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