Galère requête MySQL

$query_eleves_lundi=mysql_query("SELECT DISTINCT ia.id_eleve FROM ins_activity ia, activity a WHERE ia.id_activity=a.id_activity AND a.day='1'");
while ($eleves_lundi=mysql_fetch_array($query_eleves_lundi)) {
    $query_act_par_el_lundi=mysql_query("SELECT a.hour_start, a.hour_end FROM activity a, ins_activity ia WHERE ia.id_activity=a.id_activity AND a.day='1' ia.id_eleve='".$eleves_lundi['id_eleve']."'");
    $count_act_par_el_lundi=mysql_num_rows($query_act_par_el_lundi);
    if ($count_act_par_el_lundi!='1') {
        $res_prec='01:00:00';
        while ($act_par_el_lundi=mysql_fetch_array($query_act_par_el_lundi)) {
            if (($res_prec!='01:00:00') && ($act_par_el_lundi['hour_end']>$res_prec)) {
                echo $eleves_lundi['id_eleve'];
            }
            $res_prec=$act_par_el_lundi['hour_start'];
        }
    }
}

select * from ins_activity ia
where ia.ID_ACTIVITY in (
  select a.id_activity
  from activity a
  join activity ab on a.day=ab.day
  where a.HOUR_START between ab.HOUR_START and ab.HOUR_END
  and a.id_activity != ab.id_activity
  and a.id_activity = (select ID_ACTIVITY from ins_activity iab where iab.ID_ELEVE=ia.ID_ELEVE and iab.ID_ACTIVITY=a.id_activity)
  and ab.id_activity = (select ID_ACTIVITY from ins_activity iac where iac.ID_ELEVE=ia.ID_ELEVE and iac.ID_ACTIVITY=ab.id_activity)
  )