Re: Can sets have order?

Elliotte Rusty Harold writes:
 > >>  I'd also agree with my original statement. Terminology isn't always
 > >>  consistently applied.
 > >
 > >Agreed here, but I can't think of a respectable terminology that restricts
 > >sets to be unordered.

 > I don't know if he's correct or not, but the eminently respectable C. 
 > J. Date writes on p. 92 of the sixth edition of his well respected 
 > text "Introduction to Database Systems":
 > 
 > "2. Tuples are unordered (top to bottom)
 > 
 > This property follows from the fact that the body of the relation is 
 > a mathematical set; sets in mathematics are not ordered."

In fact, in databases, relations are *multisets*, allowing for
duplicates.   I.e., tuples with the same values but different 
internal row-ids are allowed.

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
(obtained from different nodes in the document). Here, the row-id
corresponds to the internal position of the node in the source
document.
E.g., when applying id(...), these duplicates are removed.

Wolfgang

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