Contoh Code : 

$this->db->select('SUBSTRING(zsmonth, 5, 2) as month', FALSE)
        ->from('tblsales_month'); 

Contoh Code Lain :

public function count_setor_bulan()
 {   
  $bulan = date('Y-m');
     $this->db->select_sum('value');
     $this->db->join('t_nasabah', 't_nasabah.kd_nasabah = t_transaksi.kd_nasabah');
     $this->db->where('jenis_trans','Setor');
     $this->db->where('SUBSTRING(tanggal, 1, 7)=',$bulan);
     $query = $this->db->get('t_transaksi');
     if($query->num_rows()>0)
     {
       return $query->row()->value;
     }
     else
     {
       return 0;
     }
 }

Referensi : https://stackoverflow.com/questions/20486162/substring-query-in-codeigniter