数据库系统概念全套配套课件PPT ch25.ppt

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1、Chapter25:AdvancedDataTypesandNewApplicationsChapter25:AdvancedDataTypesandNewApplicationsTemporalDataSpatialandGeographicDatabasesMultimediaDatabasesMobilityandPersonalDatabasesTimeInDatabasesWhilemostdatabasestendtomodelrealityatapointintime(atthe“current”time),temporaldatabasesmodelthesta

2、tesoftherealworldacrosstime.Factsintemporalrelationshaveassociatedtimeswhentheyarevalid,whichcanberepresentedasaunionofintervals.Thetransactiontimeforafactisthetimeintervalduringwhichthefactiscurrentwithinthedatabasesystem.Inatemporalrelation,eachtuplehasanassociatedtimewhenitistrue;thetimem

3、aybeeithervalidtimeortransactiontime.Abi-temporalrelationstoresbothvalidandtransactiontime.TimeInDatabases(Cont.)Exampleofatemporalrelation:Temporalquerylanguageshavebeenproposedtosimplifymodelingoftimeaswellastimerelatedqueries.TimeSpecificationinSQL-92date:fourdigitsfortheyear(1--9999),two

4、digitsforthemonth(1--12),andtwodigitsforthedate(1--31).time:twodigitsforthehour,twodigitsfortheminute,andtwodigitsforthesecond,plusoptionalfractionaldigits.timestamp:thefieldsofdateandtime,withsixfractionaldigitsforthesecondsfield.TimesarespecifiedintheUniversalCoordinatedTime,abbreviatedUTC

5、(fromtheFrench);supportstimewithtimezone.interval:referstoaperiodoftime(e.g.,2daysand5hours),withoutspecifyingaparticulartimewhenthisperiodstarts;couldmoreaccuratelybetermedaspan.TemporalQueryLanguagesPredicatesprecedes,overlaps,andcontainsontimeintervals.Intersectcanbeappliedontwointervals,

6、togiveasingle(possiblyempty)interval;theunionoftwointervalsmayormaynotbeasingleinterval.Asnapshotofatemporalrelationattimetconsistsofthetuplesthatarevalidattimet,withthetime-intervalattributesprojectedout.Temporalselection:involvestimeattributesTemporalprojection:thetuplesintheprojectioninhe

7、rittheirtime-intervalsfromthetuplesintheoriginalrelation.Temporaljoin:thetime-intervalofatupleintheresultistheintersectionofthetime-intervalsofthetuplesfromwhichitisderived.Itintersectionisempty,tupleisdiscardedfromjoin.TemporalQueryLanguages(Cont.