SQL & PL/SQL :: Create A Temporary Table With Same Structure

create table my_rows
(
my_envvarchar2(100),
anumber(2),
bnumber(2)
)
/
insert into my_rows values ('A', 10, 20);
insert into my_rows values ('A', 10, 20);
insert into my_rows values ('A', 10, 20);
insert into my_rows values ('A', 10, 20);
insert into my_rows values ('A', 10, 20);
insert into my_rows values ('A', 10, 20);
insert into my_rows values ('A', 10, 20);
insert into my_rows values ('A', 10, 20);
[code]....

The first row means that the value 10 represents 40% in the couple (10,20). Meaning if I have 100 rows with the couple (10,20), 40 rows will be marked with the value 10 and 60 will be marked with the value 20. To do this, I used to create a temporary table with the same structure as the my_rows table with a new column "the_value" and I used to update this new column wth a PL/SQL for loop. But I think it is doable in a signle SQL.