query that gives a return

[brandie willis via AccessMonster.com]

I am having problems writing a query that when the user inputs information
such as a user name, it will return an id number or vice versa.

for example

UserInput - brandie
OutPut - 1234

the information is located on the same table we'll call it userdata. i
have tried using the criteria field to ask the question please input user
name or ID number and when i do it that way, it returns both the user name
and the ID. I want one or the other based on the information entered.

I hope this made sense to some one.
Thanks for any help you can give