Subject: Re: [xsl] Can sets have order? From: Wolfgang May <may@xxxxxxxxxxxxxxxxxxxxxxxxxx> Date: Wed, 31 Jan 2001 15:35:32 +0100 (MET) |
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 XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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