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> > I don't think you mean "the absence of a tuple". The
> relational model
> > has the vast edifice of "null" to support absent data in a
> cell of a
> > table, but it has never had any way of representing a missing or
> > unknown row.
>
> This depends on whether on not you are making what C.J Date
> calls the "Closed World" assumption about your relational
> database (which he prefers to). Thus the absence of a
> particular row in a relation is taken to imply the falsity of
> the predicate that relation represents.
Yes, exactly. You can either assume that the absence of the row implies
the nonexistence of the employee, or you can assume that the absence of
the row implies that no information is available, but there is no
explicit way of distinguishing the two cases.
Michael Kay
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