Dates and times in a subquery with a group by

Question

I have some data that I need to group to the Month, Day, Year, Hour and minute in a subquery. Then I need to summarize it to the Month and year in the parent query.
If I try to group by the field itself, it is not taking it down to hour and minutes - just the day, so I am losing records.
if I do a TO_char(visitdate, 'DD-MON-YY HH:MI AM') in the subquery, then the main query no longer sees it as a date, so cannot do my TO_CHAR(VISITDATE,'MON-YYYY') in the parent. I could parse out the pieces using string manipulation, but that seems rather silly.
Is there a way to keep as a date in my sub query and then convert to a string?
it looks a little like this, with some other fields that I have to max, sum ...
visit     provider     person     visitdate
1     2     1     12/20/2012 10:30
2     2     2     12/20/2012 10:30
3     2     5     12/20/2012 11:30
4     3     3     12/21/2012 11:30
5     3     4     12/21/2012 11:30
I need to boil this down to
provider     visitdate
2     12/20/2012 10:30
3     12/21/2012 11:30
2     12/20/2012 11:30
Then I use that in a subquery where I use just the month and year
TO_CHAR(VISITDATE,'MON-YYYY') AS APPT_MO_YR
right now if I do a group by visitdate on the subquery it returns
provider     visitdate
2     12/20/2012
3     12/21/2012
even if I do a group by to_date(visitdate, 'DD-MON-YY HH:MI AM')
it is still returning :
provider     visitdate
2     12/20/2012
3     12/21/2012

Answer 1

Hi,
Sorry, it's unclear what you want.
What is the output you want from the given sample data?
Do you want to see data from individual times, and then summaries for the entire month in the same result set? If so, you can use GROUP BY ROLLUP or GROUP BY GROUPING SETS. For example:
SELECT        provider
,        NVL ( TO_CHAR ( visit_date
                        , 'MM/DD/YYYY  HH24_MI'
            , TO_CHAR ( TRUNC (visit_date, 'MONTH')
                        , '   MM/YYYY "Total for month"'
            )           AS d
,        COUNT (*)             AS total_visits
FROM        sample_table
GROUP BY   provider
,        TRUNC (visit_date, 'MONTH')
,             ROLLUP (visit_date)
ORDER BY   provider
,        TRUNC (visit_date, 'MONTH')
,             visit_date
; Output:
` PROVIDER D                          TOTAL_VISITS
         2 12/20/2012  10_30                     2
         2 12/20/2012  11_30                     1
         2    12/2012 Total for month            3
         3 12/21/2012  11_30                     2
         3    12/2012 Total for month            2