code: def get_mysql_update_depth_in_branch(self, path):
          
          vendor = self.node_cls.get_database_vendor('write')
          sql = ("UPDATE %s SET depth=" + sql_length("path", vendor=vendor) + "/%%s WHERE path LIKE %%s") % (
              connection.ops.quote_name(
                  get_result_class(self.node_cls)._meta.db_table), )
          vals = [self.node_cls.steplen, path + '%']
          return sql, vals
docs: :returns: The sql needed to update the depth of all the nodes in a
                    branch.