How many female students have enlisted for the Army?
female students refers to enlist.name who are NOT in male.name; Army refers to organ = 'army';
SELECT SUM(IIF(T3.name IS NULL, 1, 0)) AS "result" FROM enlist AS T1 INNER JOIN person AS T2 ON T1.name = T2.name LEFT JOIN male AS T3 ON T2.name = T3.name WHERE T1.organ = 'army'