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设有如下关系: 与元组演算表达式t| (u)(v)(R(u)∧S(v)Au[3]=v[1]∧u[4]=v[2]∧u[1]> v[3]∧t[i]=u[2])等价的关系代数表达式是...
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设有如下关系 则与关系代数表达式π14R S等价的元组演算表达式为t|
A
B
C
D
关系RS如下图所示关系代数表达式=它与元组演算表达式Sv∧}等价 关系代数表达式=
A
B
C
D
关系RS如下所示关系代数表达式π156σ1>5R×S= 它与元 组演算表达式t| u
u[2]=v[1]∧t[1]=u[1]∧t[2]=v[2]
u[2]=v[1]∧t[1]=u[1]∧t[2]=v[1]
u[1]=v[1]∧t[1]=u[1]∧t[2]=v[2]
u[1]=v[1]∧t[1]=u[1]∧t[2]=v[1]
关系RS如下图所示元组演算表达式{t|uRt∧Su∧t[3]>u[1]}的结果为 R S
A
B
C
D
设有如下变量定义doublea=3.500000floatb=2.5intm=2n=3则表达式flo
2.5
3.5
4.5
表达式不正确
关系RS如下图所示关系代数表达式=它与元组演算表达式Sv∧}等价 它与元组演算表达式
A
B
C
D
设有如下关系则与关系代数表达式等价的元组演算表达式为关系代数表达式RS的结果集为34 34处
设有如下关系 与元组演算表达式 t| u vRu Sv u
A
B
C
D
设有如下关系与元组演算表达式 等价的关系代数表达式是关系代数表达式R÷S的
设有关系RS和T如下图所示则元组演算表达式{t|uRu∨Su∧vTv→wRw∨Sw∧W[1]=u[
A
B
C
D
设有如下关系 则与关系代数表达式π14RS等价的元组演算表达式为t| u
u[2]=v[1]∧t[1]=u[1]∧t[2]=v[2]
u[2]=v[1]∧t[1]=u[1]∧t[2]=v[1]
u[1]=v[1]∧t[1]=u[1]∧t[2]=v[2]
u[1]=v[1]∧t[1]=u[1]∧t[2]=v[1]
设有关系R和S与R-S等价的元组关系演算表达式是
{t
R(t)∨┐s(t)}
{t
R(t)∧┐S(t)}
{t
(u)R(t∨┐S(u))
{t
(u)R(t)∧┐S(u)}
设有如下关系则与关系代数表达式等价的元组演算表达式为关系代数表达式RS的结果集为34 33处
u[2]=v[1]∧ t[1] =u[1] ∧t[2] =v[2]
u[2]=v[1]∧ t[1] =u[1] ∧t[2] =v[1]
u[1]=v[1]∧ t[1] =u[1] ∧t[2] =v[2]
u[1]=v[1]∧ t[1] =u[1] ∧t[2] =v[1]
设有如下变量定义doublea=3.500000floatb=2.5intm=2n=3则表达式flo
2.5
3.5
4.5
表达式不正确
设有如下关系与元组演算表达式 等价的关系代数表达式是关系代数表达式R÷S的
关系RS如下图所示元组演算表达式的结果为
A
B
C
D
设有如下关系 与元组演算表达式 t| u vRu Sv u
A
B
C
D
设有如下定义inta=4;则计算表达式a+=a-a*a后的值是
-12
-24
-8
设有如下变量定义doublea=3.500000floatb=2.5intm=2n=3则表达式flo
2.5
3.5
4.5
表达式不正确
设有如下关系 与元组演算表达式t|uvRu∧SvAu[3]=v[1]∧u[4
A
B
C
D
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判断下列叙述是否正确①在数据库系统中数据独立性指数据之间的相互独立互不依赖②SQL语言的视图定义和视图操作功能不支持逻辑数据的独立性③SQL语言中不提供显式地使用索引的功能支持了物理数据的独立性④用户对脏数据的读出是由于数据库完整性规则受到了破坏⑤在数据库系统中数据的安全性是指保护数据以防止被末授权用户的蓄意或者无意使用⑥实体完整性规则指主关键字值的任何组成部分都不可以是空值参数完整性规则则不允许引用不存在的实体即元组⑦在数据库系统中数据的完整性是指数据的正确性和有效性⑧授权是数据库系统中采用的完整性措施之一⑨事务处理Transaction是数据库运行的基本单位如果一个事务处理成功则全部数据应更新和提交如果失败则已做的全部更新被恢复成原状好像整个事务处理未进行过一样这样使数据库保持了一致性⑩对数据库的查找增添删除修改等操作都需由数据库管理员进行完整性定义和完全性授权由数据库系统具体执行
Themostaccurateandmostboringwayto105JavaisthatitisanewcomputerprogramminglanguagedevelopedbySunMicrosystemsthatcreates106independentprogramsthatcanbedistributedandnmremotely.TorunJavaprogramsacomputermusthaveaJavainterpreter.Currently107Javaprogramsaresmall"applets"thatare107aspartofwebpages. WhenyouuseaJava108browsertoviewaWebpagethatincludesaJavaappletthebrowserloadstheappletontoyourcomputerthroughyourmodemornetwork.ThentheJavainterpreterrunstheappletwhichcouldincludeanimationorsoundonyourcomputerratherthantransmittingthecodebitbybitoverInternet.AfewthousandbytesofJavacodecantamintoapowerfulprogramonyourcomputer. Soanappletcouldinclude109withJavainterpreter. 106是
Alloftheseapplicationswillenhancethe130oflifeandspureconomicgrowth.OverhalfoftheU.S.workforceisnowinjobsthatareinformation131.ThetelecommunicationandinformationsectoroftheU.S.economynow132for12percentoftheGrossDomesticProductgrowingmuchfasterthananyothersectoroftheeconomy.Lastyearthe133inthissectorexceeded700billiondollars.TheU.S.exportedover48billiondollarsoftelecommunicationequipment134. 134是
