SSRS - Subtracting column values without knowing row details - reporting-services

Question

Got a question regarding SSRS. Using 2005 version.
I've got 2 tables in my report, one with the current months data and one with data for the last 3 months.
The last 3 months table always returns 3 rows, 1 row for each month. What I want to do is subtract a value from the 2nd oldest month, basically it will always be row 2 of the table with the equivalent column in the current months table. Hope this kind of makes sense.
I will use an example here;
Table 1
Month Value Var
August 5 ?
Table 2
Month Value
July 4
June 7
May 10
I want to use the Value in Table 1 and subtract the Value of the June column in Table 2 therefore 5 - 7, returning the value -2.
There is a month parameter so the Tables data would change depending on the month but it was always be the 2nd rows Value column that I want to use.
Is this possible in SSRS?
Thanks

Answer 1

Method 1:
It is better to solve this in the SQL (dataset) then in the report design.
If data comes from two different tables then
Select C.Month, C.Value, (C.Value - L.Value) AS Var
From CurrentMonthData C
Left JOIN (Select * Last3MonthData where Month = CurrentMonth - 2) L
ON 1=1
If data comes from same table
Select C.Month, C.Value, (C.Value - L.Value) AS Var
From (Select * MonthData Where Month = #CurrentMonth) C
Left JOIN (Select * MonthData where Month = #CurrentMonth - 2) L
ON 1=1
Method 2:
If you prefer to do it in the Report Itself you can use ReportItems collection.
In the variance text box value put
=ReportItems!CurrentMonthValueTextBox.Value - ReportItems!Prev2MonthValueTextBox.Value
HTH.

Answer 2

you may try using LookupSet or MultilookupSet