Sort Order

Hello,

I need to add a sort to an out of the box report. The report already sorts on the first 3 Fields. I have added a fourth. A-BKTRAND.POSTDATE.

I am trying to add a Parameter. Sort by Date: Yes or No and if Yes Sort first by A-BKTRAND-POSTDATE if no keep the original sort order.

Can someone explain to me the correct way to do this ?

Thanks,

Debbie