingcommondefinition:

adatabaseisacollectionofrelated,logicallycoherent,datausedbytheapplicationprogramsinanorganization.

II.DatabaseManagementSystems

Adatabasemanagementsystem（DBMS）defines,create,andmaintainsadatabase.TheDBMSalsoallowscontrolledaccesstodatainthedatabase.ADBMSisacombinationoffivecomponents:

hardware,software,data,users,andprocedures.

1.Hardware

Thehardwareisthephysicalcomputersystemthatallowsaccesstodata.Forexample,theterminals,harddisk,maincomputer,andworkstationsareconsideredpartofthehardwareinaDBMS.

2.Software

Thesoftwareistheactualprogramthatallowsuserstoaccess,maintain,andupdatedata.Inaddition,thesoftwarecontrolswhichusercanaccesswhichpartsofthedatainthedatabase.

3.Data

Thedatainadatabaseisstoredphysicallyonthestoragedevice.Inadatabase,dataisaseparateentityfromthesoftwarethataccessesit.Thisseparationallowsanorganizationtochangethesoftwarewithouthavingtochangethephysicaldataorthewayinwhichitisstored.IfanorganizationdecidestouseaDBMS,thenalltheinformationneededbytheorganizationshouldbekepttogetherasoneentity,tobeaccessiblebythesoftwareintheDBMS.

4.Users

ThetermusersinaDBMShasabroadmeaning.Wecandivideusersintotwocategories:

endusersandapplicationprograms.

Endusersarethosehumanswhocanaccessthedatabasedirectlytogetinformation.Therearetwotypesofendusers:

databaseadministrators（DBAs）andnormalusers.DatabaseadministratorshavethemaximumlevelofprivilegesandcancontrolotherusersandtheiraccesstotheDBMS,grantsomeoftheirprivilegestosomebodyelse,butretaintheabilitytorevokethematanytime.Anormaluser,ontheotherhand,canonlyusepartofthedatabaseandhaslimitedaccess.

Theotherusersofdatainadatabaseareapplicationprograms.Applicationsneedtoaccessandprocessdata.Forexample,apayrollapplicationprogramneedstoaccesspartofthedatainadatabasetocreatepaychecksattheendofthemonth.

5.Procedures

ThelastcomponentofaDBMSisasetofproceduresorrulesthatshouldbeclearlydefinedandfollowedbytheusersofthedatabase.

III.DatabaseArchitecture

TheAmericanNationalStandardsInstituteStandardsPlanningandRequirementsCommittee（ANSI/SPARC）hasestablishedathree-levelarchitectureforaDBMS:

internal,conceptual,andexternal（Figure6A-1）.

1.InternalLevel

Theinternalleveldetermineswheredataisactuallystoredonthestoragedevices.Thisleveldealswithlow-levelaccessmethodsandhowbytesaretransferredtoandfromstoragedevices.Inotherwords,theinternallevelinteractsdirectlywiththehardware.

2.ConceptualLevel

Theconceptualleveldefinesthelogicalviewofthedata.Thedatamodelisdefinedonthislevel,andthemainfunctionsoftheDBMS,suchasqueries,arealsoonthislevel.TheDBMSchangestheinternalviewofdatatotheexternalviewthatusersneedtosee.Theconceptuallevelisanintermediaryandfreesusersfromdealingwiththeinternallevel.

3.ExternalLevel

Theexternallevelinteractsdirectlywiththeuser（enduserorapplicationprograms）.Itchangesthedatacomingfromtheconceptualleveltoaformatandviewthatisfamiliartotheusers.

IV.DatabaseModels

Adatabasemodeldefinesthelogicaldesignofdata.Themodelalsodescribestherelationshipsbetweendifferentpartsofthedata.Inthehistoryofdatabasedesign,threemodelshavebeeninuse:

thehierarchicalmodel,thenetworkmodel,andtherelationalmodel.

1.HierarchicalDatabaseModel

Inthehierarchicalmodel,dataisorganizedasaninvertedtree.Eachentityhasonlyoneparentbutcanhaveseveralchildren.Atthetopofthehierarchy,thereisoneentity,whichiscalledtheroot.Figure6A-2showsalogicalviewofanexampleofthehierarchicalmodel.Thehierarchicalmodelisnowobsolete.

2.NetworkDatabaseModel

Inthenetworkmodel,theentitiesareorganizedinagraph,inwhichsomeentitiescanbeaccessedthroughseveralpaths（Figure6A-3）.Thereisnohierarchy.Thismodelisnowalsoobsolete.

