《计算机专业英语》Database Management

ComputerEnglishChapter11DatabaseManagementKeypoints:Keypoints:uusefultermsanddefinitionssefultermsanddefinitionsofofDatabaseManagementDatabaseManagementDifficultpoints:Difficultpoints:CClassificationlassificationofofthelogicalthelogicalstructuringtechniquesofDBMSstructuringtechniquesofDBMSRequirements:Requirements:1.Principleofeasiestpenetration2.Thekindsofcomputersecuritybreaches3.Whatisfirewall4.NewWords&Expressions:facilitateretrievaln.fieldn.recordalphabeticallychronologicallybreakdownv.buildupencyclopedian.referencen.bibliographyperiodicaln.literaturen.professionaln.11.1OverviewAbbreviations:Adatabaseconsistsofafileorasetoffiles.Theinformationinthesefilesmaybebrokendownintorecords,eachofwhichconsistsofoneormorefields.Fieldsarethebasicunitsofdatastorage,andeachfieldtypicallycontainsinformationpertainingtooneaspectorattributeoftheentitydescribedbythedatabase.Usingkeywordsandvarioussortingcommands,userscanrapidlysearch,rearrange,group,andselectthefieldsinmanyrecordstoretrieveorcreatereportsonparticularaggregatesofdata.11.1Overview11.1OverviewDatabaserecordsandfilesmustbeorganizedtoallowretrievaloftheinformation.Earlysystemswerearrangedsequentially(i.e.,alphabetically,numerically,orchronologically);thedevelopmentofdirect-accessstoragedevicesmadepossiblerandomaccesstodataviaindexes.Queriesarethemainwayusersretrievedatabaseinformation.Typically,theuserprovidesastringofcharacters,andthecomputersearchesthedatabaseforacorrespondingsequenceandprovidesthesourcematerialsinwhichthosecharactersappear.Ausercanrequest,forexample,allrecordsinwhichthecontentofthefieldforaperson’slastnameisthewordSmith.Inflatdatabases[2],recordsareorganizedaccordingtoasimplelistofentities;manysimpledatabasesforpersonalcomputersareflatinstructure.Therecordsinhierarchicaldatabasesareorganizedinatreelikestructure,witheachlevelofrecordsbranchingoffintoasetofsmallercategories.Unlikehierarchicaldatabases,whichprovidesinglelinksbetweensetsofrecordsatdifferentlevels,networkdatabasescreatemultiplelinkagesbetweensetsbyplacinglinks,orpointers,toonesetofrecordsinanother;thespeedandversatilityofnetworkdatabaseshaveledtotheirwideuseinbusiness.11.1OverviewRelationaldatabasesareusedwhereassociationsamongfilesorrecordscannotbeexpressedbylinks;asimpleflatlistbecomesonetable,or“relation”,andmultiplerelationscanbemathematicallyassociatedtoyielddesiredinformation.Object-orienteddatabasesstoreandmanipulatemorecomplexdatastructures,called“objects”,whichareorganizedintohierarchicalclassesthatmayinheritpropertiesfromclasseshigherinthechain;thisdatabasestructureisthemostflexibleandadaptable.“”11.1OverviewNewWords&Expressions:sequentialintegrateaccessv.retrievev.employv.liststructuren.invoicen.pointern.hierarchicalstructuresubordinateadj.networkstructurerelationalstructurephysicalstructure11.2DBMSSTRUCTURINGTECHNIQUESAbbreviations:DBMSDatabaseManagementSystem11.2DBMSSTRUCTURINGTECHNIQUESDBMSisabletoaccessandretrievedatafromnonkeyrecordfields.Thatis,theDBMSisabletostructureandtietogetherthelogicallyrelateddatafromseverallargefiles.LogicalStructures.Identifyingtheselogicalrelationshipsisajobofthedataadministrator.Adatadefinitionlanguageisusedforthispurpose.TheDBMSmaythenemployoneofthefollowinglogicalstructuringtechniquesduringstorageaccess,andretrievaloperations[1]:11.2DBMSSTRUCTURINGTECHNIQUES1.Liststructures.Inthislogicalapproach,recordsarelinkedtogetherbytheuseofpointers.Apointerisadataiteminonerecordthatidentifiesthestoragelocationofanotherlogicallyrelatedrecord.Recordsinacustomermasterfile,forexample,willcontainthenameandaddressofeachcustomer,andeachrecordinthisfileisidentifiedbyanaccountnumber.Duringanaccountingperiod,acustomermaybuyanumberofitemsondifferentdays.Thus,thecompanymaymaintainaninvoicefiletoreflectthesetransactions.11.2DBMSSTRUCTURINGTECHNIQUESAliststructurecouldbeusedinthissituationtoshowtheunpaidinvoicesatanygiventime.Eachrecordinthecustomerfilewouldcontainafieldthatwouldpointtotherecordlocationofthefirstinvoiceforthatcustomerintheinvoicefile(fig11-1).Thisinvoicerecord,inturn,wouldbelinkedtolaterinvoicesforthecustomer.Thelastinvoiceinthechainwouldbeidentifiedbytheuseofaspecialcharacterasapointer.PointerInvoicenumberInvoicenumberPointerInvoicenumberInvoicenumberInvoicenumberInvoicenumber11.2DBMSSTRUCTURINGTECHNIQUESHierarchical(tree)structures.Inthislogicalapproach,dataunitsarestructuredinmultiplelevelsthatgraphicallyresembleanupsidedowntreewiththerootatthetopandthebranchesformedbelow.There'sasuperior-subordinaterelationshipinahierarchical(tree)structure.Belowthesingle-rootdatacomponentaresubordinateelementsornodes,eachofwhich,inturn,ownoneormoreotherelements(ornone)[3].Eachelementorbranchinthisstructurebelowtheroothasonlyasingleowner.Thus,asweseeinfig11-2,acustomerownsaninvoice,andtheinvoicehassubordinateitems.Thebranchesinatreestructurearenotconnected.“”“”11.2DBMSSTRUCTURINGTECHNIQUESNetworkStructures.Unlikethetreeapproach,whichdoesnotpermittheconnectionofbranches,thenetworkstructurepermitstheconnectionofthenodesinamultidirectionalmanner(seefig1