How do I get a column value from the last row of this thing only?
LOOKUP( MAX( SELECT( My Table[_ROWNUMBER], ([_THISROW].[Thing] = [Thing]) ) ), "My Table", "_ROWNUMBER", "Wanted Column" )
My Table with the name of the table from which you want the column value;
Thing with the name of the column containing a value that identifies the thing you want (e.g.,
Order ID); and
Wanted Column with the name of the column whose value you want.
See also: MAX()