Sql 忽略与给定列匹配的行的 LAG 函数

在我的Postgres 14数据库中，我有一个类似下面的表.

在我的SELECT中，我想添加两列来显示last_contract_id和next_contract_id，如果它们存在的话.棘手的部分是，如果rental_start_date在下面或上面的行上是相同的，我想跳过该行，直到它改变.所以在本例中，我希望807的last_contract_id为空，而不是806，因为第一行的id为3,rental_start_date发生了变化.

我试图添加这两行，但这并没有跳过具有相同rental_start_date的同行:

lag(contract_id) over (partition by objekt_id order by id) as last_contract_id, 
lead(contract_id) over (partition by objekt_id order by id) as next_contract_id 

Schema:个

CREATE TABLE object_history (
    objekt_id int4 NOT NULL,
    id serial NOT NULL,
    rental_start date NULL,
    rental_end date NULL,
    contract_id int4 NULL
);

INSERT INTO public.object_history
(objekt_id, id, rental_start, rental_end, contract_id)
VALUES(77920, 6, '2023-06-01', '2100-01-01', 807);
INSERT INTO public.object_history
(objekt_id, id, rental_start, rental_end, contract_id)
VALUES(77920, 5, '2023-06-01', '2100-01-01', 806);
INSERT INTO public.object_history
(objekt_id, id, rental_start, rental_end, contract_id)
VALUES(77920, 4, '2023-06-01', '2100-01-01', 803);
INSERT INTO public.object_history
(objekt_id, id, rental_start, rental_end, contract_id)
VALUES(77920, 3, '2023-05-01', '2023-05-31', NULL);
INSERT INTO public.object_history
(objekt_id, id, rental_start, rental_end, contract_id)
VALUES(77920, 2, '2022-01-01', '2023-04-30', 802);
INSERT INTO public.object_history
(objekt_id, id, rental_start, rental_end, contract_id)
VALUES(77920, 1, '2017-11-01', '2021-12-31', NULL);

Select * from object_history;

| objekt_id | id  | rental_start             | rental_end               | contract_id |
| --------- | --- | ------------------------ | ------------------------ | ----------- |
| 77920     | 6   | 2023-06-01T00:00:00.000Z | 2100-01-01T00:00:00.000Z | 807         |
| 77920     | 5   | 2023-06-01T00:00:00.000Z | 2100-01-01T00:00:00.000Z | 806         |
| 77920     | 4   | 2023-06-01T00:00:00.000Z | 2100-01-01T00:00:00.000Z | 803         |
| 77920     | 3   | 2023-05-01T00:00:00.000Z | 2023-05-31T00:00:00.000Z |             |
| 77920     | 2   | 2022-01-01T00:00:00.000Z | 2023-04-30T00:00:00.000Z | 802         |
| 77920     | 1   | 2017-11-01T00:00:00.000Z | 2021-12-31T00:00:00.000Z |             |

View on DB Fiddle个