SQL & PL/SQL :: How To Show Sequential Numbers For Each Group

MyTable has missing records:

id_group id_column
-------------------------
1 1
1 2
1 3
1 5
1 9
2 1
2 2
2 4
2 6

As you see, it could be missed record 4,6,7 and 8 of group 1 and record 3 and 5 of group 2. That's is an example.

I need to show all sequential records, incluing missing ones, like this:

Id_group Id_column
-------------------------------------------------
1 1
1 2
1 3
1 4
1 5
1 6
1 7
1 8
1 9
2 1
2 2
2 3
2 4
2 5
2 6

I try this query:

/*---*/
Select G.id_group, C.rownum id_column
from (select distinct id_group from MyTable) G
cross join (SELECT rownum FROM dual CONNECT BY LEVEL <= (select
max(id_column) from MyTable)) C
/*---*/

but it shows record 7, 8 and 9 as members of group 2.