RE: Can sets have order?

> 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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