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RE: Can sets have order?
- To: <xsl-list at lists dot mulberrytech dot com>
- Subject: RE: [xsl] Can sets have order?
- From: "Michael Kay" <mhkay at iclway dot co dot uk>
- Date: Wed, 31 Jan 2001 16:14:01 -0000
- Reply-To: xsl-list at lists dot mulberrytech dot com
> In fact, in databases, relations are *multisets*, allowing for
> duplicates. I.e., tuples with the same values but different
> internal row-ids are allowed.
In Codd's relational model, relations are sets.
In SQL, relations are multisets, because a relation can contain two tuples
that are indistiguishable.
In every real RDBMS that I know of, relations are sets, because two tuples
in the same relation are always distinguishable by some kind of id, which
means that the relation no longer contains duplicates.
> For XPath/XSL I would say that it is somewhere between sets and
> multisets since a result set can contain the same values several times
Unlike the pure relational model, but like most relational implementations,
nodes in XPath/XSLT have identity, so a node-set cannot contain "the same
node" twice. It can contain two nodes that are equal, but not two that are
identical.
Mike Kay
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