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;
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'