Don't know if it fits into your criteria of one query, but this might work.

Create a second table with all the dates which need to be in the output. (This means you could have daily,weekly, monthly, etc.). In your case

CREATE CURSOR dRange (dDate D)
dStart={4/1/2006)   && Using American Date formats
FOR I = 0 TO 14
  INSERT INTO dRange (dStart + I)
ENDFOR

SELECT dRange.dDate,count(*) AS cnt;
 FROM myData,dRange;
 WHERE dRange.dDate between myData.open AND NVL(myData.close,{12/31/9999});
 GROUP BY dRange.dDate

>I've been asked it is possible to do this with ONE SELECT. First I answered NO, but here is about 2 PM and my mind doesn't works properly, so ask you :-)
>Here the conditions:
>
>The database is a bug tracking database.
>Table has several columns: bugID, open_date, close_date
>A bug is considered open on a given day if it's open date is on or before that day, and it's close date is on or after that day or it is NULL.
>So the goal is to generate a record set to show the number of open bugs for a range of dates.
>The ideal result set would have two columns: date and open bug count on that date.
>Some data:
>
>(all dates here are in German Format - dd.mm.yyyy)
>

>BugId   Open_Date       Close_Date
>1	01.01.2006	02.01.2006	
>3	21.01.2006	NULL
>4	05.01.2006	10.02.2006	
>5	01.12.2005	NULL
>7	13.12.2005	13.02.2006
>8	12.02.2006	NULL
>12	10.02.2006	10.02.2006
>14	07.04.2006	NULL
>20	18.11.2005	NULL
>21	10.05.2006	NULL
>22	15.02.2006	NULL
>23	01.10.2004	03.02.2006	
>24	31.12.2005	31.12.2005	
>27	09.01.2005	09.01.2006	
>32	01.01.2006	NULL
>45	13.02.2006	NULL
>55	12.12.2005	12.12.2005
>61	31.12.2005	NULL
>88	09.01.2006	09.01.2006
>99	13.03.2006	NULL
>

>
>So if the period is 01.04.2006 - 15.04.2006 desired result is:
>

>Date                    BugsCount
>2006-04-01 00:00:00.000	        9
>2006-04-02 00:00:00.000	        9
>2006-04-03 00:00:00.000	        9
>2006-04-04 00:00:00.000	        9
>2006-04-05 00:00:00.000	        9
>2006-04-06 00:00:00.000	        9
>2006-04-07 00:00:00.000	       10
>2006-04-08 00:00:00.000	       10
>2006-04-09 00:00:00.000	       10
>2006-04-10 00:00:00.000	       10
>2006-04-11 00:00:00.000	       10
>2006-04-12 00:00:00.000	       10
>2006-04-13 00:00:00.000	       10
>2006-04-14 00:00:00.000	       10
>2006-04-15 00:00:00.000	       10
>

>
>TIA