Question: Remove all instructor IDs who did not teach in Spring 2010 from the list. | Did  you  mean  retain  the instructor IDs who  both teach in Spring 2010   and  in Fall 2009 | Retain the instructor IDs who did not teach in Spring 2010 but did teach in Fall 2009
SQL: SELECT id FROM teaches WHERE semester  =  'Fall' AND YEAR  =  2009 EXCEPT SELECT id FROM teaches WHERE semester  =  'Spring' AND YEAR  =  2010