public function field($field, $alias = null) {
        if (is_string($field)) {
            $fields = explode(',', trim($field));
            foreach ($fields as $field) {
                $this->fields [] = Condition::cleanField($field . ($alias ? ' AS ' . $alias : ''));
            }
        } else if ($field instanceof Query) {
            if ($alias) {
                $field->alias($alias);
            }
            $this->fields [] = $field;
        } else if ($field instanceof ImmutableValue) {
            if ($alias) {
                $field->alias($alias);
            }
            $this->fields [] = $field;
        }

        return $this;
    }