Why does OBI include ROW_NUMBER function and then only take values=1?

Hi, We have a query that derives a couple of columns using CASE statements and also return some measures that are summed.

For some reason the columns that are used in the CASE statements are alos included in the SELECT list and the GROUP BY clause. In addition, a ROW_NUMBER() function is also included and any rows without a value of 1 are excluded.

Why is the ROW_NUMBER function included? Does anybody have any ideas?

Thanks,
Ian