Oracle sql set end date based on previous start date - oracle

Question

I have one table where I need to add new column endDate for future implementation but since we have currently only start date for all records I need to set endDate which should be equal to start date from previous record that are connected by userId and if it is only one record for that user than end date will have some value in future.
For example:
Table structure:
ID | USER_ID | START_DATE | END_DATE
-------------------------------------
1 | 1 | 01.01.2015 |
2 | 1 | 01.01.2016 |
3 | 1 | 01.07.2018 |
4 | 1 | 01.08.2021 |
5 | 2 | 01.01.2015 |
6 | 3 | 01.01.2016 |
7 | 3 | 01.07.2018 |
8 | 4 | 01.08.2021 |
Expected result should be like this
ID | USER_ID | START_DATE | END_DATE
-------------------------------------
1 | 1 | 01.01.2015 | 01.01.2016
2 | 1 | 01.01.2016 | 01.07.2018
3 | 1 | 01.07.2018 | 01.08.2021
4 | 1 | 01.08.2021 | 01.01.2050
5 | 2 | 01.01.2015 | 01.01.2050
6 | 3 | 01.01.2016 | 01.07.2018
7 | 3 | 01.07.2018 | 01.01.2050
8 | 4 | 01.08.2021 | 01.01.2050
Can someone help me with how query in oracle databse should look to update it like this?
I've tried something with for loop but not sure how to continue from this step
DECLARE
CURSOR c_contract
IS
SELECT
USER_ID
FROM
CONTRACT
ORDER_BY START_DATE

BEGIN
FOR r_contract IN c_contract
LOOP

dbms_output.put_line( r_contract.USER_ID );
END LOOP;

END;

Answer 1

Use the LEAD analytic function with the default date as the third argument:
SELECT t.*,
LEAD( start_date, 1, DATE '2050-01-01') OVER (
PARTITION BY user_id
ORDER BY start_date
) AS end_date
FROM table_name t
Which, for the sample data:
ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name ( ID, USER_ID, START_DATE ) AS
SELECT 1, 1, DATE '2015-01-01' FROM DUAL UNION ALL
SELECT 2, 1, DATE '2016-01-01' FROM DUAL UNION ALL
SELECT 3, 1, DATE '2018-07-01' FROM DUAL UNION ALL
SELECT 4, 1, DATE '2021-08-01' FROM DUAL UNION ALL
SELECT 5, 2, DATE '2015-01-01' FROM DUAL UNION ALL
SELECT 6, 3, DATE '2016-01-01' FROM DUAL UNION ALL
SELECT 7, 3, DATE '2018-07-01' FROM DUAL UNION ALL
SELECT 8, 4, DATE '2021-08-01' FROM DUAL;
Outputs:
ID
USER_ID
START_DATE
END_DATE
1
1
2015-01-01 00:00:00
2016-01-01 00:00:00
2
1
2016-01-01 00:00:00
2018-07-01 00:00:00
3
1
2018-07-01 00:00:00
2021-08-01 00:00:00
4
1
2021-08-01 00:00:00
2050-01-01 00:00:00
5
2
2015-01-01 00:00:00
2050-01-01 00:00:00
6
3
2016-01-01 00:00:00
2018-07-01 00:00:00
7
3
2018-07-01 00:00:00
2050-01-01 00:00:00
8
4
2021-08-01 00:00:00
2050-01-01 00:00:00
If you want to add a column then:
ALTER TABLE table_name ADD (end_date DATE);
MERGE INTO table_name dst
USING (
SELECT ROWID AS rid,
LEAD( start_date, 1, DATE '2050-01-01') OVER (
PARTITION BY user_id
ORDER BY start_date
) AS end_date
FROM table_name
) src
ON (dst.ROWID = src.rid)
WHEN MATCHED THEN
UPDATE SET end_date = src.end_date;
fiddle