SQL & PL/SQL

create table a
(
cont_number number,
range_from number,
range_to number
)
/
insert into a values (1,0,100);
insert into a values (2,0,100);
insert into a values (3,0,100);
insert into a values (4,0,1000);
insert into a values (5,0,100);
insert into a values (6,0,100);
/
i want the output shoulld be displayed as
1-2-3   0  100
4-4      0  1000
5-6      0  100