SQL & PL/SQL

This is a SQL (not PL/SQL) question...

Let's say I have the following:

SELECT NULL full_name, NULL address, NULL last_update_date
FROM dual
UNION ALL
SELECT full_name, address, last_update_date
FROM employees
WHERE TRUNC(sysdate)=TRUNC(last_update_date)

If the second part of the query retrieves no rows, this works wonderfully and I get an empty row; however, if the second part of the query returns records, I will get an empty row plus my dataset that I want.

How can I create a SQL statement that would only execute the first query (before the union all) when no results are returned from the second. Also, I don't want to exceute the query twice just so I can get a count of the records because my actual query is much more complex than the example I've provided here.

Thanks in advance!
Emily