Displaying a zero in a field

Question

Here is how my application works:
Essentially, I have permit numbers, most of which own tags.
I want to show a report for each permit number, which lists those tags if they exist. But if the permit does not have tags, the frame with the "Number of Tags Issued" data should report a 0, instead of not showing up at all.
In my report, I have a group based on a query that selects a COUNT of tags from the join of 2 tables. I display this count in the "Number of Tags Issued" field in a frame that is based on that group.
Problem: If the count is null, then no row will be returned by the query. Thus the label in the frame will not show up at all. But that is not what I want. I want the field in my frame to display a zero, indicating that there are no tags. I'm not sure how to do that.
Here is the SQL in my group:
SELECT COUNT (t.tag_number) Tag_Count, t.tag_status, tow.permit_number
FROM tag t, tag_owner tow
WHERE t.tag_number = tow.tag_number
AND t.order_number = tow.order_number
AND tow.tag_owner_end_date IS NULL
GROUP BY t.tag_status, tow.permit_number;
As you can see, if there simply is no Tag_Count for a given permit number, then no row will return and the frame will not even display.
Can someone show me how to handle this situation. Thanks.

Answer 1

Thanks for all of your suggestions. After doing some Metalink research and some trial and error, I discovered that the solution is to:
1. Create a formula column in my master query. This column is based on a function that uses a SQL query with an outer join. If the given permit has no tags, then the function returns a 0.
You were essentially on the right track regarding the outer join, but I didn't describe all of the queries in my report, so it was not possible for you to understand the data structure the report is based on, and thus you couldn't get the SQL right.
To clarify: My master query selects rows from a print_queue table, which contains one row for each report request, as well as from a table called gillnet_vessel. Thus, the formula column had to be created in that query. Notice that the formula column has to reference gillnet_vessel, since it contains the permit numbers -- the inclusion of this table is what lets me do the outer join. The function looks like this:
function CF_1Formula return Number is
-- Declare a variable to hold the tag count.
tag_count NUMBER;
BEGIN
-- Return a count of tags. If the given permit number has no tags, then return 0.
SELECT COUNT (t.tag_number)
INTO tag_count
FROM tag t, tag_owner tow, gillnet_vessel gv
WHERE t.tag_number = tow.tag_number
AND t.order_number = tow.order_number
AND gv.permit_number(+) = tow.permit_number
AND gv.permit_number = :permit_number
AND tow.tag_owner_end_date IS NULL
GROUP BY t.tag_status, tow.permit_number;
RETURN (tag_count);
EXCEPTION
WHEN NO_DATA_FOUND THEN
RETURN(0);
END;
2. Create a boilerplate text item with a format trigger that controls when it is displayed. If the formula column returned 0 then display the text item. And the text item's text is of course, '0'. Here is the function in the trigger:
function B_1FormatTrigger return boolean is
BEGIN
IF :CF_Tag_Count = 0 THEN
RETURN (TRUE);
ELSE
RETURN (FALSE);
END IF;
END;
Edited by: Prohan on Sep 15, 2009 1:05 PM