Subject: RE: [xsl] Anyone can explain me this syntax? From: "Jaime Stuardo" <jstuardo@xxxxxxxxxxx> Date: Tue, 13 Jan 2004 15:01:07 -0400 |
Thanks Wendell for your complete explanation. While reading your help, I tested another alternative: ROW[. = key('relacion_x_cobertura', REL_ID)[1]] and it worked too. The actual XML structure has 35 ROW's, the first 7 ROW's has REL_ID = 1, the following 7 ROW's has REL_ID = 2, and so on (this was a particular case where exist 7 ROW's for each REL_ID). I think that the new way above worked because all of the ROW's are different (besides REL_ID, the ROW has other fields), and since key('relacion_x_cobertura', REL_ID)[1] returns only 1 node where the key is equal to REL_ID, by comparing . with the key value, returns only 1 ROW, the ROW that allows me to group by REL_ID=1, REL_ID=2, until REL_ID = 5. I think that case is equivalent to generate-id method since all ROW's has at least 1 field different, making the whole ROW different. Is that true? Jaime -----Mensaje original----- De: owner-xsl-list@xxxxxxxxxxxxxxxxxxxxxx [mailto:owner-xsl-list@xxxxxxxxxxxxxxxxxxxxxx]En nombre de Wendell Piez Enviado el: Martes, 13 de Enero de 2004 13:28 Para: xsl-list@xxxxxxxxxxxxxxxxxxxxxx Asunto: Re: [xsl] Anyone can explain me this syntax? Jaime, To add to others' responses ... the point of the expression is to select a single node that can serve as a representative or "flagbearer" node for a group of nodes. If we know exactly which group we want, this is not hard -- for example, in your case if we want the group of nodes matching the key 'relacion_x_cobertura' with REL_ID value of 'x', we'd say key('relacion_x_cobertura', 'x')[1] -- that is, the first node (the [1] predicate) in document order of the set returned by the key function with value 'x' as the key. But what if we don't want a particular unique flagbearer, but one flagbearer each for *all* the key values in our document? The only way to do this in XSLT 1.0 is to collect all the candidates and test each one to see if it's the designated flagbearer for its set (throwing away the ones that aren't). This means we have to compare each node in turn to key('relacion_x_cobertura', 'x')[1], and see if it's the same node. (Fortunately this test can be performed in the predicate of a path expression that would otherwise select all the candidates.) Since XSLT 1.0 does not have a real node-identity test, we can't do this by simply comparing them. Accordingly we have a workaround test, which takes the form of count($node | $nodeset[1]) = 1 this will be the case if $node and $nodeset[1] are the same, but not if they are two different nodes. (This is why your test worked when you said [2] instead of [1], since [2] picks a single node just as [1] does. But yes, you were lucky: if any of your sets didn't have a second member, returning an empty node set from $nodeset[2], you'd be sunk.) Another way to perform this same test is to say generate-id($node)=generate-id($nodeset[1]) which is why we also see the Muenchian technique used in the form ROW[generate-id(.) = generate-id(key('relacion_x_cobertura', REL_ID)[1])] which can be abbreviated as ROW[generate-id() = generate-id(key('relacion_x_cobertura', REL_ID))] since the generated ID (a string) of a set of nodes will be the generated ID of the first node in the set. The essence of the Muenchian grouping technique could be summarized as "collect the nodes to be grouped and de-duplicate by the grouping criterion to select a set of unique flagbearers; iterate over that set and with each iteration take the flagbearer and all the nodes in its group". I hope this helps-- Wendell At 10:58 AM 1/13/2004, you wrote: >Hi all... > > >From Muenchian method of grouping, I always use something like this: > >ROW[count(. | key('relacion_x_cobertura', REL_ID)[1]) = 1] > >That always works, but I want to understand he reason of that syntax, so >that if I don't have the possibility of doing copy & paste from other >code, I could be able to write that expression by myself. > >I know that instruction only returns the different elements contained in >ROW. That difference is determined by REL_ID value. > >But the actual questions are: > >- Why the . (dot) is used? why if I omit it, it doesn't work (it returns >all elements)? >- What's the meaning of the | (pipe)? >- What's the meaning of [1]? I have always used things like >[FIELD_NAME=some_value]. I understand that perfectly, but what about >placing only that number in the brackets? I tried by using [2] and it >worked too... or, perhaps I was lucky? ====================================================================== Wendell Piez mailto:wapiez@xxxxxxxxxxxxxxxx Mulberry Technologies, Inc. http://www.mulberrytech.com 17 West Jefferson Street Direct Phone: 301/315-9635 Suite 207 Phone: 301/315-9631 Rockville, MD 20850 Fax: 301/315-8285 ---------------------------------------------------------------------- Mulberry Technologies: A Consultancy Specializing in SGML and XML ====================================================================== XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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