将sql转换为codeigniter db

Question

有人可以帮我翻译成codeigniter db query

SELECT COUNT(SALES_NO),shop_fc_no  
FROM (
  SELECT MIN(SALES_NO) AS SALES_NO,MIN(SEQ) AS SEQ,shop_fc_no
  FROM TBL_FOOD_ORDER 
  WHERE shop_fc_no = '99997' AND 
        check_flg =0
  GROUP BY SALES_NO,SEQ) AS a

到目前为止，我已经提出了

$this->db->select('SHOP_FC_NO, count(SALES_NO) AS ORDER_COUNT'); 
$this->db->from('TBL_FOOD_ORDER'); 
$this->db->where('SHOP_FC_NO',$shop_fc_no); 
$this->db->where('CHECK_FLG = 0'); 
$query = $this->db->get(); 
return $query->result_array();