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关系操作的特点是( )。
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关系操作的特点是______操作
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OneoftheguidelinesinwritingtheCcodeforasoftwaretoolisasfollows:writecodethatisasclearandassimplea76.TheClanguagecanbedifficulttoreadifyoucombineall77featuresinasinglestatement.Breakcomplicated78intoseveraleasytounderstandstatementsforthe79ofreadability.Thisstylehelpstomakeyourprogramsmore80anderror-free. 79
Therelationaldatabasemodelrequiresthedatabe110throughprogramsthatdon’trelyonthepositionofthedatainthedatabase.Thisisindirect111totheotherdatabasemodewheretheprogramhastofollowaseriesofpointerstothedataitseeks.Aprogram112arelationaldatabasesimplyasksforthedataitseeks;theDBMSperformsthenecessarysearchesand113theinformation.The114onhowthesearchisdonearespecifictotheDBMSandvaryfromproducttoproduct. 114是
Alocal-areanetworkLANisacommunicationsnetworkthat125avarietyofdevicesandprovidesa126forinformationexchangeamongthosedevices.ThescopeoftheLANissmalltypicallyasinglebuildingoracluster丛ofbuildings.TheLANisusuallyownedbythesameorganizationthatownsthe127devices.TheinternaldatarateofLANisatleastseveralMb/s. ThebasicofLANcommunicationisbroadcasting.Ateachstationthereisatransmitter/receiverthatcommunicatesovera128sharedByotherstations.Atransmissionfromanyonestationis129toandreceivedbyallotherstations. 128是
Thegrowthofswitchinghas135anewgenerationofnetworkmanagementtoolsthathelp136copewiththechallenges137themtocorrelatedevicealarmsinorderto138pinpointrootcausesortomonitorservicelevelswithoutdependingonIPsubnetschemes.Thesenewtoolshavebecomeessentialtocopingwiththeprimary139effectofanytreatmentthatsignificantlyincreasesnetworkflexibility:addedcomplexity. 138
Alocal-areanetworkLANisacommunicationsnetworkthat125avarietyofdevicesandprovidesa126forinformationexchangeamongthosedevices.ThescopeoftheLANissmalltypicallyasinglebuildingoracluster丛ofbuildings.TheLANisusuallyownedbythesameorganizationthatownsthe127devices.TheinternaldatarateofLANisatleastseveralMb/s. ThebasicofLANcommunicationisbroadcasting.Ateachstationthereisatransmitter/receiverthatcommunicatesovera128sharedByotherstations.Atransmissionfromanyonestationis129toandreceivedbyallotherstations. 126是
Networkmanagershavelong140practicalvoice-over-IPVOIPsolutions.VOIP141easenetworkmanagementanddecreasescostsbyconvergingacompany’stelephonyanddatainfrastructuresintoonenetwork.AndaVOIPsolutionimplementedatacompany’shead-quarterswithfar-reachingbranchofficescan142tremendousamountsof143inlongdistancephonebillsprovidedthatsolutiondeliversPOTS-likevoice144overtheInternet. 143是
OriginallyintroducedbyNetscapeCommunications21areageneralmechanismwhichHTTPServersideapplicationssuchasCGI22canusetobothstoreandretrieveinformationontheHTTP23sideoftheconnection.BasicallyCookiescanbeusedtocompensateforthe24natureofHTTP.Theadditionofasimplepersistentclient-sidestatesignificantlyextendsthecapabilitiesofWWW-based25. 21
In85softwareonequestionofnear-universalinterestishowrapidlyWindowsNTwhichbeganshippinglastsummerwillbe86inthemarketplaceandforwhatuses.Forthemostpartobserversare87abouttheoperatingsystem’slongtermprospectsbutexpectittoremainon88ratherthandesktopsduring1994sayingthatitwon’tbecomeamainstreamproductuntil89PCstypicallyhave16to24MBofRAMwhichisunlikelytohappennextyear. 85是
OriginallyintroducedbyNetscapeCommunications21areageneralmechanismwhichHTTPServersideapplicationssuchasCGI22canusetobothstoreandretrieveinformationontheHTTP23sideoftheconnection.BasicallyCookiescanbeusedtocompensateforthe24natureofHTTP.Theadditionofasimplepersistentclient-sidestatesignificantlyextendsthecapabilitiesofWWW-based25. 25
