select count(*)
from answer_table a_1, answer_table a_2
where a_1.student_id = a_2.student_id
  and a_1.exam_id = ?
  and a_1.question_id = ?
  and a_1.answer = ?
  and a_2.exam_id = ?
  and a_2.question_id = ?
  and a_2.answer = ?