SQL check if contracts are active in each month of the quarter (year). Sum values/(duration in mths) - sql

Question

I am working with an excel file that contains data and a vba to filter the data via ADO using an SQL string.
My database contains:
Surname; Start_date (of the contract), Dur_mths (contract duration in months), Value (Total value of the contract).
A contract may have the duration of 2-3 years. Therefore I would like to divide the total value of each contract by duration(Value/m), check if the contract is active in the specific period and then sum the results for each Surname. Each Surname may have several contracts active in a given period. I would like to see the sum of all contract values divided by their durations for each Surname for a given period.
My current working code for one month (January 2012) looks like this:
SELECT [Surname],
Sum (iif([Start_date] <= '2012-1-1'
AND DateAdd ("m",[Dur_mths], [Start_date]) >= DateSerial(2012,1+1, 0),[Value]/[Dur_mths],0))
AS [SumValue] FROM [Data$]
GROUP BY [Surname]
Now, I would like to make it work for a quarter (or a year).
I understand this still requires checking for each month in a quarter if the contract is active betw. start of contract and end of contract (start plus duration in months).
Here comes my question:
Is there an easy way to develop the code to do the same for a quarter (a year, any period)? Ie. to check if the contract is active for several months in a given period (quarter, year)? And then sum the values of active contracts(/duration in months) for each customer?
Thank you very much for all the help.
Jacek
EDIT! : In the case of one quarter, for one customer or surname, the calculation should test: in how many months in the quarter (or in the year, or in the specified period) the contract is active.
m1 m2 m3
contr1 50 50 0 (contract ended in m2)
contr2 0 0 20 (contract starts in m3)
contr3 10 10 10 (contract started 1y ego and will continue well into 2015) etc.
For this single surname, the sum for the quarter should give 150.
EDIT2:
I am now thinking how to perhaps use a loop to calculate how many months in a quarter (or a year, or a given period) the contract was active. Eg. I am trying to develop a loop, sopmething like (It is not actually any working code but an idea what I think needs to be done, I am not a programmer unfortunately):
(??? i do not know what to do with it yet...)
DECLARE #I INT, #N INT;
DECLARE #StartDate DATETIME #EndDate DATETIME
SET #I = 1
SET #N = 0
SET #QueryStartDate = 2012-01-01
SET #EndDate = DateAdd ("m",[Dur_mths], [Start_date])
WHILE #I <= 3
BEGIN
SET #N = #N + iif(DateSerial(year[Start_date];month[Start_date]+#I-1;1); <= #QueryStartDate AND #EndDate >= DateSerial(year(#StartDate),month(#StartDate+#I, 0),1,0))
SET #I = #I + 1
END
(??? i do not know what to do with it yet...)
It would return the number of months the contract was active - #N. I would use it to multiply my contract/(duration in months) in the initial code. I am trying now to read how to incorporate it in my initial code.

Answer 1

You can, perhaps, use the same DateAdd function with various parameters to get the data for a quarter or a year, as below:
AND DateAdd ("q",[Dur_quarter], [Start_date]) >= DateSerial(2012,1+1, 0),[Value]/[Dur_quarter],0))
and
AND DateAdd ("yyyy",[Dur_year], [Start_date]) >= DateSerial(2012,1+1, 0),[Value]/[Dur_year],0))
The complete details of the DATEADD function are provided on MSDN here.

Answer 2

For the time being I arrived at a following solution for a single quarter -by simply repeating the code for the month. The date range and region is entered to sql string from excel cells...I am still wondering if it can be made somehow universal for any period (with a loop?) In addition I added a left join
SELECT [Data$].[Surname],[DataInternal$].[Code], [reg],
Sum (iif([Start_date]
<= '2012-1-1'
AND DateAdd ("m",[Dur_mths], [Start_date])
>= DateSerial(2012,1+1, 0)
,[Value]/[Dur_mths],0)) AS [M1],
Sum (iif([Start_date]
<= '2012-2-1'
AND DateAdd ("m",[Dur_mths], [Start_date])
>= DateSerial(2012,1+2, 0)
,[Value]/[Dur_mths],0)) AS [M2],
Sum (iif([Start_date]
<= '2012-3-1'
AND DateAdd ("m",[Dur_mths], [Start_date])
>= DateSerial(2012,1+3, 0)
,[Value]/[Dur_mths],0)) AS [M3],
([M1]+[M2]+[M3]) AS [Q]
FROM [Data$]
LEFT JOIN [DataInternal$] ON [Data$].[Surname] = [DataInternal$].[Surname]
WHERE 1 = 1 AND [reg] = 'N'
GROUP BY [Data$].[Surname],[Code],[reg] ORDER BY [Data$].[Surname]
Empty lines added only for readability. The strings are first concatenated in excel to form a one SQL query string executed in vba macro. I will be grateful for all the comments and corrections. My question remains unadressed yet, if it is perhaps possible to loop such a solution in EXCEL ADO VBA.