Networkmanagershavelong140practicalvoice-over-IPVOIPsolutions.VOIP141easenetworkmanagementanddecreasescostsbyconvergingacompany’stelephonyanddatainfrastructuresintoonenetwork.AndaVOIPsolutionimplementedatacompany’shead-quarterswithfar-reachingbranchofficescan142tremendousamountsof143inlongdistancephonebillsprovidedthatsolutiondeliversPOTS-likevoice144overtheInternet. 141是
TheRationalUnifiedProcessRUPisasoftwareengineeringprocesswhichcapturesmanyofbestpracticesinmodemsoftwaredevelopment.Thenotionsof36andscenarioshavebeenproventobeanexcellentwaytocapturefunctionrequirements.RUPcanbedescribedintwodimensions-timeandcontent.Inthetimedimensionthesoftwarelifecycleisbrokenintocycles.Eachcycleisdividedintofourconsecutive37whichisconcludedwithawell-defined38andcanbefurtherbrokendowninto39-acompletedevelopmentloopresultinginareleaseofanexecutableproductasubsetofthefinalproductunderdevelopmentwhichgrowsincrementallytobecomethefinalsystem.Thecontentstructurereferstothedisciplineswhichgroup40logicallybynature. 37
OriginallyintroducedbyNetscapeCommunications21areageneralmechanismwhichHTTPServersideapplicationssuchasCGI22canusetobothstoreandretrieveinformationontheHTTP23sideoftheconnection.BasicallyCookiescanbeusedtocompensateforthe24natureofHTTP.Theadditionofasimplepersistentclient-sidestatesignificantlyextendsthecapabilitiesofWWW-based25. 23
Object-orientedDBMSintegratedavarietyof81datatypes-suchasbusinessproceduresgraphicspicturesvoiceandannotatedtext. Objectorientationalsomakesa82toapplicationdevelopmentefficiency.Itmakesthedatafunctionsattributesandrelationshipsanintegralpartofthe83.Inthiswayobjectscanbereusedandreplicated. SomeleadingRDBMSvendorssupporttheconceptofintegratingobjectmanagementcapabilitieswiththeircurrentlineofrelationalproducts.Thatcapabilityenableuserstothedevelopmentcyclesinceintegritylogicandbusinessrolesnolongerneedtobeprogrammed84eachapplication. 82是
100whethertogowithasharedorswitchedfastLANtechnologyonceyoustarttomaxoutyoursharedEthernetoftoken-ringLANistoughcall.Usersandanalystsinvolvedin101high-speedLANshavelearnedthebenefitsofeach.Howevertherearetrade-offstoeachand102whichfastLANtechnologytochooseiscriticalfor103networkperformanceproblemsand104costlypurchasingmistakes. 102
Withthewidespreaduseofthepersonalcomputermanyauthoritiesinthefieldof56havepointedouttheneedforcomputerliteracy. Unfortunatelythereisno57agreementastowhattheterm"computerliteracy"means.Somefeelthatcomputerliteracymeansknowinghowtomakethecomputer"compute";thatisknowinghowtoprogramcomputersinoneormoreprogramminglanguages. Othersfeelthatknowinghowtoprogramismerelyasmallsegmentofcomputerliteracy.Thesepeople58themajoremphasisinschoolsshouldbeonteachinghowtoeffectivelyusethemanysoftwarepackagesthatareavailable. Stillotherssuggestthatcomputerliteracyeducationisnotrequired.Theysuggestthatcomputersarebeingsorapidlyintegratedintooursocietythatusingacomputerwillbeas59asusingatelephoneoravideotaperecorderandthatspecialeducationwillnotbenecessary.60ofone’sdefinitionofcomputerliteracyitisrecognizedbymostthatlearningtouseacomputerisindeedanimportantskillinmodernsociety. 58
31analysisemphasizesthedrawingofpictorialsystemmodelstodocumentandvalidatebothexistingand/orproposedsystems.Ultimatelythesystemmodelsbecomethe32fordesigningandconstructinganimprovedsystem.33issuchatechnique.Theemphasisinthistechniqueisprocess-centered.Systemsanalystsdrawaseriesofprocessmodelscalled34.35isanothersuchtechniquethatintegratesdataandprocessconcernsintoconstructscalledobjects. 31是
