selecting rows tagged with "a" but not "b"

Hi,

I have a two tables:

article
articleID, name, content

tags
articleID, tag

I want to find all articles that are tagged with "a" but not "b"

how do I do this?

what I'd like to do is:

<wishful thinking>
select articleID from tags where tag="a"
SUBTRACT
select articleID from tags where tab="b"
</wishful thinking>

how do I do this in real SQL?

thanks

Darrell