$query="SELECT
s.name AS name, sm.mark AS mark,
sm.date AS date_mark, s.id_student AS id_student FROM yu_student s
LEFT JOIN yu_student_mark sm ON (sm.id_student=s.id_student AND
sm.date>='$date_start' AND
sm.date<='$date_finish' AND sm.id_subject='$id_subject')
WHERE id_group='$id_group'
ORDER BY
s.name";