Oracle 18c

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CREATE TABLE table_name ( id, col1, source_id ) AS
SELECT 'a1',  765.3, 'a5' FROM DUAL UNION ALL
SELECT 'a2', 3298.3, 'a4' FROM DUAL UNION ALL
SELECT 'a3', 8762.1, 'a8' FROM DUAL UNION ALL
SELECT 'a4', 3298.3, null FROM DUAL UNION ALL
SELECT 'a5',   null, 'a6' FROM DUAL UNION ALL
SELECT 'a6',   null, null FROM DUAL UNION ALL
SELECT 'a7',     10, 'a5' FROM DUAL;
7 rows affected

SELECT id,
       COALESCE(
         col1,
         (
           SELECT SUM( COALESCE( col1, 0 ) )
           FROM   table_name s
           START WITH s.ROWID = t.ROWID
           CONNECT BY source_id = PRIOR id
         )
       ) AS col1,
       source_id
FROM   table_name t;


ID
COL1
SOURCE_ID


a1
765.3
a5

a2
3298.3
a4

a3
8762.1
a8

a4
3298.3
null

a5
775.3
a6

a6
775.3
null

a7
10
a5


SELECT id,
       COALESCE(
         col1,
         (
           SELECT SUM( COALESCE( col1, 0 ) )
           FROM   table_name s
           START WITH s.ROWID = t.ROWID
           CONNECT BY source_id = PRIOR id
                  AND PRIOR col1 IS NULL
         )
       ) AS col1,
       source_id
FROM   table_name t;


ID
COL1
SOURCE_ID


a1
765.3
a5

a2
3298.3
a4

a3
8762.1
a8

a4
3298.3
null

a5
775.3
a6

a6
775.3
null

a7
10
a5

ID	COL1	SOURCE_ID
a1	765.3	a5
a2	3298.3	a4
a3	8762.1	a8
a4	3298.3	null
a5	775.3	a6
a6	775.3	null
a7	10	a5