Wouldn't your Enter New Product and Update Existing Product pages be different? Basically, you enter a new product - including SKU number - when the product is brought to the shelf , so to speak (addNew.php). However, you have to choose the SKU number in order to update or edit information associated with the specific product, right (editExisting.php)?
So it shouldn't be a problem - you've just got the addNew.php page and the editExisting.php, each with a separate query.
Or am I missing something important here?
Don't get me wrong, dfr574's snippet is sound from a logical standpoint, but user error is rampant and you could easily end up with boatloads of wrong product SKUs with full information, and SKUs with incomplete information for the actual products. This is alleviated by making the user select the specific product to edit.