WINDOW version

gistfile1.txt

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;