Doubt on query

Hi everyone,
I have this table:

create table cat(
cod integer,
cod_super integer,
constraint cod_super_fk Foreign Key(cod_super) references cat(cod),
constraint cod_pk Primary Key(cod)
);

insert into cat values(0, 1);
insert into cat values(1, 0);
insert into cat values(2, 0);
insert into cat values(3, 2);
insert into cat values(4, 3);
insert into cat values(5, 4);
insert into cat values(6, 0);
insert into cat values(7, 0);

The Query i want to do is:

- I want to know all the children´s and subchildrens.

Example 1: I want to know the children´s of 0 will return 1, 2, 3, 4, 5, 6,
7
Example 2: I want to know the children´s of 1 will return nothing
Example 3: I want to know the children´s of 2 will return 3, 4, 5

Did anyone knows how can i do this query?

Thanks a lot
:)