Apologies to the list if this is off-topic. We'll continue this
privately if that's the case.

> That depends on the requirements.
> Care to tell em?

Simple invoice data model. Each line item has an item_id (of course), a
qty, a unit_price, and a discount. The discount can be a fixed amount
(i.e. $10.00,-) or a percentage (i.e. 10%).

There might be some editing to 'finalize' the unit_price as long as the
invoice is still 'open'. A discount percentage need to be retained so
that the percentage value can survive changes to the unit_price.

For example :

item_id qty unit_price discount subtotal(calculated)
=========================================================
001 5 $1.00 $0.50 $4.50
002 10 $2.00 20% $16.00
=========================================================

The $4.50 figure might seem wrong if you interpret the 'discount' as
'discount per unit'. In this case, the discount was applied to the
subtotal.

At a later date, the unit_price might be modified (see the second line
item below) :
item_id qty unit_price discount subtotal(calculated)
=========================================================
001 5 $1.00 $0.50 $4.50
002 10 $2.50 20% $20.00
=========================================================

That's all there is to it, actually. There was never a formal
requirement on this, so i hope i explained it clearly enough. Of course,
there is another 'discount' applied to the master level (INVOICE) that
works similarly.

Thanks in advance,
Regards,
sugi.