code: def get_sql_update_numchild(self, path, incdec='inc'):
          
          sql = "UPDATE %s SET numchild=numchild%s1"\
                " WHERE path=%%s" % (
                    connection.ops.quote_name(
                        get_result_class(self.node_cls)._meta.db_table),
                    {'inc': '+', 'dec': '-'}[incdec])
          vals = [path]
          return sql, vals
docs: :returns: The sql needed the numchild value of a node