3.RelationalDatabaseModel

Intherelationalmodel,dataisorganizedintwo-dimensionaltablescalledrelations.Thereisnohierarchicalornetworkstructureimposedonthedata.Thetablesorrelationsare,however,relatedtoeachother（Figure6A-4）.Therelationaldatabasemanagementsystem（RDBMS）organizesthedatasothatitsexternalviewisasetofrelationsortables.Thisdoesnotmeanthatdataisstoredastables:

thephysicalstorageofthedataisindependentofthewayinwhichthedataislogicallyorganized.Figure6A-5showsanexampleofarelation.ArelationinanRDBMShas

thefollowingfeatures:

●Name.Eachrelationinarelationaldatabaseshouldhaveanamethatisuniqueamongotherrelations.

●Attributes.Eachcolumninarelationiscalledanattribute.TheattributesarethecolumnheadingsinthetableinFigure6A-5.Eachattributegivesmeaningtothedatastoredunderit.Eachcolumninthetablemusthaveanamethatisuniqueinthescopeoftherelation.Thetotalnumberofattributesforarelationiscalledthedegreeoftherelation.Forexample,inFigure6A-5,therelationhasadegreeof3.Notethattheattributenamesarenotstoredinthedatabase:

theconceptuallevelusestheattributestogivemeaningtoeachcolumn.

●Tuples.Eachrowinarelationiscalledatuple.Atupledefinesacollectionofattributevalues.Thetotalnumberofrowsinarelationiscalledthecardinalityoftherelation.Notethatthecardinalityofarelationchangeswhentuplesareaddedordeleted.Thismakesthedatabasedynamic.

Therelationalmodelisoneofthecommonmodelsinusetoday.Theothertwocommonmodelsthatarederivedfromtherelationalmodelarethedistributedmodelandtheobject-orientedmodel.

4.DistributedDatabaseMode）

Thedistributeddatabasemodelisnotanewmodel,butisbasedontherelationalmodel.However,thedataisstoredonseveralcomputersthatcommunicatethroughtheInternetoraprivatewideareanetworkEachcomputer（orsite）maintainseitherpartofthedatabaseorthewholedatabase.Inotherwords,dataiseitherfragmented,witheachfragmentstoredatonesite,ordataisreplicatedateachsite.

Inafragmenteddistributeddatabase,dataislocalized—locallyuseddataisstoredatthecorrespondingsite.However,thisdoesnotmeanthatasitecannotaccessdatastoredatanothersite,butaccessismostlylocal,butoccasionallyglobal.Althougheachsitehascompletecontroloveritslocaldata,thereisglobalcontrolthroughtheInternetorawideareanetwork.

Forexample,apharmaceuticalcompanymayhavemultiplesitesinmanycountries.Eachsitehasadatabasewithinformationaboutitsownemployees,butacentralpersonneldepartmentcouldhavecontrolofallthedatabases.

Inareplicateddistributeddatabase,eachsiteholdsanexactreplicaofanothersite.Anymodificationtodatastoredinonesiteisrepeatedexactlyateverysite.Thereasonforhavingsuchadatabaseissecurity.Ifthesystematonesitefails,usersatthesitecanaccessdataatanothersite.

5.Object-OrientedDatabaseModel

Therelationaldatabasehasaspecificviewofdatathatisbasedonthenatureofthedatabase'stuplesandattributes.Thesmallestunitofdatainarelationaldatabaseistheintersectionofatupleandanattribute.However,someapplicationsneedtolookatdataasotherforms,forexa.'dataasastructure,suchasarecordcomposedoffields.

Anobject-orienteddatabasetriestokeeptheadvantagerelationalmodelandatthesametimeallowsapplications|structureddata.Inanobject-orienteddatabase,objectsandtheiraredefined.Inaddition,eachobjectcanhaveattributesthaexpressedasfields.

Forexample,inanorganization,onecoulddefineobject!

employee,department,andcustomer.Theemployeeclasscoulddattributesofanemployeeobject（firstname,lastname,socialnumber,salary,andsoon）andhowtheycanbeaccessed.Thedeobjectcoulddefinetheattributesofthedepartmentandhowthe]accessed.Inaddition,thedatabasecouldcreatearelationbeemployeeobjectandadepartmentobjecttodenotethattheworksinthatdepartment.

V.DatabaseDesign

Thedesignofanydatabaseisalengthyandinvolvedtaskthatabedonethroughastep-by-stepprocess.Thefirststepnormallyinvilotofinterviewingofpotentialusersofthedatabasetocollainformationneededtobestoredandtheaccessrequirementodepartment.Thesecondstepistobuildanentity-relationshipmodel（thatdefinestheentitiesforwhichsomeinformationmustbemaintainattributesoftheseentities,andtherelationshipbetweentheseentities.

Thenextstepindesignisbasedonthetypeofdat