What are the ids of instructors who taught in the Fall of 2009 but not in the Spring of 2010?
SELECT id FROM teaches WHERE semester  =  'Fall' AND YEAR  =  2009 EXCEPT SELECT id FROM teaches WHERE semester  =  'Spring' AND YEAR  =  2010
taught in the Fall of 2009 refers to semester = 'Fall' AND YEAR = 2009; not in the Spring of 2010 refers to an EXCEPT operation with semester = 'Spring' AND YEAR = 2010;