SQL & PL/SQL :: Grouping Results By Ranges?

I have this table,

Create table TBL_OK_HIST
(
DATE_KEY NUMBER,
A_N VARCHAR2(22 BYTE),
R_DUR VARCHAR2(8 BYTE),
CH_DUR VARCHAR2(8 BYTE),
REV VARCHAR2(20 BYTE)
)

insert into TBL_OK_HIST
values (20101010,123456768,5,20,2);
insert into TBL_OK_HIST
values (20101010,123496568,15,20,2);
insert into TBL_OK_HIST
values (20101012,122235768,25,25,3);
[Code] ......

Thus, applying the following would yeld:

Select * from TBL_OK_HIST

DATE_KEYA_N R_DUR CH_DUR REV
201010101234567685202
2010101012349656815202
2010101212223576825253
201010112345676819202
2010101234567681252527
2010101323456768136365
2010101056768123411202
2010101134681256717202
2010101068123456755559

generate the following results:

range_start_rdur range_end_rdur no_of_an sum_of_rdur sum_of_chdur sum_of_rev
1 5 1 5 20 2
6 10 1 9 20 2
11 15 2 26 40 4
16 20 1 17 20 2
21 25 1 25 25 3
26 30 0 0 0 0
31 35 0 0 0 0
36 40 1 36 36 5
41 45 0 0 0 0
46 50 0 0 0 0
51 55 2 107 107 16

I thought I would make use of the following query, but I am not getting the proper results when applying it to a real table with more than 20 mln records:

SELECT trunc(R_DUR/6)*5+1 as range_start_rdur,
trunc(R_DUR/6)*5+5 range_end_rdur,
sum(noofan) as no_of_an,
sum(sumofrdur) as sum_of_rdur,
sum(sumofchdur) as sum_of_chdur,
[Code] ...........