SQL & PL/SQL

Hi,

I have a query like below:

SELECT COLUMNS, SALES_PERIOD, SUM(AMOUNT) FROM(
SELECT SALES_TABLE.COLUMNS, 'SALES-1' SALES_PERIOD, SALES_TABLE.AMOUNT
FROM SALES_TABLE
WHERE SALES_PERIOD = @PARAMETER ( for example JANUARY_2011)
UNION ALL
SELECT SALES_TABLE.COLUMNS, 'SALES-2' SALES_PERIOD, SALES_TABLE.AMOUNT * -1
FROM SALES_TABLE
WHERE SALES_PERIOD = @PARAMETER ( for example FEBRUARY_2011)
)

My query is retrieving sales difference between periods. So end users can see the difference between sales.
But they want to see this difference as percentage.
How can I implement it?

Thank you & Bye..