II.Reading.(2x4=8)TheUnitedStatesandCanadahaveagreedtokeeptheirsharedborderclosedfornonessentialtravelforanother30daystopreventthespreadofthecoronavirus.CanadianPrimeMinisterJustinTrudeauannouncedtheextensionduringabriefingSaturdayinOttawa.Therestrictionsontheworld’slongestfrontiertookeffectonMarch21,whileallowingtradeandothertraveldeemedessentialtocontinue.Thepartialbanwastoexpiresoon,buttheneighboringcountrieshavedecideditisnotsafetoallowtraffictofullyresume.“Thisisanimportantdecisionandonethatwillkeeppeopleonbothsidesofthebordersafe,”Trudeautoldanewsconferenceoutsidetheresidencewherehehasbeenlivingandworkinginrecentweeks.Canada,withapopulationofunder38million,hashadmorethan32,000casesofinfectionand1,340deathsfromCOVID-19,asofSaturday.TheU.S.hasidentifiedalmost720,000casesandover37,000deaths,asoflateSaturdayafternoon.SeveralofAmerica’shardest-hitareasincludingNewYorkstateshareaborderwithCanadianprovinces.Bothcountrieshavetestingshortages.Bordercrossingshavefallendramatically,evendespitetheessentialtravelexemptions.AtthePeaceBridgebetweenBuffalo,N.Y.,andFortErie,Ontario,“passengertraffic[is]down95%,bustrafficdown...99.6%,soessentially,it’sshutdown,”KathrynFriedman,aglobalfellowattheWilsonCenter’sCanadaInstitute,saidinaconferenceTuesday.“Commercialtrafficisdownalittleover28%.”“Itissurrealtoseesuchlittletraffic,”RonRienas,whorunstheauthoritythatmanagesthePeaceBridge,toldTheBuffaloNews.SinceApril1,tunneltrafficbetweentheheavilyintegratedDetroitandWindsor,Ontarioregionsisdown88%overthesameperiodlastyear,accordingtoFriedman.ConcernshaveriseninCanadaaboutdailytripsbyaround1,600healthcareworkersfromOntariotohard-hitDetroit.SomenurseswhoworkinfacilitiesinboththeU.S.andCanadahavehadtochooseone.Theextensionoftheborderrestrictionscameafteraweekinwhichthegovernmentsgavemixedsignalsaboutthenextstep.PresidentTrumpsaidonWednesdaythattheCanadianborder“willbeoneoftheearlyborderstobereleased.Canadaisdoingwell.We’redoingwell.”Butthenextday,PrimeMinisterTrudeautoldreporters,“Thereisasignificantamountoftime,still,beforewecantalkaboutlooseningsuchrestrictions.”6.WhendidTrudeauannouncetheextension?
设有一个SPJ数据库，其包括S，P，J，SPJ四个关系:S(SNO,SNAME,STATUS,CITY);P(PNO,PNAME,COLOR,WEIGHT);J(JNO,JNAME,CITY);SPJ(SNO,PNO,JNO,QTY);供应商表S由供应商代码（SNO）、供应商姓名（SNAME）、供应商状态（STATUS）、供应商所在城市（CITY）组成；零件表P有零件代码（PNO）、零件名（PNAME）、颜色（COLOR）、重量（WEIGHT）组成；工程项目表J由工程项目代码（JNO）、工程项目名（JNAME）、工程项目所在城市（CITY）城市组成；供应情况表SPJ由供应商代码（SNO）、零件代码（PNO）、工程项目代码（JNO）、供应数量（QTY）组成，表示某供应商供应某种零件给某工程项目的数量为QTY。供应工程J1零件为红色的供应商号码SNO用关系代数表示为（）。A.∏SNO(σJNO=’J1’(SPJ)∧(σColor=’红’(P))B.∏SNO(σJNO=’J1’,Color=’红’(P∞SPJ∞S))C.∏SNO(σJNO=’J1’(SPJ)∞(σColor=’红’(P)))D.∏SNO(σJNO=’J1’(SPJ)Ú(σColor=’红’(P))
C
在“学生-选课-课程”数据库中的3个关系如下：S（SNO,SNAME,SEX,AGE）；SC（SNO,CNO,GRADE）；C（CNO,CNAME,TEACHER），查找选修“数据库技术”这门课程学生的学生名和成绩，若用关系代数表达式来表示为（）。A.ПCNAME,GRADE(σCNAME=’数据库技术’(C)∞ПSNO,CNO,GRADE(SC)∞ПSNO,SNAME(S))B.ПCNAME,GRADE(σCNAME=’数据库技术’(C)∞SC∞S)C.ПCNAME,GRADE(C∞SC∞SC)D.ПCNAME,GRADE(σCNAME=’数据库技术’(ПCNAME(C))∞ПSNO,CNO,GRADE(SC)∞ПSNO,SNAME(S))
AB