def unique(self):
        """
        Returns all unique values as a DataFrame. This is executing:
            SELECT
                DISTINCT
                    <name_of_the_column>
            FROM
                <name_of_the_table>

        Examples
        --------
        >>> from db import DemoDB
        >>> db = DemoDB()
        >>> db.tables.Customer.FirstName.unique().head(10)
        0         Luis
        1       Leonie
        2     Francois
        3        Bjorn
        4    Franti\u0161ek
        5       Helena
        6       Astrid
        7         Daan
        8         Kara
        9      Eduardo
        Name: FirstName, dtype: object
        >>> len(db.tables.Customer.LastName.unique())
        59
        """
        q = self._query_templates['column']['unique'].format(column=self.name, schema=self.schema,
                                                             table=self.table)
        return pd.read_sql(q, self._con)[self.name]