retrieve the oldest post id

Question

I'm trying to get the ID of the oldest post with the post_type = job_listing and post_status = publish. I'm using the code below but it does not work. Could anyone please help me

$oldestID = mysql_query("select id from wp_posts where post_type = 'job_listing' and post_status = 'publish' order by post_date ASC");

Thanks!