Add condition if the order product is out of stock

Add condition if the order product is out of stock, order status still process.

how to write the code：

????
$count=$wpdb->get_var($wpdb->prepare(“SELECT license_key FROM $wpdb->bff_lmfwc_licenses where ???? order_id() ?????;”));
????
if ( is_a($order, ‘WC_Order’), $count>0 ) {
// Change order status to “completed”
$order->update_status( ‘completed’, __(‘Approved by the customer’, ‘woocommerce’) ) ;

sorry, i don’t know how to write, please help, thanks.