Full outer join unexpected results

Question

Warning, long message - I've searched the forums and found nothing similar. I've cut out as much as possible. We have been given the following schema to work with:
CREATE TABLE CALLS (
TRUNKIN VARCHAR2 (10),
TRUNKOUT VARCHAR2 (10),
DURATION FLOAT)
Here is some test data:
insert into calls values(null, 'a', 3);
insert into calls values(null, 'a', 2);
insert into calls values('a', null, 1);
insert into calls values(null, 'a', 0);
insert into calls values(null, 'a', 0);
insert into calls values(null, 'a', 7);
insert into calls values(null, null, 0);
This is horribly unnormalized, but basically this table represent phone calls. trunkin and trunkout represent the two ends, and duration is length of a call. While obviously every call has two ends, the nulls above represent ends we don't care about in this example.
The goal is to end up with data that looks like this. In English, we want the in and out summary statistics for each trunk to be summarized into a single row. (Sorry, these are supposed to be columns, but they got wrapped, so I reposted them as rows):
TRUNKIN a
IN_CALLS_ATTEMPTED 1
IN_CALLS_COMPLETED 1
IN_AVERAGE_DURATION 1
TRUNKOUT a
OUT_CALLS_ATTEMPTED 5
OUT_CALLS_COMPLETED 3
OUT_AVERAGE_DURATION 4
Indeed, with the data given above, these are the results returned with the query at the end of this message. However, when I changed the one non-null trunkin value to null, I got very strange results. First, I independently ran the two subqueries. The first of course returns no rows, the second returns 1 with the same out values above; this is exactly what I would expect. However, when I run the full query, I get **5** rows back, each with just the trunkout column set to "a" and **all** other columns set to null. This makes no sense to me. The 5 rows are obviously the 5 rows from the original data set where trunkout = "a", but I'm not full outer joining those; I'm full outer joining the result of the group by, which only has 1 row. But even given that I'm getting 5 rows back, shouldn't all five of those have the remaining out columns filled in with the values above?
Here is the query:
select
from
SELECT
trunkin as trunk,
COUNT(*) AS in_calls_attempted,
SUM
CASE
WHEN duration > 0 THEN 1
ELSE 0
END
) AS in_calls_completed,
SUM(duration)/
SUM
CASE
WHEN duration > 0 THEN 1
ELSE 0
END
) AS in_average_duration
FROM CALLS
WHERE trunkin IS NOT NULL
GROUP BY trunkin
) callsin
full outer join
SELECT
trunkout as trunk,
COUNT(*) AS out_calls_attempted,
SUM
CASE
WHEN duration > 0 THEN 1
ELSE 0
END
) AS out_calls_completed,
SUM(duration)/
SUM
CASE
WHEN duration > 0 THEN 1
ELSE 0
END
) AS out_average_duration
FROM CALLS
WHERE trunkout IS NOT NULL
GROUP BY trunkout
) callsout
on callsin.trunk = callsout.trunk;

Answer 1

I am not entirely sure why you are getting the results you are, but I strongly suspect that it is a result of outer joining on null columns. I would write the query as follows to avoid the outer join problem. The CASE statements in the outer query around the average duration calulations avoid the divide by zero error that would occur when some trunk has only in or out calls.
SELECT trunk,SUM(in_calls_attempted) in_calls_attempted,
       SUM(in_calls_completed) in_calls_completed,
       CASE WHEN SUM(in_calls_completed) <> 0 THEN
            SUM(in_duration)/SUM(in_calls_completed)
            ELSE 0 END ave_in_duration,
       SUM(out_calls_attempted) out_calls_attempted,
       SUM(out_calls_completed) out_calls_completed,
       CASE WHEN SUM(out_calls_completed) <> 0 THEN
            SUM(out_duration)/SUM(out_calls_completed)
            ELSE 0 END ave_out_duration
FROM (
   SELECT trunkin trunk,COUNT(*) in_calls_attempted,
          SUM(CASE WHEN duration > 0 THEN 1 ELSE 0 END) in_calls_completed,
          SUM(duration) in_duration,0 out_calls_attempted,
          0 out_calls_completed,0 out_duration
   FROM calls
   GROUP BY trunkin
   UNION ALL
   SELECT trunkout trunk,0 in_calls_attempted,0 in_calls_completed,
          0 in_duration,COUNT(*) out_calls_attempted,
          SUM(CASE WHEN duration > 0 THEN 1 ELSE 0 END) out_calls_completed,
          SUM(duration) out_duration
   FROM calls
   GROUP BY trunkout)
GROUP BY trunkTTFN
John