# validate fields//////////////////////////////////////////
    if (!is_null($fields)) {
        $fields_arr=explode(",",$fields);
        $default_fields=['province_id','id','name'];
        if(!empty(array_diff($fields_arr,$default_fields)))
            return [];
    $fields_str=implode(",",$fields_arr);
    }else $fields_str='*';
    #/////////////
    $sql = "select $fields_str from city $where $limit";