max_value и min_value не совпадают с базой данных Codeigniter

Question

я пытаюсь вызвать поле tb_pemeliharaan.isi_kriteria max_value и min_value с соединением tb_pemeliharaan.id_kriteria tb_kriteria.id_kriteria

это мой контроллер:

function Index()
    {
      $data['data'] = $this->PenilaianModel->GetPerKendaraan();
      if($this->session->userdata['logged_in']['level']=="1"){
      $id_merk = $this->uri->segment(3);
      $data['data'] = $this->PenilaianModel->GetPerKendaraan();
      $data['kriteria'] = $this->PenilaianModel->GetKriteria();
      $data['getsifat'] = $this->PenilaianModel->getDataSifat(); //call model
      $this->template->load('template', 'Data/DataPenilaian', $data);
      }else{
        $data['error'] = 'Anda tidak dapat mengakses halaman tersebut.';
        $this->template->load('template', 'error', $data);
      } 
    }

это моя модель:

public function getDataSifat()
    {
        $this->db->select('DISTINCT(tb_pemeliharaan.id_kendaraan), nama, platno');
        $this->db->from('tb_pemeliharaan');
        $this->db->join('tb_kendaraan', 'tb_kendaraan.id_kendaraan = tb_pemeliharaan.id_kendaraan');
        $query = $this->db->get();
        $data = array();
        $index = 0;
        if($query->num_rows() > 0)
        {
            foreach($query->result() as $kr_key => $x){
                $data[$kr_key] = array(
                    'id_kendaraan'   => $x->id_kendaraan,
                    'nama'           => $x->nama,
                    'platno'         => $x->platno,
                );
                $id_kendaraann = $this->db->where('id_kendaraan', $x->id_kendaraan);
                $this->db->select('*');
                $this->db->from('tb_pemeliharaan');
                $query = $this->db->get();
                $id_kendaraann = $query->result();
                foreach($id_kendaraann as $z){
                    $data[$kr_key]['sub'][] = array(
                        'id_kendaraan'      => $z->id_kendaraan,
                        'id_kriteria'       => $z->id_kriteria,
                        'isi_kriteria'      => $z->isi_kriteria // key
                    );
                    $this->db->select_max('tb_pemeliharaan.isi_kriteria', 'max_value'); //my error
                    $this->db->select_min('tb_pemeliharaan.isi_kriteria', 'min_value'); // my error
                    $this->db->select('tb_kriteria.id_kriteria, nama_kriteria, sifat, bobot');
                    $this->db->from('tb_pemeliharaan');
                    $this->db->where('tb_pemeliharaan.id_kriteria',$z->id_kriteria); // i call for key
                    $this->db->join('tb_kriteria', 'tb_kriteria.id_kriteria = tb_pemeliharaan.id_kriteria');
                    $query = $this->db->get();
                    $kriteria = $query->result();
                        foreach($kriteria as $k){
                            // 0 cost, 1 benefit
                            if($k->sifat == "1" ){
                           $hasil =  $k->max_value; // max value not same with database
                            }
                            else if ($k->sifat == "0" ) {
                           $hasil =    $k->min_value; // min value not same with database
                            }
                            $data[$kr_key]['sub']['dari'][] = array(
                                'isi'      => $hasil
                            );
                        }
                }
                $index++;
            }
            return $data;
            // var_dump($data);
        } else{
            return  NULL;
        }
    }

в моей модели max_value и min_value должны быть получены данные isi_kriteria из tb_pemeliharaan where tb_pemeliharaan.id_kriteria == $z->id_kriteria пример: в моем критерии tb_kriteria значение моего поля равно item_broken в базе данных равно 2 , 5,8 но если я попробую эту модель max_value результат будет 5 .... почему? должен быть ответ 8 верно?

мой взгляд:

<!-- Basic datatable -->
<div class="panel panel-flat">
  <div class="panel-heading">
    <h5 class="panel-title">Conversi berdasarkan sifat</h5>
    <div class="heading-elements">
      <ul class="icons-list">
                <li><a data-action="collapse"></a></li>
                <li><a data-action="reload"></a></li>
                <li><a data-action="close"></a></li>
              </ul>
            </div>
  </div>
  <table class="table datatable-basic">
    <thead>
      <tr>
      <th>No</th>
       <th>Kendaraan</th>
       <th>Plat Nomor</th>
       <?php 
      $no=1; 
      if(!empty($kriteria)) {
      foreach ($kriteria as $isi) { ?>
       <th><?php echo $isi['nama_kriteria'] ?></th>  
      <?php }} ?>             
      </tr>
    </thead>
    <tbody>
    <?php 
      $no=1; 
      if(!empty($getsifat)) {
        foreach ($getsifat as $kr_key => $kendaraan) { ?>
      <tr>
      <td><?php echo $no ?> </td>
      <td><?php echo $kendaraan['nama'] ?> </td>
      <td><?php echo $kendaraan['platno'] ?></td>
      <?php foreach ($kendaraan['sub']['dari'] as $data)     { ?> // i call isi from model
      <td><?php echo $data['isi'] ?></td>
      <?php } ?>
      </tr> 
      <?php $no++; }} ?>
    </tbody>
  </table>
</div>
<!-- /basic datatable -->

спасибо за ваш ответ ...