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an interpreter approach or a generator approach. The former is a direct transla-
tion from one data item to another. The latter is to provide a generator that gener-
ates a program to accomplish the physical data conversion.
• Logicalconversion—Thelogicalapproachistounloadthenonrelationaldata-
basetosequentialfilesinthelogicalsequence,similartotherelationalmodel.
Thesequentialfilescanthenbeuploadedbacktoatargetrelationaldatabase.
Thisapproachisconcernedwiththelogicalsequenceofthedataratherthephys-
icalattributesofeachdataitem.
• Bridgeprogram—Eachnonrelationalfilerequiresabridgeprogramtoconvertit
to the relational model.
Chapter 4 describes in detail the methods for data conversion.
In program conversion,thefiveapproachestotranslatingnonrelationaldatabase
programs to relational database programs are as follows:
• Rewrite—Onecantranslatethenonrelationalschemaintoarelationalschema,
mapanonrelationaldatabaseintoarelationaldatabase,andrewritealltheap-
plicationprogramstorunontherelationaldatabase.
• Bridgeprogram—Onecanmapthenonrelationalschemaintoarelationalsche-
ma,thenaddarelationalinterface software layer on the top of the nonrelational
DBMS.TherelationalinterfacelayertranslatestherelationalprogramDMLinto
nonrelational program DMLstatementstoaccesstheexistingnonrelationaldata-
base.Theusercanthenviewthenonrelationaldatabaseasarelationaldatabase,
anduserelationalDMLcommandstoextractandmanipulatetheunderlying
nonrelational database system.
• Emulation—Thisisthetechniqueofprovidingsoftwareorfirmwareinthetarget
systemthatmapssourceprogramcommandsintofunctionallyequivalentcom-
mandsinthetargetsystem.EachnonrelationalDMLissubstitutedbyrelational
DMLstatementstoaccesstheconvertedrelationaldatabase.
• Decompilation—Decompilation is the process of transforming a program writ-
teninalowlevellanguageintoanequivalentbutmoreabstractversionandthe
implementationofthenewprogramstomeetthenewenvironment,database
files,andDBMSrequirements.
• Coexistence—Onecancontinuetosupportanonrelationaldatabasewhilede-
veloping an information capacity equivalentrelationaldatabaseforthesameap-
plication.
Chapter 5 describes in detail the methods for program translation.
1.4.2
Adding a Relational Interface to Nonrelational Database
Eventhoughalotofproblemshavebeenresolvedindatabaseconversion,thedif-
ficultyarisesinthetranslationofsemantics.Notonlydowenotknowwhetherthere
is a 1:1 or a 1:n relationship between the parent (owner) and the child (member)
segments(records)inthehierarchical(network)schema,butwealsocannotobtain
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