code: def sum(self):
          
          res = self._runQuery('SELECT', 'SUM(%s)' % (self._queryTarget,)) or [(0,)]
          assert len(res) == 1, "more than one result: %r" % (res,)
          dbval = res[0][0] or 0
          return self.attribute.outfilter(dbval, _FakeItemForFilter(self.store))
docs: Return the sum of all the values returned by this query.  If no results
          are specified, return None.
  
          Note: for non-numeric column types the result of this method will be
          nonsensical.
  
          @return: a number or None.