Oracle group by and the empty resulting set - oracle

Question

I have the following SQL problem.
Scenario:
I have two tables: Change and ChangeTicket.
There is a 1:n relationship. One Change has cero or many changeTickets.
No change means no changeTickets.
A changeTicket has a status (open/closed/...)
A changeTicket has a field representing how many minutes took this change.
A Change has a company and a month. There is at most ONE change in the month.
I have to report for a given company and for a given month the sum of minutes
a given change took.
I wrote the following SQL statement.
select nvl(sum(service_req), 0) as SUM_REQ
from Change_Ticket, Change
where Change.company_id (+) = '0'
and Change.month (+)='07'
and Change.Id (+) = Change_Ticket.Change_Id
This is OK.
If for the given month and the given company there are neither changes nor tickets
then I get a null value which is converted to cero usgin the NVL function.
The problem arises when I want to group the information using the status.
If I add a grup clause
select Change_Ticket.status, nvl(sum(service_req), 0) as SUM_REQ
from Change_Ticket, Change
where Change.company_id (+) = '0'
and Change.month (+)='07'
and Change.Id (+) = Change_Ticket.Change_Id
group by Change_Ticket.status
then my result is the empty set.
I understand that there is no status and then the resulting set is
somehow consistent and then an empty resulting set is returned.
How can I avoid this problem. In this case I need to report an
empty status and cero as the the sum.
(BTW, I've also tried putting nvl(Change_Ticket.status, 'none') but didn't work)
Thanks a lot in advance.
Luis

Answer 1

I think to achieve what you want you would need to do this:
select Change_Ticket.status, nvl(sum(service_req), 0) as SUM_REQ
from Change_Ticket, Change
where Change.company_id (+) = '0'
and Change.month (+)='07'
and Change.Id (+) = Change_Ticket.Change_Id
group by Change_Ticket.status
union all
select '' as STATUS, 0 as SUM_REQ
from dual
where not exists (select null from Change_ticket)

Answer 2

Assuming you have the list of your statuses in a table called statuses:
SELECT statuses.id, nvl(sum(service_req), 0) as SUM_REQ
FROM statuses
LEFT JOIN
Change
ON Change.company_id = '0'
AND Change.month = '07'
AND Change.status = statuses.id
LEFT JOIN
Change_Ticket
ON Change_Ticket.Change_Id = Change.Id
GROUP BY
statuses.id
or this (only if you are using 8i, this syntax is deprecated in higher versions):
SELECT statuses.id, nvl(sum(service_req), 0) as SUM_REQ
FROM statuses, Change, Change_Ticket
WHERE Change.company_id(+) = '0'
AND Change.month(+) = '07'
AND Change.status(+) = statuses.id
AND Change_Ticket.Change_Id(+) = Change.Id
GROUP BY
statuses.id
If you only want to show existing statuses or a NULL when no records exist, use this:
SELECT statuses.id, nvl(sum(service_req), 0) as SUM_REQ
FROM dual
LEFT JOIN
Change
ON Change.company_id = '0'
AND Change.month = '07'
LEFT JOIN
Change_Ticket
ON Change_Ticket.Change_Id = Change.Id
GROUP BY
statuses.id