How many items that were shipped via air were returned in 1994?
items refer to l_linenumber; shipped via air in 1994 refers to year(l_shipdate) = 1994 where l_shipmode = 'AIR'; returned refer to l_returnflag = 'R';
SELECT COUNT(l_linenumber) FROM lineitem WHERE l_returnflag = 'R' AND l_shipmode = 'AIR' AND STRFTIME('%Y', l_shipdate) = '1994'