Спасибо всем, это решено левым соединением.
SELECT
sm.`student_id`, sm.`subject_id`, s.`name` AS subject_name, sm.`marks_obtained` AS e_11 , e2.`marks_obtained` AS e_14
FROM
ims_studentmarks sm INNER JOIN ims_subject s ON s.`id` = sm.`subject_id`
LEFT JOIN ims_studentmarks e2 ON e2.`subject_id` = sm.`subject_id` AND e2.`student_id` = sm.`student_id` AND e2.`exam_id` = 14
WHERE sm.`exam_id` = 11
AND sm.student_id IN (1789)