set pages 12

with A as
(
  select ceil(rownum/3) as i, mod(rownum -1 , 3) + 1 as j, rownum as val from dual connect by level <= 9
),
B as
(
  select ceil(rownum/3) as i, mod(rownum -1 , 3) + 1 as j, 12 - rownum as val from dual connect by level <= 9
)
select A.i, A.j, A.val from A union all
select B.i, B.j, B.val from B
;

         I          J        VAL
---------- ---------- ----------
         1          1          1
         1          2          2
         1          3          3
         2          1          4
         2          2          5
         2          3          6
         3          1          7
         3          2          8
         3          3          9

         I          J        VAL
---------- ---------- ----------
         1          1         11
         1          2         10
         1          3          9
         2          1          8
         2          2          7
         2          3          6
         3          1          5
         3          2          4
         3          3          3

with A as
(
  select ceil(rownum/3) as i, mod(rownum -1 , 3) + 1 as j, rownum as val from dual connect by level <= 9
),
B as
(
  select ceil(rownum/3) as i, mod(rownum -1 , 3) + 1 as j, 12 - rownum as val from dual connect by level <= 9
)
----------------------------------------------------------------
select
  A.i as i,
  B.j as j,
  sum (A.val * B.val) as val
from
  A, B
where A.j = B.i
group by A.i, B.j
order by
  1, 2
;

         I          J        VAL
---------- ---------- ----------
         1          1         42
         1          2         36
         1          3         30
         2          1        114
         2          2         99
         2          3         84
         3          1        186
         3          2        162
         3          3        138

with A AS (...), B as (...),
     C AS (select i, j from A union i, j from B)
select C.i, C.j, COALESCE(A.val * B.val, 0) AS val
from C
left join A on (A.i = C.i AND A.j = C.j)
left join B on (B.i = C.i AND B.j = C.j)
order by 1, 2;