# sql = %Q(delete from exercise_answers where (exercise_question_id, user_id, exercise_choice_id) in
# (select * from (select exercise_question_id, user_id, exercise_choice_id from exercise_answers group by exercise_question_id, user_id, exercise_choice_id having count(*) > 1) a)
# and id not in (select * from (select min(id) from exercise_answers group by exercise_question_id, user_id, exercise_choice_id having count(*) > 1 order by id) b))