select to group date and time in 5-minute intervals

I wish count the rows of a table grouped by date and time intervals of 5 minutes:

for example, if the minutes portion of HH:MM falls between 00 mins and 04 min it will be counted as 00, eg. 08:04 will be counted as 08:00

if the minutes portion falls between 05 mins and 09 mins it will be counted as 05, eg. 08:06 will be counted as 08:05

Table Data

Date       Time
18/01/18    08:00
18/01/18    08:01
18/01/18    08:02
18/01/18    08:03
18/01/18    08:04

18/01/18    08:05
18/01/18    08:06
18/01/18    08:08

18/01/18    08:10

19/01/18    17:01
19/01/18    17:03
19/01/18    17:04

Expected Output

DATE         TIME  COUNT
18/01/2018   08:00   5
18/01/2018   08:05   3
18/01/2018   08:10   1
19/01/2018   17:00   3

Table Creation

create table TAB1 (tDATE DATE,tTIME VARCHAR2(5));

Data

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:00');

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:01');

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:02');

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:03');

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:04');

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:05');

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:06');

insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:08');


insert into TAB1(tDATE,tTIME) values (to_date('18/01/2018','DD/MM/YYYY'),'08:10');

insert into TAB1(tDATE,tTIME) values (to_date('19/01/2018','DD/MM/YYYY'),'17:01');

insert into TAB1(tDATE,tTIME) values (to_date('19/01/2018','DD/MM/YYYY'),'17:03');

insert into TAB1(tDATE,tTIME) values (to_date('19/01/2018','DD/MM/YYYY'),'17:04');

Solution

Thank you all. As to why there is a separate VARCHAR2 column for the time component, the tables were originally migrated from some legacy database that had a date type but without a time component the latter which was stored as a string. Here is my own idea which gives me exactly what I want:

select tDATE,substr(tTIME,1,3)||
         case 
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 0 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 5 then '00' 
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 5 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 10 then '05'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 10 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 15 then '10'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 15 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 20 then '15'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 20 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 25 then '20'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 25 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 30 then '25'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 30 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 35 then '30'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 35 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 40 then '35'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 40 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 45 then '40'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 45 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 50 then '45'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 50 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 55 then '50'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 55 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 60 then '55'
           else '00'
         end as tTIME
,count(*)
from TAB1
group by tDATE,substr(tTIME,1,3)||
         case 
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 0 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 5 then '00' 
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 5 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 10 then '05'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 10 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 15 then '10'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 15 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 20 then '15'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 20 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 25 then '20'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 25 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 30 then '25'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 30 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 35 then '30'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 35 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 40 then '35'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 40 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 45 then '40'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 45 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 50 then '45'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 50 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 55 then '50'
           when to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) >= 55 and to_number(regexp_substr(substr(tTIME,4,2),'^[-]?[[:digit:]]*\.?[[:digit:]]*$')) < 60 then '55'
           else '00'
         end
order by 3 desc;