Find the id of instructors who taught a class in Fall 2009 but not in Spring 2010.
SELECT id FROM teaches WHERE semester  =  'Fall' AND YEAR  =  2009 EXCEPT SELECT id FROM teaches WHERE semester  =  'Spring' AND YEAR  =  2010
taught a class in Fall 2009 refers to semester = 'Fall' AND year = 2009; not in Spring 2010 refers to EXCEPT SELECT id WHERE semester = 'Spring' AND year = 2010;