Withthewidespreaduseofthepersonalcomputermanyauthoritiesinthefieldof56havepointedouttheneedforcomputerliteracy. Unfortunatelythereisno57agreementastowhattheterm"computerliteracy"means.Somefeelthatcomputerliteracymeansknowinghowtomakethecomputer"compute";thatisknowinghowtoprogramcomputersinoneormoreprogramminglanguages. Othersfeelthatknowinghowtoprogramismerelyasmallsegmentofcomputerliteracy.Thesepeople58themajoremphasisinschoolsshouldbeonteachinghowtoeffectivelyusethemanysoftwarepackagesthatareavailable. Stillotherssuggestthatcomputerliteracyeducationisnotrequired.Theysuggestthatcomputersarebeingsorapidlyintegratedintooursocietythatusingacomputerwillbeas59asusingatelephoneoravideotaperecorderandthatspecialeducationwillnotbenecessary.60ofone’sdefinitionofcomputerliteracyitisrecognizedbymostthatlearningtouseacomputerisindeedanimportantskillinmodernsociety. 56
100whethertogowithasharedorswitchedfastLANtechnologyonceyoustarttomaxoutyoursharedEthernetoftoken-ringLANistoughcall.Usersandanalystsinvolvedin101high-speedLANshavelearnedthebenefitsofeach.Howevertherearetrade-offstoeachand102whichfastLANtechnologytochooseiscriticalfor103networkperformanceproblemsand104costlypurchasingmistakes. 100
100whethertogowithasharedorswitchedfastLANtechnologyonceyoustarttomaxoutyoursharedEthernetoftoken-ringLANistoughcall.Usersandanalystsinvolvedin101high-speedLANshavelearnedthebenefitsofeach.Howevertherearetrade-offstoeachand102whichfastLANtechnologytochooseiscriticalfor103networkperformanceproblemsand104costlypurchasingmistakes. 104
Therelationaldatabasemodelrequiresthedatabe110throughprogramsthatdon’trelyonthepositionofthedatainthedatabase.Thisisindirect111totheotherdatabasemodewheretheprogramhastofollowaseriesofpointerstothedataitseeks.Aprogram112arelationaldatabasesimplyasksforthedataitseeks;theDBMSperformsthenecessarysearchesand113theinformation.The114onhowthesearchisdonearespecifictotheDBMSandvaryfromproducttoproduct. 112是
OneoftheguidelinesinwritingtheCcodeforasoftwaretoolisasfollows:writecodethatisasclearandassimplea76.TheClanguagecanbedifficulttoreadifyoucombineall77featuresinasinglestatement.Breakcomplicated78intoseveraleasytounderstandstatementsforthe79ofreadability.Thisstylehelpstomakeyourprogramsmore80anderror-free. 77
In85softwareonequestionofnear-universalinterestishowrapidlyWindowsNTwhichbeganshippinglastsummerwillbe86inthemarketplaceandforwhatuses.Forthemostpartobserversare87abouttheoperatingsystem’slongtermprospectsbutexpectittoremainon88ratherthandesktopsduring1994sayingthatitwon’tbecomeamainstreamproductuntil89PCstypicallyhave16to24MBofRAMwhichisunlikelytohappennextyear. 87是
In85softwareonequestionofnear-universalinterestishowrapidlyWindowsNTwhichbeganshippinglastsummerwillbe86inthemarketplaceandforwhatuses.Forthemostpartobserversare87abouttheoperatingsystem’slongtermprospectsbutexpectittoremainon88ratherthandesktopsduring1994sayingthatitwon’tbecomeamainstreamproductuntil89PCstypicallyhave16to24MBofRAMwhichisunlikelytohappennextyear. 89是
Thepurposeoftherequirementsdefinitionphaseistoproduceaclearcompleteconsistentandtestable6ofthetechnicalrequirementsforthesoftwareproduct. Duringtherequirementsdefinitionphasetherequirementsdefinitionteamusesaniterativeprocesstoexpandabroadstatementofthesystemrequirementsintoacompleteanddetailedspecificationofeachfunctionthatthesoftwaremustperformandeach7thatitmustmeet.Thestartingpointisusuallyasetofhigh-levelrequirementsfromthe8thatdescribetheprojectorproblem. Ineithercasetherequirementsdefinitionteamformulatesanoverallconceptforthesystemandthendefines9showinghowthesystemwillbeoperatedpublishesthesystemandoperationsconceptdocumentandconductsasystemconceptreviewSCR. FollowingtheSCRtheteamderives10requirementsforthesystemfromthehighlevelrequirementsandthesystemandoperationsconcept.Usingstructuredorobject-orientedanalysistheteamspecifiesthesoftwarefunctionsandalgorithmsneededtosatisfyeachdetailedrequirement. 7
