What are the ids for courses that were offered in both Fall of 2009 and Spring of 2010?
SELECT course_id FROM SECTION WHERE semester  =  'Fall' AND YEAR  =  2009 INTERSECT SELECT course_id FROM SECTION WHERE semester  =  'Spring' AND YEAR  =  2010
courses offered in both Fall of 2009 and Spring of 2010 refers to the use of INTERSECT between two SELECT statements, one for each semester and year condition;