Oracle 21c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (LineNumber, item, lvl, numofdays, start_date) AS
  select 1, 123, 1, 2, sysdate from dual union all
  select 2, 123, 2, 2, sysdate + 10 from dual union all
  select 3, 123, 3, 1, null from dual union all
  select 4, 123, 4, 3, null from dual union all
  select 5, 123, 5, 2, null from dual union all
  select 6, 345, 1, 1, sysdate+2 from dual union all
  select 7, 345, 2, 2, null from dual union all
  select 8, 345, 3, 1, null from dual;
8 rows affected

SELECT LineNumber,
       item,
       lvl,
       numofdays,
       first_start_date + COALESCE(total_days, 0) AS start_date
FROM   table_name
MATCH_RECOGNIZE(
  PARTITION BY item
  ORDER BY lvl
  MEASURES
    FIRST(start_date) AS first_start_date,
    SUM(null_start_date.numofdays) AS total_days
  ALL ROWS PER MATCH
  PATTERN (not_null_start_date null_start_date*)
  DEFINE
    not_null_start_date AS start_date IS NOT NULL,
    null_start_date     AS start_date IS NULL
)


LINENUMBER
ITEM
LVL
NUMOFDAYS
START_DATE


1
123
1
2
2023-01-24 18:23:54

2
123
2
2
2023-02-03 18:23:54

3
123
3
1
2023-02-04 18:23:54

4
123
4
3
2023-02-07 18:23:54

5
123
5
2
2023-02-09 18:23:54

6
345
1
1
2023-01-26 18:23:54

7
345
2
2
2023-01-28 18:23:54

8
345
3
1
2023-01-29 18:23:54

LINENUMBER	ITEM	LVL	NUMOFDAYS	START_DATE
1	123	1	2	2023-01-24 18:23:54
2	123	2	2	2023-02-03 18:23:54
3	123	3	1	2023-02-04 18:23:54
4	123	4	3	2023-02-07 18:23:54
5	123	5	2	2023-02-09 18:23:54
6	345	1	1	2023-01-26 18:23:54
7	345	2	2	2023-01-28 18:23:54
8	345	3	1	2023-01-29 18:23:54