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】
    Please calculate the percentage of Sub-Saharan African countries which are in the Special trade system. Sub-Saharan African is the name of the region; SystemOfTrade = 'Special trade system'; countries refer to CountryCode; DIVIDE(COUNT (CountryCode where SystemOfTrade = 'Special trade system' and Region = 'Sub-Saharan Africa'), COUNT(CountryCode where Region = 'Sub-Saharan Africa')) as percentage;

    【Evidence】
    Sub-Saharan African is the name of the region; SystemOfTrade = 'Special trade system'; countries refer to CountryCode; DIVIDE(COUNT (CountryCode where SystemOfTrade = 'Special trade system' and Region = 'Sub-Saharan Africa'), COUNT(CountryCode where Region = 'Sub-Saharan Africa')) as percentage;

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

SQL: SELECT  CAST(SUM(CASE WHEN Region = 'Sub-Saharan Africa' THEN 1 ELSE 0 END) AS REAL) * 100 / COUNT(CountryCode) FROM Country WHERE SystemOfTrade = 'Special trade system'