After child record deleted update master

Hi,

I'm using PHPR 10.91 (Build 41974 x64)

After record/s delete from child to update the master table subtotal & below is my code.

//UPDATE MASTER RECORD
$sum = DBLookup("Select SUM(Total) AS subtotal from OrderDetails where OrdersID=".$deleted_values["OrdersID"]);
CustomQuery("Update Orders set subtotal='".$sum."+0' where OrderID=".$deleted_values["OrdersID"]);

I've also tried setting CustomQuery differently

1. CustomQuery("Update Orders set subtotal=".$sum."+0 where OrderID=".$deleted_values["OrdersID"]); //removed single quotes then resorted to single quotes again as it's a numeral field that I'm updating.
2. CustomQuery("Update Orders set subtotal=".$sum." where OrderID=".$deleted_values["OrdersID"]); // without +0, if all child records are deleted, the update to master table the values are incorrect.

If you can navigate me in the correct direction i'll appreciate.

Regards
Warren