How Can Used Union Statment According Condition In Report?

How can used UNION STATMENT ACCORDING CONDITION in REPORT?

I mean that I have two Table , If the date > year(2000) will use one table but if the date < year (2000) will use tow table
IN FORM THAT's NOT problme BUT in report how it's will do it ?

CASE WHEN DATE  > '01-JAN-2000' then
SELECT ALL FROM INVOICE WHERE INVOICE_DATE >= DATE
ELSE
SELECT ALL FROM INVOICE WHERE INVOICE_DATE >= DATE
       UNION ALL
SELECT ALL FROM ARCHIVE_INVOICE WHERE INVOICE_DATE >= DATE
END

ARE IT'S AVAILABLE ??
THANKS FOR ALL