Oracle SQL - do something if found in table

[htcilt]

I have a table containing many rows of usernames. I want to say:

if current user's username is found in the usernames table, then do something
else do nothing

I use upper(PORTAL.wwctx_api.get_user) to get the username of whoever is logged in.

This is my code so far

select 'some html' from domain.table where upper(PORTAL.wwctx_api.get_user) not like upper(username)

This doesn't seem to work. I tried using != <> and not like, but they all return 'some html'. I've tried = and although its the wrong way around (I want it to return 'some html' if current user is NOT found in usernames) it does seem to work better, but I get "no rows returned" if the username isn't found.

I've tried doing case statements, but I get an output for every row in the table.

Hope I've explained this clearly? Can anyone help?