Assuming I haven't misunderstood what you are trying to do (and there is a decent chance I have), I don't think this is going to work as long as you have to have both Event ID and Task ID in the same list table. With both columns present, there is no way to aggregate the way you're wanting, at least that I can find.
Borrowing on Bruehl's suggestion, I made up some sample data that looks like this:
So let us say you want to be able to filter to Event ID 2 because it has zero active tasks. I then created a calculated item to say if Task Activity = Active Task, return 1, else return 0:
Then I added a new List Table with just Event ID. I then added a local filter to this List Table like this:
where I'm filtering on my new Task Activity Number = 0 AND it's filtering on aggregated values. This gives me a List Table that is filtered to just Event IDs that have zero active tasks:
I believe this is more or less what Bruehl was getting at.
I could then use this list table and create an action to another list table that has Task Details for example, so that clicking a row in my Event IDs table filters my Task table. Again once you add Task ID to this first Event table, the whole thing breaks but if you keep them in separate table objects, it might be doable.
Another option I have not fully explored myself yet is making use of Aggregated Datasources to potentially create the relational tables you mentioned earlier. Assuming all of the relevant columns exist in the single table you currently have you, you could go to the Data Options menu and select "New data from aggregation of <table name>":
Then you could pick only the event columns. Create table.
Repeat again and pick only the task columns plus the event ID column. Create that table.
Then maybe you could join these together in the way you're wanting, or work with them separately in separate report objects?
One downside to this approach, there is a performance hit when creating tables like this. Each time you open the report this way, these tables are generated in the user's personal CAS library so there might be a delay opening the report each time, and these tables take up space on the CAS server too that you may or may not have.
I haven't fully thought this approach out yet though so this may not be feasible either, but just something to possibly explore.
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