Dynamically add / remove group column in crosstable?

temphan

Hey

I need to design a cross tab which provides multiple level of grouping. For example, to counter the number of people, user may want to count at department level, or count at the group level within each department. So I provide a checkbox, based on the value of the check, the desired results should be like

Deparment
total_number (group1 + group2 + group3)

OR

Department
group1 group2 group3
total_number total_number total_number

I saw a previous post saying that I can use key expression to achieve this. I tried but failed. Could some please give me a more detailed instruction on how to do this. Thanks a lot!