Required property of Primary Key fields

[Jamie Collins]

While usnig another table to be the junction between the backordered items
table and the purchase order items table is certainly feasible, it seemed to
be an unnecessary structure for my design.

Design is in part art IMO. I think my design is better in principle
because I don't consider 'backorder item ID' (or whatever) to be an
attribute of a 'purchase order item' (surely 'backorder item ID' comes
into existence before the purchase order item' therefore how can it be
its attribute?) and better in practice because it doesn't involve
redundantly storing a bunch of NULLs to indicate that no relationship
exists (I can generate those NULLs on demand with an outer join). Is
better not good enough? Must it be proved "necessary" <g>?

Jamie.

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