Query to combine 2 tables?

[HakkieDEV]

I guess I wasn't really clear about what I would like to do, so I hope this info will make it a bit easier to help me.

I'm trying to combine 2 tables like this:

I have 2 tables, postcount and user
postcount has field userid
user has field userid
user has field username

How do I combine these so I can display the username together with the postcount for each userid?

I tried to do:
[sql]
$postcountuser = $DB_site->query("
SELECT userid, user FROM postcount, user where user.username=postcount.username
order BY username");


[/sql]

But, mysql considers this ambigious.
What am I doing wrong here?