SQL Group Distinct Count - asp.net

Question

I have the following table
User ID Start Date End Date
-------------------------------------
John Doe Mar 11 2011 May 28 2011
Robret S Mar 21 2011 Jun 29 2011
Tina T Feb 01 2011 August 20 2011
I want to show how many people I have available for the past 6 months, even if the month has no people. How can this be possible. I know I have to do grouping and use distinct count.
Expected Output:
February = 1 Resource
March = 3 Resources
April = 3 Resources
May = 3 Resources
June = 2 Resources
July = 1 Resource
August = 1 Resource

Answer 1

With Calendar As
(
Select Cast('20110501' As DateTime) As [Date]
Union All
Select DateAdd(m,-1,[Date])
From Calendar
Where [Date] > DateAdd(m,-5,'20110501')
)
Select DateName(m, C.Date) + ' ' + Cast(Year(C.Date) As char(4))
, Case Count(*)
When 1 Then Cast(Count(*) As varchar(10)) + ' Resource'
Else Cast(Count(*) As varchar(10)) + ' Resources'
End
From Calendar As C
Left Join MyTable As T
On C.Date Between T.StartDate And T.EndDate
Group By C.Date
Results:
December 2010 | 1 Resource
January 2011 | 1 Resource
February 2011 | 1 Resource
March 2011 | 1 Resource
April 2011 | 3 Resources
May 2011 | 3 Resources

Answer 2

You will need the existing data records for the last 6 months, so that you can merge the two sets of data. You can generate the last 6 months in a CTE and do a left join with your data. That will allow you to show the last 6 months even if you have no data.

Answer 3

I don't think you can do what you want using a "simple" select statement (even using GROUPing etc.) The following is off the top of my head, so you'll have to experiment with it a little, and preferably read Joe Celko's excellent SQL for Smarties book.
You need to create a second table that contains all of your months (start/end dates). You only need one table for all types of similar queries, and it must contain all the months in the date ranges your interested in querying:
CREATE TABLE months (id, start DATE, end DATE);
INSERT INTO months (id, start, end)
values ( (1, 2011-01-01, 2011-01-31),
(2, 2011-02-01, 2011-02-28), ...);
You then LEFT OUTER JOIN from your user table to this month table. That will give you a row for each user for each month they were available, which you can GROUP as required:
SELECT months.id, COUNT(user.id)
FROM months LEFT OUTER JOIN users
ON user.start_date < months.end
AND user.end_date > months.start
GROUP BY months.id;
Hope that helps.

Answer 4

WITH resources AS (
SELECT
Date = DATEADD(month, v.number, [Start Date])
FROM atable t
INNER JOIN master.dbo.spt_values v ON v.type = 'P'
AND v.number BETWEEN 0 AND DATEDIFF(month, t.[Start Date], t.[End Date])
)
SELECT
Month = DATENAME(month, Date),
ResourceCount = CAST(COUNT(*) AS varchar(30)) +
CASE COUNT(*) WHEN 1 THEN ' Resource' ELSE ' Resources' END
FROM resources
WHERE Date > DATEADD(month, -6, DATEADD(day, -DAY(GETDATE()), GETDATE()))
GROUP BY YEAR(Date), MONTH(Date), DATENAME(month, Date)
ORDER BY YEAR(Date), MONTH(Date)