I’ve reached the database limit on my hosting platform. I want to add a function to my functions.php that deletes the oldest post in my database anytime that I publish a new post.

function deleteOldestPost(){
    global $wpdb;
    $prefix = $wpdb->prefix;
    $wpdb->query("DELETE code");
}
add_action('publish_post', 'deleteOldestPost');

Can someone please help me with the appropriate query?