Graphical Filter in Calculation View?

I have a Calculation View like

and Flag=1 in Projection_1 and Flag=2 in Projection_2 (both introduced in the Projection as a Calculated Column).

What I now want to add is some filter for result, e.g. Flag=1 .But somehow I don't find a mean to create a filter. On the Aggregation Level there is no item:

nor in the Union. Since I have seen screenshot with applying Filter-Expression on the Aggregation Level, I wonder how this is done. I'm using SPS08 - perhaps this was possible in prior versions, and now not?

NOTE: I'm not looking for the SQL-script, I'm curious how this could be done on the graphical level. With the search here I wasn't neither successful.