Hi Gilbert,
Thanks for your question, I've actually created an article that shows the SQL required to do this in either SQL Server or Oracle. This is available in the following article:
Select Random Users
What you'll need to do in PS Query is create a sub-query that produces the SQL shown above for your database platform. If you are on SQL server this will be fairly straightforward.
If you are on Oracle, this will be a lot harder. I tried messing around with it for a bit but the catch is making it order by dbms_random.value in a sub-query, which isn't possible as you can only have one field in a sub-query and you can only order by that field.
The way to make this work in Oracle is to create a view with the logic that generates the random EMPLIDs. The SQL in this view will be:
SELECT EMPLID
FROM (
SELECT EMPLID
FROM PS_PERSON
ORDER BY dbms_random.value )
And you will need to save it as a record and add it to the query tree so you can see it through query manager. Join to this view in your existing query, and then add one more expression: rownum and add that expression to your criteria equal to a constant (which will = 450).
Good luck with this and please post any more questions.