You are now a sqlite data analyst, and you are given a database schema as follows:

    【Schema】
    【DB_ID】 world_development_indicators
【Schema】
# Table: Series
[
(SeriesCode:TEXT, Primary Key, Examples: [AG.AGR.TRAC.NO, AG.CON.FERT.PT.ZS, AG.CON.FERT.ZS]),
(Topic:TEXT),
(IndicatorName:TEXT),
(ShortDefinition:TEXT),
(LongDefinition:TEXT),
(UnitOfMeasure:TEXT, Examples: [`, %, 2005 PPP $]),
(Periodicity:TEXT, Examples: [Annual]),
(BasePeriod:TEXT, Examples: [2005, varies by country, 2011]),
(OtherNotes:INTEGER),
(AggregationMethod:TEXT, Examples: [Weighted average, Sum, Gap-filled total]),
(LimitationsAndExceptions:TEXT),
(NotesFromOriginalSource:TEXT),
(GeneralComments:TEXT),
(Source:TEXT),
(StatisticalConceptAndMethodology:TEXT),
(DevelopmentRelevance:TEXT),
(RelatedSourceLinks:TEXT),
(OtherWebLinks:INTEGER),
(RelatedIndicators:INTEGER),
(LicenseType:TEXT, Examples: [Open, Restricted])
]
# Table: Footnotes
[
(Countrycode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(Seriescode:TEXT, Primary Key, Examples: [AG.LND.FRST.K2, BX.KLT.DINV.CD.WD, DC.DAC.AUSL.CD]),
(Year:TEXT, Primary Key, Examples: [YR1990, YR2000, YR2005]),
(Description:TEXT)
]
# Table: Indicators
[
(CountryName:TEXT, Examples: [Arab World]),
(CountryCode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(IndicatorName:TEXT),
(IndicatorCode:TEXT, Primary Key, Examples: [AG.LND.AGRI.K2, AG.LND.AGRI.ZS, AG.LND.ARBL.HA]),
(Year:INTEGER, Primary Key, Examples: [1961, 1962, 1963]),
(Value:INTEGER, Examples: [133, 87, 6])
]
# Table: SeriesNotes
[
(Seriescode:TEXT, Primary Key, Examples: [DT.DOD.PVLX.CD, IC.CUS.DURS.EX, IC.FRM.BNKS.ZS]),
(Year:TEXT, Primary Key, Examples: [YR2014, YR2002, YR2003]),
(Description:TEXT)
]
# Table: Country
[
(CountryCode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(ShortName:TEXT, Examples: [Afghanistan, Albania, Algeria]),
(TableName:TEXT, Examples: [Afghanistan, Albania, Algeria]),
(LongName:TEXT, Examples: [Islamic State of Afghanistan]),
(Alpha2Code:TEXT, Examples: [AF, AL, DZ]),
(CurrencyUnit:TEXT, Examples: [Afghan afghani, Albanian lek, Algerian dinar]),
(SpecialNotes:TEXT),
(Region:TEXT, Examples: [South Asia]),
(IncomeGroup:TEXT, Examples: [Low income, Upper middle income, High income: nonOECD]),
(Wb2Code:TEXT, Examples: [AF, AL, DZ]),
(NationalAccountsBaseYear:TEXT, Examples: [2002/03]),
(NationalAccountsReferenceYear:TEXT, Examples: [1996, 2013/14, 2010]),
(SnaPriceValuation:TEXT, Examples: [Value added at basic prices (VAB)]),
(LendingCategory:TEXT, Examples: [IDA, IBRD, Blend]),
(OtherGroups:TEXT, Examples: [HIPC, Euro area]),
(SystemOfNationalAccounts:TEXT),
(AlternativeConversionFactor:TEXT, Examples: [199196, 197184, 199095]),
(PppSurveyYear:TEXT, Examples: [Rolling]),
(BalanceOfPaymentsManualInUse:TEXT, Examples: [IMF Balance of Payments Manual, 6th edition.]),
(ExternalDebtReportingStatus:TEXT, Examples: [Actual, Preliminary, Estimate]),
(SystemOfTrade:TEXT, Examples: [General trade system, Special trade system]),
(GovernmentAccountingConcept:TEXT, Examples: [Consolidated central government]),
(ImfDataDisseminationStandard:TEXT, Examples: [General Data Dissemination System (GDDS)]),
(LatestPopulationCensus:TEXT, Examples: [1979, 2011, 2008]),
(LatestHouseholdSurvey:TEXT, Examples: [Multiple Indicator Cluster Survey (MICS), 2010/11]),
(SourceOfMostRecentIncomeAndExpenditureData:TEXT),
(VitalRegistrationComplete:TEXT, Examples: [Yes]),
(LatestAgriculturalCensus:TEXT, Examples: [2013/14, 2012, 2007]),
(LatestIndustrialData:INTEGER, Examples: [2011, 2010, 2002]),
(LatestTradeData:INTEGER, Examples: [2013, 2006, 2012]),
(LatestWaterWithdrawalData:INTEGER, Examples: [2000, 2006, 2001])
]
# Table: CountryNotes
[
(Countrycode:TEXT, Primary Key, Examples: [ABW, ADO, AFG]),
(Seriescode:TEXT, Primary Key, Examples: [EG.EGY.PRIM.PP.KD, EG.ELC.RNEW.ZS, EG.FEC.RNEW.ZS]),
(Description:TEXT)
]
【Foreign keys】
Footnotes.Seriescode=Series.SeriesCode
Footnotes.Countrycode=Country.CountryCode
Indicators.CountryCode=Country.CountryCode
SeriesNotes.Seriescode=Series.SeriesCode
CountryNotes.Seriescode=Series.SeriesCode
CountryNotes.Countrycode=Country.CountryCode

    【Question】
    Which countries in the upper middle income category still have unfinished external debt reporting? Please provide the country codes in your answer. in the upper middle income category refers to incomegroup = 'Upper middle income'; still have unfinished external debt reporting refers to ExternalDebtReportingStatus = 'Preliminary'

    【Evidence】
    in the upper middle income category refers to incomegroup = 'Upper middle income'; still have unfinished external debt reporting refers to ExternalDebtReportingStatus = 'Preliminary'

    Please read and understand the database schema carefully, and generate an executable SQL based on the user's question and evidence. 

SQL: SELECT CountryCode FROM Country WHERE IncomeGroup = 'Upper middle income' AND ExternalDebtReportingStatus = 'Preliminary'