MIMDsystemscanbeclassifiedinto150-orientedsystemshigh-availabilitysystemsandresponse-orientedsystems. Thegoalof150-orientedmultiprocessingistoobtainhigh150151minimalcomputingcost.Thetechniquesemployedbymultiprocessoroperatingsystemstoachievethisgoaltakeadvantageofaninherentprocessingversusinput/outputbalanceintheworkloadtoproduce152and153.loadingofsystem154 150是
Basicallyafirewallisa145processorasetofintegratedprocessesthatrunsonarouteror146tocontroltheflowofnetworkedapplicationtraffic147throughit.Typicallyfirewallsareplacedontheentrypointtoa148networksuchastheInternet.Theycouldbeconsideredtrafficcops.Thefirewall’s149istoensurethatallcommunicationbetweenanorganization’snetworkandtheInternetconformtotheorganization’spolicies. 148是
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SoonmoreoftheinformationwereceiveviatheInternetcouldcome155indigitalwrappers. Wrappersaremadeup156softwarecodethat’stargetedtodospecificthingswiththedata157withinthemsuchashelpingtodefinequeriesforsearchengines.Theyalsokeep158from159accesstothatcode. 156是
Networkscanbeinterconnectedbydifferentdevices.Inthephysicallayernetworkscanbeconnectedby1orhubswhichjustmovethebitsfromonenetworktoanidenticalnetwork.Onelayerupwefindbridgesandswitcheswhichoperateatdatalinklayer.Theycanaccept2examinetheMACaddressandforwardtheframestoadifferentnetworkwhiledoingminorprotocoltranslationintheprocess.Inthenetworklayerwehaveroutersthatcanconnecttwonetworks.Iftwonetworkshave3networklayertheroutermaybeabletotranslatebetweenthepacketformats.Inthetransportlayerwefindtransportgatewaywhichcaninterfacebetweentwotransportconnections.Finallyintheapplicationlayerapplicationgatewaystranslatemessage4.AsanexamplegatewaysbetweenInternete-mailandX.400e-mailmust5thee-mailmessageandchangevariousheaderfields. 5是
MIMDsystemscanbeclassifiedinto150-orientedsystemshigh-availabilitysystemsandresponse-orientedsystems. Thegoalof150-orientedmultiprocessingistoobtainhigh150151minimalcomputingcost.Thetechniquesemployedbymultiprocessoroperatingsystemstoachievethisgoaltakeadvantageofaninherentprocessingversusinput/outputbalanceintheworkloadtoproduce152and153.loadingofsystem154 152是
DOMisaplatform-andlanguage-11APIthatallowsprogramsandscriptstodynamicallyaccessandupdatethecontentstructureandstyleofWWWdocumentscurrentlydefinitionsforHTMLandXMLdocumentsarepartofthespecification.Thedocumentcanbefurtherprocessedandtheresultsofthatprocessingcanbeincorporatedbackintothepresented12.DOMisa13-basedAPItodocumentswhichrequiresthewholedocumenttoberepresentedin14whileprocessingit.AsimpleralternativetoDOMistheevent-basedSAXwhichcanbeusedtoprocessverylarge15documentsthatdonotfitinfothememoryavailableforprocessing. 14是
