reflexiblethanhierarchicalDBMS,butaccesspathsmuststillbespecifiedinadvance.Therearepracticallimitationstothenumberoflinks,orrelationships,thatcanbeestablishedamongrecords.Iftheyaretoonumerous,thesoftwarewillnotworkefficiently.Neithernetworknohierarchicaldatabasemanagementmodelscaneasilycreatenewrelationshipsamongdataelementsornewpatternsofaccesswithoutmajorprogrammingefforts.
TheRelationalModelIntheearly1970stherelationaldatabaseapproachwasdevelopedtoprovideamuchmoreuser-friendlyorganization.Insteadofusingcomplexstructuresofpointerstherelationaldatabasestoresinformationsimplyastablescalledrelations.Thesetablesaresometimescalledflatbecausetherowsofthetablereallyareverymuchthesameastherecordsofafile.
Eachrowinarelationiscalledarecord.Eachcolumncorrespondstoaparticularfieldwithintherecord(fieldsarealsocalleddomains).Thefieldsareheadedbyattributes,whichdescribetheentriesinthecolumn.Arelationaldatabaseconsistsofoneormorerelations.ArelationalDBMSperformsthreeprimaryoperationsonrelationstoformnewrelations:
*Tworelationsmaybejoined(combined).
*Arelationalmaybeprojected(someofthecolumnsareextractedformtherelationsandusedtoformthecolumnsofthenewrelation).
*Recordsmaybeselectedaccordingtovarioususerspecifiedcriteria's(StructuredQueryLanguage)isthemostimportantquerylanguagebasedontherelationalmodel.Forinstance,arelationnamedAccounts,recordbankaccounts,theirbalance,andtypemightlooklike:
Accounts
Account No
Balance
Type
173921
1700.00
Checking
251101
888.00
Savings
...
...
...
Headingthecolumnsarethethreeattributes:AccountNo,Balance,andType.Belowtheattributesaretherows,orrecords.Thefirstrowsaysthataccountnumber173921hasabalanceofonethousandandsevenhundredsdollars,anditischeckingaccount.Supposewewantedtoknowthebalanceofaccount173921.WecouldaskthisQueryinSQLasfollows:
SELECTbalanceFORMAccountsWHEREaccountNo=173921
IBM'sDB2andOraclefromtheOracleCorporationareexamplesofmainframerelationaldatabasemanagementsystems.MicrosoftAccessisaPCrelationaldatabasemanagementsystem.(OraclealsohasaPCversion).
Client-ServerArchitecture
Manyvarietiesofmodernsoftwareuseaclient-serverarchitecture,inwhichrequestsbyoneprocess(theclient)aresenttoanotherprocess(theserver)forexecution.Databasesystemsarenoexception.Inthesimplestclient/serverarchitecture,theentireDBMSisaserver,exceptforthequeryinterfacesthaninteractwithuserandsendqueriesorothercommandsacrosstotheserver.Forexample,relationalsystemsgenerallyusetheSQLlanguageforrepresentingrequestsfromtheclienttotheserver.Thedatabaseserverthensendstheanswer,intheformofatableorrelation,backtotheclient.Therelationshipbetweenclientandservercangetmorecomplexespeciallywhenanswersareextremelylarge.Thereisalsoatrendtoputmoreworkintheclient,sincetheserverwillbeaBottleneckiftherearemanysimultaneousdatabaseusers.
NewFormsofDatabaseandDataWarehouses
Object-OrientedDatabaseObject-orienteddatabasesstoredataasobjectsthatcanbeautomaticallyretrievedandshared.Includedintheobjectaretheprocessinginstructionstocompleteeachdatabasetransaction.Theseobjectscancontainvarioustypesofdata,includingsound,graphics,andvideoaswellastraditionaldataandprocessingprocedures.Theobjectscanbesharedandreused.Thesefeaturesof
Object-orienteddatabasespromisestofacilitatesoftwaredevelopmentthroughreuseandtheabilitytobuildnewmultimediaapplicationthatcombinemultipletypesofdata.BenefitofOdom'sistheirabilitytosupportapplicationfortheWorldWideWeb,asdescribedinthefocusonTechnology.
Object-orienteddatabasearestillarelativelynewtechnologyandcanbemuchslowerthanrelationalsystemsforhandinglargequantitiesofdatawherethereisahighvolumeoftransactionprocessing.Hybridobject-relationaldatabaseshavebeendevelopedthatcombinethecapabilityofhandinglargenumbersoftransactionsfoundinrelationalDamswiththecapabilityofhandingcomplexrelationshipsandnewtypesofdatafoundinOdom's.
HypermediaDatabaseHypermediadatabasemanagedatadifferentlyfromobjectorientedDBMS.buttheycanalsocontaindiversetypesofdata.Theystoredataas"chunks"ofinformation,witheachchunkinaseparatenode.Eachnodecancontaintraditionalnumericorcharacterdataorwhole
documents,softwareprograms,graphics,andevenfullmotionvideo.Eachnodeistotallyindependent-thenodesarenotrelatedbyapredeterminedorganizationschemeastheyareintraditionaldatabase.Insured,userestablishestheirownlinksbetweennodes.TherelationshipamongnodesislessstructuredthaninatraditionalDBMS.Searchingforinformationdoesnothavetofollowapredeterminedorganizationscheme.Userscanbranchdirectlyformonenodetoanotherinanyrelationshiptheyestablish.Forinstance,ahypermediadatabaseonautomobilesmightlinkbasicproductinformationwithdescriptivesalesbrochures,avideoshowingtheautomobileinaction,andthelocationofauthorizeddealers.
DataWarehousesManycompanieshaveallowedtheirdatatobestoredinmanyseparatesystemsthatareunabletoprovideaconsoledatedviewofinformationusablecompany-wide.Onewaytoaddressthisproblemistobuildadatawarehouse.Adatawarehouseisadatabasethatconsolidatesdataextractedfromvariousproductionandoperationalsystemsintoonelargeorganization'scoretransactionprocessingsystemsarereorganizedandcombinedwithotherinformation,includinghistoricaldatasothattheycanbeusedformanagementdecisionmakingandanalysis.Inmostcases,thedatainthedatawarehousecanbeusedforreporting-theycannotbeupdated-sothattheperformanceofthecompany'sunderlyingoperationalsystemisnotaffected.Thefocusonproblemsolvingdescribessomeofthebenefitscompanieshaveobtainedbyusingdatawarehouses.
DataIndependence,Integrity,andSecurity
DataIndependenceIndatabasesystem,eachprogramworkswithitsownvieworviewsofthedatabase.Ifnewfieldsareaddedtoadatabaserecord,theDBMSpreservestheexistingviewssothatexistingprogramsdon'thavetobechanged.Theabilitytomodifythestructureofthedatabasewithoutaffectingexistingprogramsthatrefertothedatabaseiscalleddataindependence
DataIntegrityDataintegrityreferstotheaccuracy,correctness,orvalidityofthedatainthedatabase.Inadatabasesystem,dataintegritymeanssafegu
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