UCFkid86 on "MySQL update wp_posts based on wp_meta value?"

I'm creating a custom plug-in for my WordPress site that will run a query. The plugin will run via WordPress' wp_schedule_event function to run daily. This query will act upon the engine post type (engine) with a custom field named wpcf-engine-days-to-go. Currently, my code is not working (see below)

Goal
I want to update the post_status to 'draft' when a custom field value, wpcf-engine-days-to-go reaches '0'. The post_status value is located in the wp_posts table, and the wpcf-engine-days-to-go is located in the wp_postmeta table.

Question
What is the correct query to update a value in one table based upon another, specifically in my example?

register_activation_hook(__FILE__, 'tdengine_my_activation');
add_action('tdengine_my_daily_event', 'tdengine_do_this_daily');

function tdengine_my_activation() {
    wp_schedule_event(time(), 'daily', 'tdengine_my_daily_event');
}

function tdengine_do_this_daily() {
    global $wpdb;
    $query = "UPDATE <code>" . $wpdb->prefix . "posts</code> SET <code>post_status</code>=<code>draft</code> WHERE <code>meta_key</code> = 'wpcf-engine-days-to-go' = 0 ";
    $wpdb->query($query);
}

register_deactivation_hook(__FILE__, 'tdengine_my_deactivation');

function tdengine_my_deactivation() {
    wp_clear_scheduled_hook('tdengine_my_daily_event');
}