TheRationalUnifiedProcessRUPisasoftwareengineeringprocesswhichcapturesmanyofbestpracticesinmodemsoftwaredevelopment.Thenotionsof36andscenarioshavebeenproventobeanexcellentwaytocapturefunctionrequirements.RUPcanbedescribedintwodimensions-timeandcontent.Inthetimedimensionthesoftwarelifecycleisbrokenintocycles.Eachcycleisdividedintofourconsecutive37whichisconcludedwithawell-defined38andcanbefurtherbrokendowninto39-acompletedevelopmentloopresultinginareleaseofanexecutableproductasubsetofthefinalproductunderdevelopmentwhichgrowsincrementallytobecomethefinalsystem.Thecontentstructurereferstothedisciplineswhichgroup40logicallybynature. 39
Object-orientedDBMSintegratedavarietyof81datatypes-suchasbusinessproceduresgraphicspicturesvoiceandannotatedtext. Objectorientationalsomakesa82toapplicationdevelopmentefficiency.Itmakesthedatafunctionsattributesandrelationshipsanintegralpartofthe83.Inthiswayobjectscanbereusedandreplicated. SomeleadingRDBMSvendorssupporttheconceptofintegratingobjectmanagementcapabilitieswiththeircurrentlineofrelationalproducts.Thatcapabilityenableuserstothedevelopmentcyclesinceintegritylogicandbusinessrolesnolongerneedtobeprogrammed84eachapplication. 84是
Thepurposeoftherequirementsdefinitionphaseistoproduceaclearcompleteconsistentandtestable6ofthetechnicalrequirementsforthesoftwareproduct. Duringtherequirementsdefinitionphasetherequirementsdefinitionteamusesaniterativeprocesstoexpandabroadstatementofthesystemrequirementsintoacompleteanddetailedspecificationofeachfunctionthatthesoftwaremustperformandeach7thatitmustmeet.Thestartingpointisusuallyasetofhigh-levelrequirementsfromthe8thatdescribetheprojectorproblem. Ineithercasetherequirementsdefinitionteamformulatesanoverallconceptforthesystemandthendefines9showinghowthesystemwillbeoperatedpublishesthesystemandoperationsconceptdocumentandconductsasystemconceptreviewSCR. FollowingtheSCRtheteamderives10requirementsforthesystemfromthehighlevelrequirementsandthesystemandoperationsconcept.Usingstructuredorobject-orientedanalysistheteamspecifiesthesoftwarefunctionsandalgorithmsneededtosatisfyeachdetailedrequirement. 9
Thegrowthofswitchinghas135anewgenerationofnetworkmanagementtoolsthathelp136copewiththechallenges137themtocorrelatedevicealarmsinorderto138pinpointrootcausesortomonitorservicelevelswithoutdependingonIPsubnetschemes.Thesenewtoolshavebecomeessentialtocopingwiththeprimary139effectofanytreatmentthatsignificantlyincreasesnetworkflexibility:addedcomplexity. 136
Withthewidespreaduseofthepersonalcomputermanyauthoritiesinthefieldof56havepointedouttheneedforcomputerliteracy. Unfortunatelythereisno57agreementastowhattheterm"computerliteracy"means.Somefeelthatcomputerliteracymeansknowinghowtomakethecomputer"compute";thatisknowinghowtoprogramcomputersinoneormoreprogramminglanguages. Othersfeelthatknowinghowtoprogramismerelyasmallsegmentofcomputerliteracy.Thesepeople58themajoremphasisinschoolsshouldbeonteachinghowtoeffectivelyusethemanysoftwarepackagesthatareavailable. Stillotherssuggestthatcomputerliteracyeducationisnotrequired.Theysuggestthatcomputersarebeingsorapidlyintegratedintooursocietythatusingacomputerwillbeas59asusingatelephoneoravideotaperecorderandthatspecialeducationwillnotbenecessary.60ofone’sdefinitionofcomputerliteracyitisrecognizedbymostthatlearningtouseacomputerisindeedanimportantskillinmodernsociety. 60
Therelationaldatabasemodelrequiresthedatabe110throughprogramsthatdon’trelyonthepositionofthedatainthedatabase.Thisisindirect111totheotherdatabasemodewheretheprogramhastofollowaseriesofpointerstothedataitseeks.Aprogram112arelationaldatabasesimplyasksforthedataitseeks;theDBMSperformsthenecessarysearchesand113theinformation.The114onhowthesearchisdonearespecifictotheDBMSandvaryfromproducttoproduct. 110是
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