Calculate difference between business that have the highest number of reviews and business that have the lowest number of reviews.
highest number of reviews refers to SUBTRACT(MAX(COUNT(business_id), MIN(COUNT(business_id))))
SELECT ( SELECT COUNT(business_id) FROM Reviews GROUP BY business_id ORDER BY COUNT(business_id) DESC LIMIT 1 ) - ( SELECT COUNT(business_id) FROM Reviews GROUP BY business_id ORDER BY COUNT(business_id) ASC LIMIT 1 ) AS DIFF