Identify the cost difference between Priority and Express shipping methods.
"Priority" and "Express" are both method_name; cost difference = Subtract (Sum(cost where method_name = 'Express'), Sum(cost where method_name 'Priority'))
SELECT SUM(CASE WHEN method_name = 'Priority' THEN cost ELSE 0 END) - SUM(CASE WHEN method_name = 'Express' THEN cost ELSE 0 END) FROM shipping_method