Oracle 21c

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/*
+----+-------+-------+-------+---------------+
| id | col_c | level | class | WANTED_OUTPUT |
+----+-------+-------+-------+---------------+
| a  | q1    | 1     | c99   | c99           |
+----+-------+-------+-------+---------------+
| a  | w2    | 2     | NULL  | c99           |
+----+-------+-------+-------+---------------+
| a  | g6    | 2     | NULL  | c99           |
+----+-------+-------+-------+---------------+
| a  | j5    | 3     | NULL  | c99           |
+----+-------+-------+-------+---------------+
| a  | x8    | 1     | c3    | c3            |
+----+-------+-------+-------+---------------+
| a  | x9    | 2     | NULL  | c3            |
+----+-------+-------+-------+---------------+
| a  | h5    | 1     | c67   | c67           |
*/
ORA-01742: comment not terminated properly

CREATE TABLE tab
AS
SELECT 1 AS rn, 'a' AS id, 'q1' AS col_c, 1 AS lvl, 'c99' AS class FROM dual
1 rows affected

INSERT INTO tab
SELECT 2,'a' AS id, 'w2' AS col_c, 2 AS lvl, NULL AS class FROM dual
1 rows affected

INSERT INTO tab
SELECT 3,'a' AS id, 'g6' AS col_c, 2 AS lvl, NULL AS class FROM dual
1 rows affected

INSERT INTO tab
SELECT 4,'a' AS id, 'j5' AS col_c, 3 AS lvl, NULL AS class FROM dual
1 rows affected

INSERT INTO tab
SELECT 5,'a' AS id, 'x8' AS col_c, 1 AS lvl, 'c3' AS class FROM dual
1 rows affected

INSERT INTO tab
SELECT 6,'a' AS id, 'x9' AS col_c, 2 AS lvl, NULL AS class FROM dual
1 rows affected

INSERT INTO tab
SELECT 7,'a' AS id, 'h5' AS col_c, 1 AS lvl, 'c67' AS class FROM dual
1 rows affected

SELECT tab.*,    
   LAST_VALUE(class) IGNORE NULLS OVER (PARTITION BY id ORDER BY rn 

    RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
) AS wanted 
FROM tab


RN
ID
COL_C
LVL
CLASS
WANTED


1
a
q1
1
c99
c99

2
a
w2
2
null
c99

3
a
g6
2
null
c99

4
a
j5
3
null
c99

5
a
x8
1
c3
c3

6
a
x9
2
null
c3

7
a
h5
1
c67
c67

RN	ID	COL_C	LVL	CLASS	WANTED
1	a	q1	1	c99	c99
2	a	w2	2	null	c99
3	a	g6	2	null	c99
4	a	j5	3	null	c99
5	a	x8	1	c3	c3
6	a	x9	2	null	c3
7	a	h5	1	c67	c67