tehmoth

SELECT name, subject, grade FROM 
  (SELECT student_id, subject_id, grade, RANK() OVER (PARTITION BY student_id ORDER BY grade) 
   FROM exams) 
 AS grades, students, subjects 
 WHERE grades.subject_id = subjects.subject_id AND grades.student_id = students.student_id 
 AND rank = 1 ORDER BY grade;

tehmoth

SELECT name, subject, grade FROM 
  (SELECT students.name, subjects.subject, grade, RANK() over (PARTITION BY students.student_id ORDER BY grade) rank 
    FROM exams, students, subjects 
    WHERE exams.subject_id = subjects.subject_id AND exams.student_id = students.student_id) AS grades 
WHERE rank = 1;