Suppose you want to know which products sold more in the U.S. than in Canada and Germany for the month of March, 2003 (3/03).
Issue the following SELECT statement:
SELECT US.PRODUCT_ITEM, US.TOTAL
FROM US_SALES AS US, CANADIAN_SALES AS CDN, GERMAN_SALES AS GERMAN
WHERE US.PRODUCT_ITEM = CDN.PRODUCT_ITEM
AND US.PRODUCT_ITEM = GERMAN.PRODUCT_ITEM
AND US.TOTAL > US_DOLLAR (CDN.TOTAL)
AND US.TOTAL > US_DOLLAR (GERMAN.TOTAL)
AND US.MONTH = 3
AND US.YEAR = 2003
AND CDN.MONTH = 3
AND CDN.YEAR = 2003
AND GERMAN.MONTH = 3
AND GERMAN.YEAR = 2003
Because you cannot directly compare U.S. dollars with Canadian dollars or Euros, you use the UDF to cast the amount in Canadian dollars to U.S. dollars, and the UDF to cast the amount in Euros to U.S. dollars. You cannot cast them all to DECIMAL and compare the converted DECIMAL values because the amounts are not monetarily comparable as they are not in the same currency.