def default_dataset(self):
        """google.cloud.bigquery.dataset.DatasetReference: the default dataset
        to use for unqualified table names in the query or :data:`None` if not
        set.

        The ``default_dataset`` setter accepts:

        - a :class:`~google.cloud.bigquery.dataset.Dataset`, or
        - a :class:`~google.cloud.bigquery.dataset.DatasetReference`, or
        - a :class:`str` of the fully-qualified dataset ID in standard SQL
          format. The value must included a project ID and dataset ID
          separated by ``.``. For example: ``your-project.your_dataset``.

        See
        https://g.co/cloud/bigquery/docs/reference/v2/jobs#configuration.query.defaultDataset
        """
        prop = self._get_sub_prop("defaultDataset")
        if prop is not None:
            prop = DatasetReference.from_api_repr(prop)
        return prop