<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def where_clause(self):
 """ convert self.restriction to the SQL WHERE clause """

        cond = self._make_condition(self.restriction)
        return '' if cond is True else ' WHERE %s' % cond