Spotfire date difference using over function

Question

I have the following data set:

Item  ||  Date       || Client ID || Date difference
A     ||  12/12/2014 || 102       || 
A     ||  13/12/2014 || 102       || 1
         


        

Answer 1

I used the following solution to deal with groups that had more than 2 rows/dates.

First create a calculated column to provide a rank order by date within each group:

RankDatePerUnit:

  Rank([EventDate],[Group_Name])

Then another calculated column to do the date diff using an over expression to reference the previous date within the group.

TimeSinceLastEvent:

DateDiff("day",
   First([EventDate]) OVER (Intersect([Group_Name], Previous([RankDatePerUnit]))),
   [EventDate])

Note: Duplicate date could be handled differently by using denserank. The above approach will not calculate a zero date diff between two rows from the same group with a duplicate time. They'll both calculate their delta from an earlier date within the same group if one exists.