Alloftheseapplicationswillenhancethe130oflifeandspureconomicgrowth.OverhalfoftheU.S.workforceisnowinjobsthatareinformation131.ThetelecommunicationandinformationsectoroftheU.S.economynow132for12percentoftheGrossDomesticProductgrowingmuchfasterthananyothersectoroftheeconomy.Lastyearthe133inthissectorexceeded700billiondollars.TheU.S.exportedover48billiondollarsoftelecommunicationequipment134. 130是
Themostaccurateandmostboringwayto105JavaisthatitisanewcomputerprogramminglanguagedevelopedbySunMicrosystemsthatcreates106independentprogramsthatcanbedistributedandnmremotely.TorunJavaprogramsacomputermusthaveaJavainterpreter.Currently107Javaprogramsaresmall"applets"thatare107aspartofwebpages. WhenyouuseaJava108browsertoviewaWebpagethatincludesaJavaappletthebrowserloadstheappletontoyourcomputerthroughyourmodemornetwork.ThentheJavainterpreterrunstheappletwhichcouldincludeanimationorsoundonyourcomputerratherthantransmittingthecodebitbybitoverInternet.AfewthousandbytesofJavacodecantamintoapowerfulprogramonyourcomputer. Soanappletcouldinclude109withJavainterpreter. 108是
SoonmoreoftheinformationwereceiveviatheInternetcouldcome155indigitalwrappers. Wrappersaremadeup156softwarecodethat’stargetedtodospecificthingswiththedata157withinthemsuchashelpingtodefinequeriesforsearchengines.Theyalsokeep158from159accesstothatcode. 158是
如果有两个事务同时对数据库中同一数据进行操作不会引起冲突的操作是A其中有一个是DELETEB一个是SELECT另一个是UPDATEC两个都是SELECTD两个都是UPDATE
DOMisaplatform-andlanguage-11APIthatallowsprogramsandscriptstodynamicallyaccessandupdatethecontentstructureandstyleofWWWdocumentscurrentlydefinitionsforHTMLandXMLdocumentsarepartofthespecification.Thedocumentcanbefurtherprocessedandtheresultsofthatprocessingcanbeincorporatedbackintothepresented12.DOMisa13-basedAPItodocumentswhichrequiresthewholedocumenttoberepresentedin14whileprocessingit.AsimpleralternativetoDOMistheevent-basedSAXwhichcanbeusedtoprocessverylarge15documentsthatdonotfitinfothememoryavailableforprocessing. 12是
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若关系中的某一属性组的值能惟一地标识一个元组则称该属性组为A主键B候选键C主属性D外部键
Alloftheseapplicationswillenhancethe130oflifeandspureconomicgrowth.OverhalfoftheU.S.workforceisnowinjobsthatareinformation131.ThetelecommunicationandinformationsectoroftheU.S.economynow132for12percentoftheGrossDomesticProductgrowingmuchfasterthananyothersectoroftheeconomy.Lastyearthe133inthissectorexceeded700billiondollars.TheU.S.exportedover48billiondollarsoftelecommunicationequipment134. 132是
从供选择的答案中选出应填入下列叙述中内的正确答案 设关系R和S的元数分别为2和3那么RS与A等价设关系P和Q具有相同的关系模式所列出的等价式中B是不正确的设关系模式RABC上成立的FD集为A→Bpl=ABBC为只的一个分解那么plC若只ABC上成立的FD集为A→CB→Cp2=ABAC为只的一个分解那么分解p2D若RABC上成立的FD集为A→Bp3=ABBC为及的一个分解那么p3E 供选择的答案 A①σ1>2RS②σ1>4RS ③σ1>2A×S④σ1>2A×S B①P∩Q=P-P-Q②P∩Q=Q-Q-P ③P∪Q=P∪Q-P④P∪Q=Q∪Q-P C://①不是无损连接分解②是无损连接分解 ③是否无损连接不能确定④是否无损连接由月的具体关系确定 D://①保持函数依赖②丢失了B→C ③丢失了A→C④是否保持函数依赖由只的具体关系确定 E://①是无损连接但不保持FD集②是无损连接且保持FD集的分解 ③是损失连接且不保持FD集④是损失连接但保持FD集的分解
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事务的ACID特性中C的含义是A一致性B邻接性C连续性D并发性
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