Grouping by time buckets

Question

I have an sql that does some counts by hour for the last 24 hours. I wanted to change this to use for the last 4 hours but instead of 24 buckets I wanted 16 buckets of 15 minutes each.
For the last 4 hours in hour buckets seem to work fine. But for the last 4 hours in 16 buckets, doesn't work fine as it seems to give me the totals for the 1 bucket and repeats the same for the rest of the 15. Could some look let me know what I'm doing wrong here, thanks.
create table test(dat date, ct1 number, ct2 number);
declare
begin
FOR i IN 1 .. 16
LOOP
insert into test values((sysdate - 4/24)+i*1/96, i, i*2);
commit;
END LOOP;
end;
with dt_minmax as (SELECT trunc(sysdate-4/24,'hh24') AS min_dt,
               trunc(sysdate,'hh24') AS max_dt
          FROM DUAL),
     dense_days AS
     (SELECT     min_dt + LEVEL / 24 AS dt, min_dt, max_dt, level lvl
            FROM dt_minmax
      CONNECT BY LEVEL <=  4)
SELECT   dt AS period,
         SUM (s.ct1 + s.ct2) AS BYTES,
         COUNT (TRUNC (s.dat, 'HH24')) AS activity
    FROM dense_days dd LEFT OUTER JOIN test s ON dd.dt = TRUNC (s.dat, 'HH24')
GROUP BY dd.dt
ORDER BY dd.dt;
PERIOD                    BYTES   ACTIVITY
15-Dec-2007 23:00:00         66          4
16-Dec-2007 00:00:00        114          4
16-Dec-2007 01:00:00        162          4
16-Dec-2007 02:00:00         48          1
with dt_minmax as (SELECT trunc(sysdate-4/24,'hh24') AS min_dt,
               trunc(sysdate,'hh24') AS max_dt
          FROM DUAL),
     dense_days AS
     (SELECT     min_dt + LEVEL / 96 AS dt, min_dt, max_dt, level lvl
            FROM dt_minmax
      CONNECT BY LEVEL <=  16)
SELECT   dt AS period,
         SUM (s.ct1 + s.ct2) AS BYTES,
         COUNT (TRUNC (s.dat, 'HH24')+lvl/96) AS activity
    FROM dense_days dd LEFT OUTER JOIN test s ON dd.dt = (TRUNC (s.dat, 'HH24')+lvl/96)
GROUP BY dd.dt
ORDER BY dd.dt;
PERIOD                    BYTES   ACTIVITY
15-Dec-2007 22:15:00         18          3
15-Dec-2007 22:30:00         18          3
15-Dec-2007 22:45:00         18          3
15-Dec-2007 23:00:00         18          3
15-Dec-2007 23:15:00         18          3
15-Dec-2007 23:30:00         18          3
15-Dec-2007 23:45:00         18          3
16-Dec-2007 00:00:00         18          3
16-Dec-2007 00:15:00         18          3
16-Dec-2007 00:30:00         18          3
16-Dec-2007 00:45:00         18          3
16-Dec-2007 01:00:00         18          3
16-Dec-2007 01:15:00         18          3
16-Dec-2007 01:30:00         18          3
16-Dec-2007 01:45:00         18          3
16-Dec-2007 02:00:00         18          3
16 rows selected.

Answer 1

I'm not sure if ntile would work in this case, since ntile puts them into equal number of buckets. But we want them to be divided by time range as in 15 minutes of 16 buckets fro 4 hours. But if there are more number of records in say one 15 minute cycle, some of these records will be pushed to the next cycle as it tries tp put them in equal buckets.
SQL> SELECT   bucket, dat
    FROM (SELECT NTILE (16) OVER (ORDER BY dat) bucket, dat
            FROM TEST
           WHERE dat >= trunc(sysdate-4/24,'hh24') and dat <= trunc(sysdate,'hh24')) s
GROUP BY bucket,dat
order by bucket;  2    3    4    5    6
    BUCKET DAT
         1 25-Dec-2007 10:32:02
         2 25-Dec-2007 10:32:03
         2 25-Dec-2007 10:32:04
         3 25-Dec-2007 10:32:05
         3 25-Dec-2007 10:38:48
         4 25-Dec-2007 10:47:02
         4 25-Dec-2007 10:53:48
         5 25-Dec-2007 11:02:02
         5 25-Dec-2007 11:08:48
         6 25-Dec-2007 11:17:02
         6 25-Dec-2007 11:23:48
    BUCKET DAT
         7 25-Dec-2007 11:32:02
         7 25-Dec-2007 11:38:48
         8 25-Dec-2007 11:47:02
         8 25-Dec-2007 11:53:48
         9 25-Dec-2007 12:02:02
         9 25-Dec-2007 12:08:48
        10 25-Dec-2007 12:17:02
        10 25-Dec-2007 12:23:48
        11 25-Dec-2007 12:32:02
        11 25-Dec-2007 12:38:48
        12 25-Dec-2007 12:47:02
    BUCKET DAT
        12 25-Dec-2007 12:53:48
        13 25-Dec-2007 13:02:02
        13 25-Dec-2007 13:08:48
        14 25-Dec-2007 13:17:02
        14 25-Dec-2007 13:23:48
        15 25-Dec-2007 13:32:02
        15 25-Dec-2007 13:38:48
        16 25-Dec-2007 13:47:02
        16 25-Dec-2007 13:53:48
31 rows selected.