Excel 2010 question: I have a table that I'm trying to write a validation formula (using Excel formula and avoid VBA if possible)for. In the example table below - the "Assignment" given to the unique combination of "Account" & "Sub-account" for an over-lapping Start and End Date must be the same. A conflict error is thrown if that rule is violated. So, "2" is not a conflict because the date range is within "1". "3" is not a conflict because there is no overlapping date range. "4" not a conflict because it is a different "Account" & "Sub-Account" combination. "5" is a conflict because a different "Assignment" is given for the same "Account" & "Sub-Account" and Date range.

NO. Account Sub-account Start Date End Date Assignment CONFLICT?
1 PET DOG 1/1/2012 1/31/2012 HOUSE1
2 PET DOG 1/15/2012 1/30/2012 HOUSE1 NO
3 PET DOG 2/1/2012 2/29/2012 HOUSE3 NO
4 PET CAT 2/1/2012 2/29/2012 HOUSE3 NO
5 PET DOG 1/1/2012 1/31/2012 HOUSE2 YES