I need help with a db query function

[DRandy]

I've been struggling with this for over a month now and I'm stumped. I'm such a hack. I don't know much about writing functions but I have been able to manipulate php and pre-written functions.

This is what I'm trying to do:

When people register on my blog, they register through a previous post. The previous post id (let's call it postid_a) follows and populates a field in the registration page and gets permanently assigned to the new user.

When the new user makes a post, the function gets the previous post id (postid_a) and then displays the postid_a user's url. It's like saying - "this is the url of the person who signed me up."

Now in my database, I have a table called "posts" which holds the field "ID" (of each post) and a table called "users" which holds the fields "user_url" and "previous_post_id".

I've tried to play with something like:

function get_the_url() {
global $db;

$query = "SELECT 
  a.*, 
  b.user_url 
FROM 
  $db->posts AS a,
    $db->users AS b
WHERE
  a.id = ?
  b.id = ?";

$posts = $db->get_results($query);

return $posts;


}

I can't make it work.

I can display the previous_post_id but I can't get the user's url of the previous_post_id.

Your help would be very much appreciated.

Thank you for reading and taking time to understand.