What is the difference between the number of communities that are located on the north and south sides with a population of more than 30,000?
population of more than 30000 refers to Population > 30000; 'North' and 'South' both are the side; different number of community refers to Subtract(Count(Community_area_no where side = 'South'), Count(Community_area_no where side = 'North'))
SELECT SUM(CASE WHEN side = 'South ' THEN 1 ELSE 0 END) - SUM(CASE WHEN side = 'North' THEN 1 ELSE 0 END) AS DIFF FROM Community_Area WHERE population > 300000