SUBSTRING_INDEX与codeigniter中的GROUP_CONCAT不起作用

Question

公共函数getProductsById（$ id）
    {

    $this->db->select('product.*,SUBSTRING_INDEX(GROUP_CONCAT(product_properties.name), " ", 4) as pname');       
    $this->db->from('product');        
    $this->db->where('product.subcategory_id',$id);                
    $this->db->join('product_properties','product_properties.product_id=product.id');
    $this->db->group_by('product_properties.product_id');
    return @$this->db->get()->result_array();
}

This is the error i see

请帮助！