Update Query Problem

Hi People!

I'm doing an application and I've chosen postgresql
for the backend db. I need to use SQL update command
like this in my application:

update mytable set x='20' where id='someid' order by
id limit 1;

In other words, I want to update just one row of the
table if there are more than one row that the query
finds.

It doesn't work for me in postgresql. What's the way I
can do this in postgresql? Please forgive me if this
has been answered before. Please point me to the right
doc. I couldn't readily find the answer.

Thanks,

Jeng Yu