#1242 - Subquery returns more than 1 row

SELECT g1.student_id, c.course_name, (SELECT g2.marks
                      FROM grades g2 JOIN courses c1
                      ON c1.id = g2.course_id
       JOIN sequences s1
       ON s1.id = g2.sequence_id
                      WHERE s1.id = 1) AS sequences_1_marks,(SELECT g3.marks
                                                              FROM grades g3 JOIN courses c2
                                                              ON c2.id = g3.course_id
                                                              JOIN sequences s2
                                                              ON s2.id = g3.sequence_id
                                                              WHERE s2.id = 2) AS sequences_2_marks
FROM grades g1 JOIN courses c
ON c.id = g1.course_id
JOIN sequences s
ON s.id = g1.sequence_id
JOIN exams e
ON e.id = s.exam_id
WHERE e.id = 1