1、DesigningaDatabaseApplicationinVB6/0AbstractBeforeyoustarttodesignadatabase/itsimportantyouunderstandexactlywhatadatabaseisandwhattypesofdatabasesyouhavetochoosefrom/Theapplicationtypewillaffectthedatabasetypethatyouchoose/WhatIsaDatabase?Adatabaseisreallyasystemthatcontainsmanydifferentobjectsusedt
2、ogethertoallowyourapplicationfastandefficientaccesstothedata/Youcanusemanyexamplesofdatabaseswithyourapplication/Themostcommonoftheseareasfollows/MicrosoftAccess；MicrosoftFoxPro；OraclePersonalEdition7ior8i；andMSSQLServer7/00isonlyafewtomention/Allthesedatabasescanrunonastandalonecomputerandallowyout
3、ocreateverycomplexdatabasesforyourapplications/Pickingtherightdatabaseforyourapplicationisextremelyimportant/becausethewrongonewillaffectperformanceandmakeyourjobasadevelopersomuchmoredifficult/Beforestartingthedesignprocess/youneedtoselectadatabase/WhenworkingwithVisualBasic/thedatabasethatyoucanch
4、oosefromfallintotwodistinctgroups/LocalDatabasescanbeaccesseddirectlyfromVisualBasicthroughVisualBasicsdatabaseJetengine；andRemotedatabasescannotbeaccessedbyusingVisualBasicsstandarddatabaseaccesscapabilities/Understandingthedifferenceswillhelpyouselectthecorrectdatabaseforyourapplication/However/in
5、VisualBasic6/thereisnowawaytoaccessbothlocalandremotedatabasesusingthesamedatabaseaccesscontrolsandobjectsknownastheActiveXDataObjects/UnderstandingUniversalDataAccess(UDA)Asdatabaseshavegrownandmovedoffthelocalcomputerandontonetworkeddatabaseservers/themethodsemployedtoaccessthedatabaseshavechanged
6、/Microsofthasaddressedthesechangesbydevelopingstrategytoallowforanycompanyordatabasetobeaccessedthesameway/ThisiscalledUniversalDataAccess/orUDA/UnderstandingwhatUDAmeansisnotassimpleasunderstandingtouseit/ThereasonforthisisthatUDAisasetofrulesandaccessmethodsthatmustbefollowedbyanycompanythatiscrea
7、tingadatabaseaccessmethodtobeused/Thisallowsyou/theprogrammer/toaccessanydatabasethesamewayButwhatisUDA?UniversalDataAccessisaplatform/applicationsandtoolsconceptthatMicrosofthasdevelopedthatdefinesanddeliversthestandardsforallfutureapplicationdevelopment/UsingUDAstandardswillprovidehighperformancea
8、ccesstomanytypesofdataandinformationthatresideonmultipleplatforms/andaprogramminginterfacethatwillworkwithalmostanytoolorlanguageusingtheskillsthatdevelopersalreadyhave/ThisisaccomplishedbyusingMicrosoftsDataAccessComponents(MDAC)/aswellasanintegratedsetoftechnologies/whichincludeActiveXDataComponen
9、ts(ADO)/OLEDB/andOpenDatabaseConnectivity(ODBC)/Thebenefitsofusingthesestandardsincludethefollowing/HighperformanceaccesstodataProvidestheabilitytoscaleanyapplicationorcomponenttosupportconcurrentlyconnecteduserswithoutlosingperformance；ReliabilityReducesthenumberofcomponentstosupportontheclientPC；a
10、ndWideindustrysupportManyvendorsincludingMicrosofthavealreadyannouncedsupportfortheUDAstandards/InpreviousreleasesofVisualBasic/youneededtochoosefromeitherDAO(local)accessmethodstodataorRDO(remote)accesstodata/Now/withthesupportofUDA/VisualBasichasathirdmethodfordataaccesscalledADO/orActiveXDataObje
11、cts/Thisallowsyoutocombinethedifferentwaysofaccessingdataintoonesetofprogrammingcallsandobjects/ADOworksinconjunctionwithOLEDBandODBCtoprovideaccesstoonedatabaseyoumightbeworkingwith/Gettingforward/MicrosoftplanstophaseoutODBCinfavourofOLEDBandtheUDAstandards/However/forthenearfuture/DAOandRDO/aswel
12、lasODBC/willstillbesupportedtoallowdeveloperstoslowlymoveuptothenewstandardsofdataaccess/LocalDatabasesLocaldatabasesaregenerallymuchsmallerthantheremotetypeofdatabase/Thefollowingdatabasesfallintothiscategory/MicrosoftAccess；MicrosoftFoxPro；LotusWorksheets；MicrosoftExcelWorksheets；dBase；Paradox；and
13、ASCII/Asyoucansee/thesePC-residentdatabasetypeshavebeenaroundforawhileandareavailablefromalmostanycomputersoftwarestore/AccessingthesedatabasesdoesntrequireanyothersoftwaretobeinstalledonthePC/nordoesitrequirespecialknowledgeonhowtoworkwiththem/However/thesizeoflocaldatabasesisrestricted/MicrosoftAc
14、cess97databasescantexceed1GB(gigabyte)/Localdatabasesaregenerallyusedforsingle-userapplicationssuchascheckbooks/addressbooks/andpersonalinformationmanagers/Someenterprisingteenagershavecreateddatabaseapplicationsthathelpthemkeeptrackofbaseballcardsorvideotapes/Soindecidingwhatyourapplicationwilladdr
15、essandhowitwillbeusedactuallyhelpsyouinthedatabaseselectionprocess/RemoteDatabasesBydefinitionremotedatabasesdontresideontheusersPC/butthisisnotalwaystrue/Infact/aremotedatabaseisanytypeofdatabasethatrequiresanODBC(OpenDatabaseConnectivity)driverforanapplicationtoaccessit/ODBCdatabasesfallintotwomai
