def _get_merged(self, tax_id):
        """Returns tax_id into which `tax_id` has been merged or `tax_id` of
        not obsolete.

        """

        cmd = """
        SELECT COALESCE(
        (SELECT new_tax_id FROM {merged}
         WHERE old_tax_id = {x}), {x})
        """.format(x=self.placeholder, merged=self.merged)

        with self.engine.connect() as con:
            result = con.execute(cmd, (tax_id, tax_id))
            return result.fetchone()[0]