Weird many to many and one to many relationship

SELECT s.name, c.className, (s.id_class = c.id_class) AS is_current
FROM s JOIN many_to_many AS mtm ON (s.id_student = mtm.id_student)
  JOIN c ON (c.id_class = mtm.id_class);

SELECT s.name, c.className,
  CASE WHEN s.id_class = c.id_class THEN 'current' ELSE 'past' END AS is_current
FROM s JOIN many_to_many AS mtm ON (s.id_student = mtm.id_student)
  JOIN c ON (c.id_class = mtm.id_class);

SELECT s.name AS "student name", c1.className AS "student class", c2.className as "class list"
FROM s
JOIN many_to_many mtm ON s.id_student = mtm.id_student
JOIN c c1 ON s.id_class = c1.id_class
JOIN c c2 ON mtm.id_class = c2.id_class

SELECT * FROM relAC RAC
  INNER JOIN tableA A ON A.id_class = RAC.id_class
  INNER JOIN tableC C ON C.id_class = RAC.id_class
    WHERE A.id_class = B.id_class