Oracle 18c

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CREATE TABLE t
AS
SELECT 'a' AS grp, 3 AS l FROM DUAL UNION ALL
SELECT 'a' AS grp, 5 AS l FROM DUAL UNION ALL
SELECT 'a' AS grp, NULL AS l FROM DUAL UNION ALL
SELECT 'a' AS grp, 7 AS l FROM DUAL UNION ALL
SELECT 'b' AS grp, 2 AS l FROM DUAL UNION ALL
SELECT 'b' AS grp, 6 AS l FROM DUAL UNION ALL
SELECT 'c' AS grp, NULL AS l FROM dual
7 rows affected

SELECT * FROM t


GRP
L


a
3

a
5

a
null

a
7

b
2

b
6

c
null


WITH cte AS (
  SELECT grp, LISTAGG(l, '*') AS product_expression
  FROM t
  GROUP BY grp
)
SELECT c.*, s.val AS product_value
FROM cte c
OUTER APPLY(SELECT *
            FROM XMLTABLE('/ROWSET/ROW/*' passing dbms_xmlgen.getXMLType('SELECT ' || c.product_expression || ' FROM dual')
                          COLUMNS val NUMBER PATH '.')
            WHERE c.product_expression IS NOT NULL) s


GRP
PRODUCT_EXPRESSION
PRODUCT_VALUE


b
2*6
12

c
null
null

a
3*5*7
105

GRP	L
a	3
a	5
a	null
a	7
b	2
b	6
c	null