Find courses that ran in Fall 2009 and in Spring 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 that ran in Fall 2009 refers to semester = 'Fall' AND YEAR = 2009; courses that ran in Spring 2010 refers to semester = 'Spring' AND YEAR = 2010; finding courses that meet both conditions requires an INTERSECT between the two selections;