Does that mean that if a lineItem record has a field named Qty with the value 10, you need to create 10 individual records in the LineItem table in place of the single record?
You'll need a script for that.
Something like this: (But change names from the example to match your actual database:)
I am assuming these tables and relationship:
Invoices::__pkInvoiceID = LineItems::_fkInvoiceID
For an explanation of the notation that I am using, see the first post of: Common Forum Relationship and Field Notations Explained
#Starting from your Invoice or Order layout
Set Variable [$InvoiceID ; Invoices::__pkInvoiceID ]
Go to Layout ["LineItems" (LineItems) ]
#Find all line items for current invoice with Qty greater than 1.
Enter Find Mode 
Set Field [LineItems::_fkInvoiceID ; $InvoiceID ]
Set FIeld [LineItems::Qty ; ">1"]
Set Error Capture [on]
Perform Find 
Exit Loop IF [ Not Get ( FoundCount ) ]
Set Variable [$Qty ; LineItems::Qty ]
Set Field [ LineItems::Qty ; 1 ]
Set Variable [$Qty ; Value: $Qty - 1 ]
Exit Loop If [ $Qty = 0 ]
Go To Layout [Original Layout]
It works like a charm... Thanks...