Hi,
There was a particularly tough question (at least for me) posted by Michal Radomski in WEBI Running Count Distinct with tooke me some time to figure out how to solve it.
The question was how to implement a RunningCount of distinct elements The OP had a list of IDs and dates and he needs to count the distinc elements over time (a date), his data is somethong like this :
| date | ID |
| 01/01/2009 | 1 |
| 01/01/2009 | 2 |
| 01/01/2009 | 3 |
| 01/01/2009 | 4 |
| 01/01/2009 | 1 |
| 01/01/2009 | 5 |
| 01/01/2009 | 2 |
| 01/01/2009 | 6 |
| 02/01/2009 | 3 |
| 02/01/2009 | 7 |
| 02/01/2009 | 8 |
| 02/01/2009 | 9 |
| 02/01/2009 | 2 |
resulting in this :
date | Running Count Distinct |
01/01/2009 | 6 |
02/01/2009 | 9 |
In order to do so,I proposed a solution using the following formula , which I´ll try to explain
[Running Count Distinct] =RunningSum(Sum(If ( ([Id] ForEach([Id];[date]) In ([Id];[date]) = Previous([Id] ForEach ([Id];[date]) )) In ([Id];[date]);0;1) ) )
The idea behind this is compare , row by row, The [id] element with its previous [id] object AS IF IT WAS ORDERED BY ID INSTEAD OF DATE , in
case it´s equal i sum 0 otherwise 1.
As the data is, in fact ordered by [date], I force the comparison to be evaluated as if was ordered by [id] and redefine the context to include [id].
This is accomplished by setting the correct order in the redefinition context ([ID];[date]).
In fact, when redefining context, the order in which the objects are set matters in terms of the final result.
Have ypu all a nice WEEK END.
Regards,
Rogerio