操纵表中的行

Question

    function getData(){
    $id = $this->uri->segment(3);
    $this->db->select("t.*, mt.mid as mid, mt.amt as mtamt, m.user");
    $this->db->from("transaction as t");
    $this->db->join("mtrans as mt", "mt.tid = t.id and mt.mid = '$id'", "left outer");
    $this->db->join("member as m", "m.id = mt.aid", "left outer");
    $query = $this->db->get();
    $data = $query->result_array();

    return $data;
}

对于初学者来说，这是我的代码。如何操作它，以便mt.amt t.id和mt.mid mt.amt显示为1行，显示mt.amt列中所有{{1}}的总和？