crystal report use Maximum as Sum condition problem

I met a problem with sum function: the data have a datetime column and I want to get sum of who's datetime is max. And also there's anoher group condition. So I do like:

1. Add form开发者_StackOverflow中文版ula maxDatetime: Maximum({datetime}, {groupcondition})
2. Add new formula for sum ValueToSum: if({datetime} = {@maxDatetime}) then value else 0
3. Sum the value, add new formula totalValue: Sum({@valueToSum}, {groupcondition})

The result is the sum can't be processed, it says: 'valueToSum can't be sumed'.

I think the causor may be using a maximum value as a condition in step 2. Because when use 1 = 1 to replace the condition, there's no problem.

Can anyone give some advice?

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We usually solve such kind of problems by creating additional queries and linking them to main data. Because Crystal Reports does only two passes over data - record reading and aggregate calculation, it can't easily aggregate over already aggregated values.

Another possibility is to accumulate your 'totalvalue' into variable. Something like next may work (needs tweaking probably):

WhilePrintingRecords();
NumberVar totalvalue;
If ({datetime} = Maximum({datetime}, {groupcondition})) 
then totalvalue:=totalvalue+value 
else 0

This formula (which displays totalvalue) needs to placed into group footer:

WhilePrintingRecords();
NumberVar totalvalue

Somewhere in group header you need another one to reset totalvalue:

WhilePrintingRecords();
NumberVar totalvalue:=0