MS Access 2010: Extract unique values - ms-access

Question

first off: I know virtually nothing about MS Access but now I'm in a situation where I have to use it (dataset is too big for Excel). The data has column names like Customer_Name, Product Name, Amount, Date
Date refers to the last day of a month, so for example for February it's 28/02/2013. Now I want to compare the amount a customer bought in February to the amount he/she bought in January and calculate the difference. So far, I've been able to this by prompting the user to enter the date.
SELECT Data.Customer_Name,
Sum(IIf(Format(Date,"yymm")=[Startdate (yymm)?],Amount,0)) AS Amount_Startdate,
Sum(IIf(Format(Date,"yymm")=[Enddate (yymm)?],Amount,0)) AS Amount_Enddate,
Amount_Enddate-Amount_Startdate AS Difference
FROM Data
GROUP BY Data.Customer_Name;
This works but is it possible for Access to recognize which dates are in the column "Date" (there are only two distinct dates) so the user does not have to enter anything? Also, I tried to replace "Amount_Startdate" with a field that has the respective date in its name (e.g. "Amount_Feb2013") and played around with ampersand but it didn't work.

Answer 1

If you create a new table called tblValues with just 2 fields; ID and TDate (always try to avoid using reserved words like "Date", "System" or other words that Access already assigns a function to), you can fill it like this:
ID TDate
-- ---------
ST 1/31/2014
EN 2/28/2014
Then you could use the DLookup function to make this code generic:
SELECT Data.Customer_Name,
Sum(IIf(Format(Date,"yymm")=DLookup(Format(TDate, "yymm"), tblValues, "ID = 'ST'"),Amount,0)) AS Amount_Startdate,
Sum(IIf(Format(Date,"yymm")=DLookup(Format(TDate, "yymm"), tblValues, "ID = 'EN'"),Amount,0)) AS Amount_Enddate,
Amount_Enddate-Amount_Startdate AS Difference
FROM Data
GROUP BY Data.Customer_Name;
Then you could just update the table with the values you want to use as start and end dates whenever you want.