How correctly am I setting the promotional price for all products?

Question

Database

I created a page in the admin panel where you can enter and apply a percentage discount to all products. In functions.php, I take all the products and run through them with this code. Tell me, is this what I do?

$cid = get_the_id();
$reg_price = get_post_meta($cid, '_regular_price', 1);
$promo_price = '';
if(!$discount) {
  $mysqli->query("UPDATE wp_postmeta SET meta_value='".$promo_price."' WHERE post_id='".$cid."' AND meta_key='_sale_price'");
  $mysqli->query("UPDATE wp_postmeta SET meta_value='".$reg_price."' WHERE post_id='".$cid."' AND meta_key='_price'");
  $mysqli->query("UPDATE wp_postmeta SET meta_value='' WHERE post_id='".$cid."' AND meta_key='_sale_price_dates_from'");
  $mysqli->query("UPDATE wp_postmeta SET meta_value='' WHERE post_id='".$cid."' AND meta_key='_sale_price_dates_to'");
} else {
  $promo_price = ceil($reg_price - ($reg_price / 100 * $discount));
  $mysqli->query("UPDATE wp_postmeta SET meta_value='".$promo_price."' WHERE post_id='".$cid."' AND meta_key='_sale_price'");
  $mysqli->query("UPDATE wp_postmeta SET meta_value='".$promo_price."' WHERE post_id='".$cid."' AND meta_key='_price'");
  $mysqli->query("UPDATE wp_postmeta SET meta_value='' WHERE post_id='".$cid."' AND meta_key='_sale_price_dates_from'");
  $mysqli->query("UPDATE wp_postmeta SET meta_value='' WHERE post_id='".$cid."' AND meta_key='_sale_price_dates_to'");
}