16、ncategories/onethatrunonasinglePCandthelargercorporateonesthatrequirelarge/verypowerfulcomputers(calledservers)withlargeamountsofavailablediskspace/DatabaseserversareusedtoseparatetheworkloadbetweentheclientPC(wheretheapplicationexecutes)andthedatabasesystem(wherethedatabasequeriesareperformed)/This
17、providestheapplicationwithfastdataaccesswithoutslowingdowntheusersPC/Mostthelargeserver-typedatabasesprovideversionsthatwillrunonstandalonePCs/Mostlargecorporationsuseoneofthefollowingdatabasesystems(thoughthisisnottosaythatothersarentoutthere)/Oracle；Sybase；andMicrosoftSQLServer/Usingaremotedatabas
18、erequiresyoutouseeithertheremotedatabaseobjectsavailableonlyintheEnterpriseversionofVisualBasic6orthenewActiveXDataObjectsavailableinallreleasesofVisualBasic6/Ifyouredesigninganapplicationthatwilluseoneoftheselargedatabases/however/thebestwaytocreatetheapplicationthatwillusethesingle-PCversionduring
19、thedesign/developmentandtestingprocess/Then/wewhentheapplicationiscompleted/switchtoODBCconnectiontothelarger/server-basedversion/BuildingtheInitialDesignCreatinganapplicationisalotlikebuildingahouse/Forawell-builthouse/youneedablueprinttofollow/buttocreateagoodblueprint/youneedtoknowwhosgoingtolive
20、inthehouseandwhattheirlivingstyleswillbe/Thiswillhelpyoudecidethenumberandsizesofthebedrooms/whatcolourstouse/andsoon/Withanapplication/theblueprintisthetechnicalspecifications；thefunctionalspecificationsarewhatyouneedtoknowtocreatethetechnicalspecifications/Now/manybooksteachtheoverallapplicationde
21、signprocessbyusingverysimplisticexampleapplications/Thedescriptionoftheapplicationwemustdevelopisasfollows/PersonalAddress/PhoneBookThisapplicationwillkeeptrackofnames/addresses/andphonenumbersbyname/Itwillallowasmanydifferentaddressesandphonenumbersasneeded/Also/itwillallowtheentryofsomepersonalinf
22、ormation(suchasbirthdate/spousesname/andchildrennames)tobedeterminedlater/Itwillalsoprovidereportsinseveralformatsandallowuserstosearchthedatabaseforaparticularperson/ChoosingtheTasksDefiningtasksfortheapplicationalsodefinesitscomplexity/Forexample/thisapplicationisanaddress/phonebook/ifyoudecidetog
23、iveuserstheabilitytoselectaphonenumberandhavethecomputerdialitforthem/Toaddthisfunctionalitytotheapplication/youneedtoknowhowtotestfortheexistenceofthemodem/connecttothatmodem/andthenactuallydialthatnumber/Ifyoudontknowhowtoaddthisfunctionality/youmightconsiderwaitingtoadditaftertheinitialversionoft
24、heapplicationiscreatedandtested/Thisiswhatreleasingnewversionsareallabout/Keeptheinitialversionofthisapplicationatasimplerlevel/Thebestwaytodesignanapplicationistosaytoyourself/“WhatwouldIwanttheapplicationtodoifIweretheuser?”Basically/yourethefirstandmostimportantuseroftheapplication/Thisapproachmi
25、ghtnotworkforthelarger/corporate-typeapplication/butitdoesworkforthesmaller/morepersonaltypes/Evenbeforeyoulistthedifferenttasksthattheapplicationwillperform/youneedtodecidewhattypeofapplicationinterfacetouseforthisapplication/Forthissampleapplication/usetheExplorer-styleinterface/Toreviewthisstyle/
26、theinitialformthatusersseeandremaininformostoftheapplicationwillresembletheoneshowninFigure1/Figure1-Thisinterfaceincludesmanyofthecontrolsthatyouhavelearnedearlier/Okay/wehavepickedtheapplicationthatyoullcreatenowyouneedtolistthemainfunctionsthatitwillperform/Thinkofwhattheapplicationisforandthenli
27、stwhatyouwantittodo/Fromthisreasoning/youshouldgetthefollowinglist/Keepnamesandaddresses；Searchforaparticularname；Printdifferenttypesofreports；Allowfordifferentusers；andEnableeasymaintenanceoftheapplication/Now/ifthislistseemssimplistic/itis!Youwouldthenexpandeachiteminthislistuntilyouresatisfiedwit
28、htheresult/Letsexpandeachoftheprecedingtasksandaddamoredetailtothedefinition/KeepingNamesandAddressesWhatdoyoureallymeanwhenyousay“Keepnamesandaddresses?”Itusuallymeansthatyouwanttokeeptrackofthestandardinformationfoundinanystandardtelephonebookyoulookat/Thefirststepindefiningallthemini-tasksrequire
29、dforthistopicistolistthedataelementsyouwanttoputinthedatabase/Table1liststhedataelementsthatyoucanuseforthistypeofapplication/NameTypeFirstNameStringLastNameStringSpouseNameStringChildNameStringAddressStringCityStringStateStringZIPIntegerE-MailAddressStringPhoneLongBirthDateDateNotesStringTable1/DataElementsThatCanBeIncludedInTheApplicationDatabase/
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