Oracle 18c

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

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, 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;
5 rows affected

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
CROSS APPLY(SELECT *
            FROM XMLTABLE('/ROWSET/ROW/*' passing dbms_xmlgen.getXMLType('SELECT ' ||c.product_expression || ' FROM dual')
                          COLUMNS val NUMBER PATH '.')) s


GRP
PRODUCT_EXPRESSION
PRODUCT_VALUE


b
2*6
12

a
3*5*7
105


-- step 1
WITH cte AS (
  SELECT grp, LISTAGG(l, '*') AS product_expression
  FROM t
  GROUP BY grp
)
SELECT * FROM cte


GRP
PRODUCT_EXPRESSION


b
2*6

a
3*5*7


-- step 2
WITH cte AS (
  SELECT grp, LISTAGG(l, '*') AS product_expression
  FROM t
  GROUP BY grp
)
SELECT *
FROM cte c
CROSS APPLY(SELECT dbms_xmlgen.getXMLType('SELECT ' ||c.product_expression || ' FROM dual') FROM dual) s


GRP
PRODUCT_EXPRESSION
DBMS_XMLGEN.GETXMLTYPE('SELECT'||C.PRODUCT_EXPRESSION||'FROMDUAL')


b
2*6
<ROWSET>
 <ROW>
  <_x0032__x002A_6>12</_x0032__x002A_6>
 </ROW>
</ROWSET>


a
3*5*7
<ROWSET>
 <ROW>
  <_x0033__x002A_5_x002A_7>105</_x0033__x002A_5_x002A_7>
 </ROW>
</ROWSET>

GRP	PRODUCT_EXPRESSION	DBMS_XMLGEN.GETXMLTYPE('SELECT'\|\|C.PRODUCT_EXPRESSION\|\|'FROMDUAL')
b	2*6	<ROWSET> <ROW> <_x0032__x002A_6>12</_x0032__x002A_6> </ROW> </ROWSET>
a	357	<ROWSET> <ROW> <_x0033__x002A_5_x002A_7>105</_x0033__x002A_5_x002A_7> </ROW> </ROWSET>