sum crystal report different month - sql

Question

I'm using Crystal Reports 2008. And I have database access 2003 "myclass" with table "Class" an output of like this:
----------
Date Total
12/2015 100
01/2016 115
02/2016 150
03/2016 180
----------
Now, my question how formula for "Previous_Total" the output should be:
----------
Date Total Previous_Total
12/2015 100 0 ----> 0 because no data before
01/2016 115 100 -----> this form total
02/2016 150 215 -----> 100 + 115
03/2016 180 365 -----> 100 + 115 + 150
----------
if i'm use record selection ex sample: {Class.date}>=Date("02/2016") And {Class.date}<=Date("03/2016")
i want to show in the report like this:
----------
Date Total Previous_Total
02/2016 150 215
----------

Answer 1

After trying 3 days, my case is done....
create group Date as "In specified order" then setting to "is in the period " and I'm selecting AllDatesFromToday (Position group is down column Page Header).
create group Total as "in ascending order" (Position group is up column Details).
Hide column details
insert Summary Date as "Maximum"
insert summary Total as "Sum"
drag max total to group Date, drag sum total to group Date
Finally create formula Previous_Total:
Sum ({MyClass.Total}, {MyClass.Date}, "daily")-{MyClass.Total}
Drag formula Previous_Total to group Date.
I try to test... Klik Menu Report, Selection Formulas, Record...
{MyClass.Date}<=date("02/2016")
Save, press F5 to refresh data and DONE...
Thank everybody for reply.... :)