[help] Merging guest posts

[Meatshield]

I am trying to merge guest posts to a current member
I have tried this quiry:

update post
set userid="2"
where (username = "bobx" and userid = "0");

The quiry itself worked as the out come was:

Affected rows: 0 (Query took 0.0005 sec)

but as you see nothing was effected

Is there any other way round this?