Which semesters do not have any student enrolled? List the semester name.
SELECT semester_name FROM Semesters WHERE semester_id NOT IN( SELECT semester_id FROM Student_Enrolment )
SELECT semester_name FROM Semesters WHERE semester_id NOT IN( SELECT semester_id FROM Student_Enrolment )
[{'semester_name': 'fall 2010'}, {'semester_name': 'spring 2013'}, {'semester_name': 'spring 2014'}, {'semester_name': 'spring 2016'}, {'semester_name': 'spring 2017'}, {'semester_name': 'winter 2018'}]