public function fetchField($field=null, $options=null)
    {
        $this->getQuery()->type('select');
        if ($options) {
            $this->getQuery()->set(static::formatQueryOptions($options));
        }
        if ($field) {
            $this->getQuery()->remove('fields')->set('fields', $field);
        }
        $fields = StringHelper::explode(',', $this->getQuery()['fields']);
        $fields = array_map(function ($v) {
            $vs = preg_split('/(as)?\s+/', trim($v));
            return $vs[count($vs)-1];
        }, $fields);
        
        $rows = $this->query();
        
        $result = [];
        if ($rows) foreach ($rows as $v) {
            $result[] = $v[$fields[0]];
        }
        return $result;
    }