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As far as I understand, keyrefs in XSD must always point to a valid
key in the instance document; there is no parallel to a C null pointer
or an SQL NULL foreign key, IOW, one cannot say, "this element *may*,
point to another element, but is not required to".
It seems that I can model this SQL in XSD:
FOO INTEGER NOT NULL REFERENCES BAR.BAZ
but not this:
FOO INTEGER NULL REFERENCES BAR.BAZ
Ari.
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