def select_join(self, table, field, join_table, join_field):
        """.. :py:method::

        Usage::

            >>> select_join('hospital', 'id', 'department', 'hospid')
            select hospital.id from hospital left join department on hospital.id=department.hospid where department.hospid is null;

        """
        sql = "select {table}.{field} from {table} left join {join_table} "\
              "on {table}.{field}={join_table}.{join_field} "\
              "where {join_table}.{join_field} is null;".format(table=table,
                                                                field=field,
                                                                join_table=join_table,
                                                                join_field=join_field)
        return super(PGWrapper, self).execute(sql, result=True).results