Use of Analytical Function

Requirement : I need list of items purchased in a month. If a item purchased more than once in a month then i need the recent purchase order number.

Solution :

SELECT *
  FROM (SELECT segment1, description, ponumber,
               ROW_NUMBER () OVER (PARTITION BY segment1 ORDER BY pocreationdate DESC) mycol
          FROM base_table
         WHERE TO_CHAR (pocreationdate, 'MM YYYY') = '04 2008')
         WHERE mycol = 1