SQL DB Design??

hey everyone...

I'm kind of stuck on what I need/should do to do what I need my program to do.

I have a table called CUSTOMERS--It contains all the Customers
I have a table called PARTS-- It contains all the Parts
I have a table called INVOICES-- It contains all the Invoices

None of the tables are linked.

I've attached some printscreens of part of my program layout...


FORM1 is the first form to load.....you click on the customer you want and then it opens FORM2.

When FORM2 loads it, selects all Invoices that = the specific customer.

You can then click the NEW INVOICE button, which opens FORM3.
The you can click the "Click Here to add new part..." label, and that opens FORM4.

Then you just click the part you want and it then adds the part to FORM3...which you can see what it looks like with FORM5.

So I have a couple concerns.....

1. If I was to delete a PART from the parts table....and 1 of my invoices contained that part...then when I went back to edit the invoice..the part would not display... I want to be able to click on one of the Invoices from FORM2, and then edit the invoice if needed?

2. I need to be able to calculate Backorders, if the there is no stock of that part. I also need to be able to calculate Amount depending on stock and the price of the part??

Any help is appreciated.

Thanks in advance!!