我有表[访问]。当order_number为空时，我需要获取按user_id分组的行和visit_duration_seconds的总和，例如，对于用户[2875636]，我将得到:61+151+33+13。每行应包含其前几行的总和。请引用下方预期结果中的RESULT列user_idstarttimevisit_duration_secondsorder_number28756362013-01-1618:03:506128756362013-01-1618:08:1815128756362013-01-1618:15:433328756362013-01-1618:16:371328