Thanks...MAGIC! MAGIC!
I wrote the same formula Sum(ThisItem.Cities, Population) but it appeared to be error. WHY?
But when I re-do as you said, OK!
I guess, sometimes, as of my experience, I need to either RETYPE, or Quit the program and re-Load.
Strange, Strange, Strange!
BUT, anyway, thanks a lot.
NB: In fact, I am grouping more than that, e.g.:ClearCollect(FarmHouse, Groupby(TableFarm, "FarmName", "HouseNo", "HouseQty), so that I can have double FILTERING of FarmName - then - HouseNo, to get the Sum of HouseQty.
This is a very useful features for multi-layer data manipulation.
Thanks.
PK Hong