modify content in databse; duplicate content of one field to another

Hi there,

Our database consists of a product table with a serial number and a registration number
Below it is a periodical check up with a relation to the registration number in both tables and a relation from serial number to serial number. In a lot of products I only have a registration number.
Unfortunately if I dont fill both fields in the periodical check up table with the same number it does not put it "below" the product item.
How can I solve this ?