Find courses that ran in Fall 2009 or in Spring 2010.
SELECT course_id FROM SECTION WHERE semester  =  'Fall' AND YEAR  =  2009 UNION SELECT course_id FROM SECTION WHERE semester  =  'Spring' AND YEAR  =  2010
courses that ran in a specific semester and year refers to filtering by semester and year; 'Fall 2009' refers to semester = 'Fall' AND year = 2009; 'Spring 2010' refers to semester = 'Spring' AND year = 2010; combining results from two different time periods uses UNION.