How do you get average of sums in SQL (multi-level aggregation)?

Question

I have a simplified table xx as follows:

rdate  date
rtime  time
rid    integer
rsub   integer
rval   integer
primary key on (rdate,rtime,rid,rsub)

and I want to get the average (across all times) of the sums (across all ids) of the values.

By way of a sample table, I have (with consecutive identical values blanked out for readability):

rdate       rtime     rid  rsub  rval
-------------------------------------
2010-01-01  00.00.00    1     1    10
                              2    20
                        2     1    30
                              2    40
            01.00.00    1     1    50
                              2    60
                        2     1    70
                              2    80
            02.00.00    1     1    90
                              2   100
2010-01-02  00.00.00    1     1   999

I can get the sums I want with:

select rdate,rtime,rid, sum(rval) as rsum
from xx
where rdate = '2010-01-01'
group by rdate,rtime,rid

which gives me:

rdate       rtime     rid  rsum
-------------------------------
2010-01-01  00.00.00    1    30  (10+20)
                        2    70  (30+40)
            01.00.00    1   110  (50+60)
                        2   150  (70+80)
            02.00.00    1   190  (90+100)

as expected.

Now what I want is the query that will also average those values across the time dimension, giving me:

rdate       rtime    ravgsum
----------------------------
2010-01-01  00.00.00      50  ((30+70)/2)
            01.00.00     130  ((110+150)/2)
            02.00.00     190  ((190)/1)

I'm using DB2 for z/OS but I'd prefer standard SQL if possible.

Answer 1

select rdate,rtime,avg(rsum) as ravgsum from (
    select rdate,rtime,rid, sum(rval) as rsum
    from xx
    where rdate = '2010-01-01'
    group by rdate,rtime,rid
) as subq
group by rdate,rtime