Any method to create a SUM Total for a column whose values based on LAG?

Question

I have a question here, and the challenge is to explain my situation well. I am trying to develop a workbook that shows by project and month, the cost this period and the change in cost from the prior period. I want a SUM total on the Cost This Period and Cost Change From Prior Mth columns. So what I want to do looks something like this -
Project Month Cost This Period Cost Chg from Prior Mth
ABCDE Jan $50,000 NULL (since no preceeding row)
ABCDE Feb $75,000 $25,000
ABCDE Mar $60,000 -15,000
Total $185,000 $10,000
I do the Cost Chg from Prior Mth as a new calculation, and use the Insert Formula from Template process to use the F(x) Difference template to create my calculation. That is working just great. If you look at the underlying code for this new column, you can see the LAG function being utilized in calculating this value. I am lagging one row, as you can see from the example above.
I can create a new total and do a SUM for the Cost This Period column and get the correct answer (the $185,000).
The problem I am running into is that I am not getting anything when I try to sum my Cost Chg from Prior Mth column. The result is just blank (does print the total's text, but no amount).
My thought is that the NULL row in this column was messing up the SUM from working. So I created a new calculation and said CASE WHEN Cost Chg from Prior Mth IS NULL THEN 0.00 ELSE Cost Chg from Prior Mth. So I get a new column with the same numbers except for the first row, where I now have a 0 instead of NULL. I then created a new total (SUM function) on this brand new column. Darn. Same result as for the other column. I do not get an amount.
Has anyone else run into an issue with trying to sum up a column of numbers that are being calculated from a LAG function in Discoverer? Any suggestions on how I could get a SUM to properly calculate for this Cost Chg from Prior Mth column? Or have I run across one of those irritating "cannot handle in Discoverer" issues?
Of course I can export the workbook run results to Excel and create the calculation in there, and that works just fine.
John Dickey

Answer 1

Hi John
Thanks for the question and the great information. That really helped. I love these kinds of questions. They make me think :-)
Just a thought but have you tried altering the SUM to be a CELL SUM?
The SUM will try to evaluate the LAG over the total which would be invalid whereas, at least in theory, the CELL SUM should literally add the values.
Best wishes
Michael