two foreach connect

Hello everyone,
I have a question.
In my website I often have to link two different "foreach".
I usually do it like this

PHP Code:

foreach ($view_category_main as $main):
    $SQL = "SELECT * FROM db_todo_mission WHERE db_todo_mission.tb_todo_mission_main = ".$main['category_id']." GROUP BY db_todo_mission.tb_todo_mission_sub";
    $query = $this->db->query($SQL);
    foreach ($query->result() as $sub):
        echo $sub->tb_todo_mission_sub;
    endforeach;
endforeach; 


Now i wonder if there is a more elegant way to store it in modal and then have it retrieved?