How many orders shipped via ship have a medium priority?
orders shipped via ship refer to o_orderkey where l_shipmode = 'SHIP'; medium priority refers to o_orderpriority = '3-MEDIUM';
SELECT COUNT(T1.o_orderkey) FROM orders AS T1 INNER JOIN lineitem AS T2 ON T1.o_orderkey = T2.l_orderkey WHERE T2.l_shipmode = 'SHIP' AND T1.o_orderpriority = '3-MEDIUM'