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(_at_)lists(_dot_)mulberrytech(_dot_)com
[mailto:owner-xsl-list(_at_)lists(_dot_)mulberrytech(_dot_)com]En nombre de
Wendell Piez
Enviado el: Martes, 13 de Enero de 2004 13:28
Para: xsl-list(_at_)lists(_dot_)mulberrytech(_dot_)com
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(_at_)mulberrytech(_dot_)com
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
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