def unique(self, *args):
        """
        Returns all unique values as a DataFrame. This is executing:
            SELECT
                DISTINCT
                    <name_of_the_column_1>
                    , <name_of_the_column_2>
                    , <name_of_the_column_3>
                    ...
            FROM
                <name_of_the_table>

        Parameters
        ----------
        *args: columns as strings

        Examples
        --------
        >>> from db import DemoDB
        >>> db = DemoDB()
        >>> db.tables.Track.unique("GenreId")
            GenreId
        0         1
        1         2
        2         3
        3         4
        4         5
        5         6
        6         7
        7         8
        8         9
        9        10
        10       11
        11       12
        12       13
        13       14
        14       15
        15       16
        16       17
        17       18
        18       19
        19       20
        20       21
        21       22
        22       23
        23       24
        24       25
        >>> len(db.tables.Track.unique("GenreId", "MediaTypeId"))
        38
        """
        q = self._query_templates['table']['unique'].format(columns=self._format_columns(args), schema=self.schema,
                                                            table=self.name)

        return pd.read_sql(q, self._con)
Returns all unique values as a DataFrame. This is executing:
            SELECT
                DISTINCT
                    <name_of_the_column_1>
                    , <name_of_the_column_2>
                    , <name_of_the_column_3>
                    ...
            FROM
                <name_of_the_table>

        Parameters
        ----------
        *args: columns as strings

        Examples
        --------
        >>> from db import DemoDB
        >>> db = DemoDB()
        >>> db.tables.Track.unique("GenreId")
            GenreId
        0         1
        1         2
        2         3
        3         4
        4         5
        5         6
        6         7
        7         8
        8         9
        9        10
        10       11
        11       12
        12       13
        13       14
        14       15
        15       16
        16       17
        17       18
        18       19
        19       20
        20       21
        21       22
        22       23
        23       24
        24       25
        >>> len(db.tables.Track.unique("GenreId", "MediaTypeId"))
        38