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>>But, I take it from this that your preference would be to store customer id.
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>That's the measure of redundance you may want to introduce, provided the data there isn't prone to changes. I've usually stored the invoice date in both invoice header and line items - because it never changed, and I had a bunch of reports which pulled from line items only, based on this date.
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>Also, I've had invoices without line items - which were corrections of previous invoices, or just lump sum charges for unspecified extras. They also had no orders - and in these cases you'd have no place to put the customer ID if it wasn't in the invoice header. Actually, back home very few things had an order at all - we usually sent a pro-forma invoice, and if it was aggreed upon (or even paid), that served as an order, but most of the time there were long term deals or just phone agreements, which were billed periodically or ad-hoc, therefore no orders most of the time.

I've had similar experiences where there isn't a neat and easy solution like an invoice without a Work Order. The redunancy makes a lot of sense. You can default customer information to the Work Order's information if there is one, but you have to allow an invoice without a Work Order.

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Equally, storing the state with the ZIP should be OK - I haven't heard of too many cities travelling between states recently.

So you don't feel store the State Id? I like the zip idea, but in some cases the user won't know the zip or it may be a new zip that would not have been added to the table yet.