Calculate the difference between the number of shipments shipped by the truck with the model year 2005 and model year 2006.
"2005" and "2006" are both model_year of truck; difference = Subtract (Count (ship_id where model_year = 2005), Count(ship_id where model_year = 2006))
SELECT SUM(CASE WHEN T1.model_year = '2005' THEN 1 ELSE 0 END) - SUM(CASE WHEN T1.model_year = '2006' THEN 1 ELSE 0 END) FROM truck AS T1 INNER JOIN shipment AS T2 ON T1.truck_id = T2.truck_id