Subject: Re: [xsl] things about grouping From: Ihe Onwuka <ihe.onwuka@xxxxxxxxx> Date: Fri, 7 Dec 2012 10:09:39 +0000 |
On Wed, Nov 28, 2012 at 12:29 PM, Michael Kay <mike@xxxxxxxxxxxx> wrote: > I think you are making two points: > > (a) the choice of keyword. Working groups sometimes have agonizing debates > over the choice of keyword, and sometimes it goes through on the nod without > significant discussion. If memory serves, in this case there wasn't much > discussion. This might have been because there was a lot of controversy > about introducing the keyword "union" as a synonym for the existing "|", > which was done because some people on the XQuery group had spent much of > their lives writing SQL and felt strongly about the choice; when working > groups have decided one controversial question, they are often inclined to > accept other parts of the same proposal without much discussion. If there > had been a discussion I guess it would have been between those who wanted to > use the language of mathematics and those who wanted something closer to > English; there is no knowing which view would have prevailed, because > committees don't apply a consistent policy on such issues, they decide each > question on its merits. As it is, the English conjunction was chosen over > the mathematical set operator. > I think it must have come from. The SQL Standard which uses Except as the difference operator although Oracle uses the far more palatable Minus (not sure whether this is instead of rather than in addition to where one flavour dedups and the other doesn't). My view is unchanged - it's just another sin SQL commits against the theoretical domain it is modelling. The situation in SQL is palliated by the fact that the syntax is always Select statement except Select statement i.e no abbreviated form to trip over. I found the following commentary in the form of a book review http://www.amazon.com/First-Course-Database-Systems-Edition/product-reviews/013600637X/ref=dp_top_cm_cr_acr_txt?ie=UTF8&showViewpoints=1 "These are classics and should be on the bookshelf of everyone who calls him/herself a computer scientist. This book is, however, a little disappointing. Most of it is good, some of it very good. But I do find some flaws in it. One of the glaring flaws deals with the attempt to extend the relational model from sets to bags (basically, to allow for duplicate tuples in relations.) This is the best attempt I've seen at formalizing "bag theory", but it introduces problems (some minor, others very serious) that aren't mentioned in the text. This review is too short and not the right place to expound on these problems. Chris Date's database text goes into most of them in substantial detail." As I recall (very very vaguely) the issues centre around situations where seemingly equivalent expressions don't yield the equivalent results because they involve multisets. One of Date's arguments (again I recall very vaguely) seems to be stick to applying set theoretic constructs to sets and you avoid such anomalies. Seeing as XPath also applies set theoretic operators to multisets the same warnings should apply. I for one would take the performance hit of deduping as a price worth paying for not having to play computer in order to reason about an expression I just wrote. Maybe the language designers sought to adopt SQL practices and subliminally did so warts n all. I wonder how many mystifying subtleties of the language that explains? > > I would strongly recommend that you get involved in the process of defining > these languages and specifications. You will find many cases where you are > able to make a difference simply by alerting the group to problems that it > might not otherwise have noticed. You will also find cases where you are > extremely frustrated because some other apparently intelligent person > passionately takes a view that is different to your own; if you put aside > the frustration and try to understand their argument you will often come to > appreciate that what is obviously right and good depends on where you are > starting from. > I will if someone will tell me where and how to sign up to any ongoing effort.
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