建表语句:create table t5_order( ord_id int COMMENT '订单ID', ord_time timestamp COMMENT '订单时间', user_id varchar(1) COMMENT '用户', is_valid int COMMENT '订单是否有效');insert into t5_order(ord_id,ord_time,user_id,is_valid)values (1,'2023-12-11 12:01:03','a',1), (2,'2023-12-11 12:02:06','a',0), (3,'2023-12-11 12:03:15','a',0), (4,'2023-12-11 12:04:20','a',1), (5,'2023-12-11 12:05:03','a',1), (6,'2023-12-11 12:01:02','b',1), (7,'2023-12-11 12:03:03','b',0), (8,'2023-12-11 12:04:01','b',1), (9,'2023-12-11 12:07:03','b',1);当我执行如下代码:select ord_id, ord_time, user_id, is_valid, lag(ord_id) over (partition by user_id order by ord_time) as last_valid_ord_idfrom (select ord_id, ord_time, user_id, is_valid from t5_order where is_valid = 1) t ;为什么执行结果中 ord_id = 6 记录对应的 last_valid_ord_id 列不是 5 呢,不应该是 null 吗?