Question: Calculate the difference in the average retail price of parts shipped via ship and air.
SQL: SELECT (CAST(SUM(IIF(T3.l_shipmode = 'SHIP', T1.p_retailprice, 0)) AS REAL) / SUM(IIF(T3.l_shipmode = 'SHIP', 1, 0))) - (CAST(SUM(IIF(T3.l_shipmode = 'AIR', T1.p_retailprice, 0)) AS REAL) / SUM(IIF(T3.l_shipmode = 'AIR', 1, 0))) FROM part AS T1 INNER JOIN partsupp AS T2 ON T1.p_partkey = T2.ps_partkey INNER JOIN lineitem AS T3 ON T2.ps_suppkey = T3.l_suppkey