relpipe-data/examples-xquery-atom.xml
author František Kučera <franta-hg@frantovo.cz>
Mon, 21 Feb 2022 00:43:11 +0100
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<stránka
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	xmlns="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/strana"
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	xmlns:m="https://trac.frantovo.cz/xml-web-generator/wiki/xmlns/makro">
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	<nadpis>Reading an Atom feed using XQuery</nadpis>
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	<perex>converting arbitrary XML into relational data using XQuery</perex>
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	<m:pořadí-příkladu>01100</m:pořadí-příkladu>
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	<text xmlns="http://www.w3.org/1999/xhtml">
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		<p>
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			Atom Syndication Format is a standard for publishing web feeds a.k.a web syndication. 
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			These feeds are usually consumed by a <em>feed reeder</em> that aggregates news from many websites and displays them in a uniform format.
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			The Atom feed is an XML with a list of recent news containing their titles, URLs and short annotations.
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			It also contains some metadata (website author, title etc.).
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		</p>
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		<p>
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			Using this simple XQuery<m:podČarou>see <a href="https://en.wikibooks.org/wiki/XQuery">XQuery</a> at Wikibooks</m:podČarou>
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			<em>FLWOR Expression</em>
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			we convert the Atom feed into the XML serialization of relational data:
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		</p>
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		<m:pre jazyk="xq" src="examples/atom.xq" odkaz="ano"/>
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		<p>
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			This is similar operation to <a href="https://www.postgresql.org/docs/current/functions-xml.html">xmltable</a> used in SQL databases.
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			It converts an XML tree structure to the relational form.
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			In our case, the output is still XML, but in a format that can be read by <code>relpipe-in-xml</code>.
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			All put together in a single shell script:
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		</p>
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		<m:pre jazyk="bash" src="examples/atom.sh"/>
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		<p>Will generate a table with web news:</p>
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		<m:pre jazyk="text" src="examples/atom.txt"/>
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		<p>Or with <code>relpipe-out-recfile</code>, we will get output in the recfile format (<a href="https://www.gnu.org/software/recutils/">GNU Recutils</a>), like this:</p>
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		<m:pre jazyk="text" src="examples/atom.rec"/>
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		<p>
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			For frequent usage we can create a script or funcrion called <code>relpipe-in-atom</code>
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			that reads Atom XML on STDIN and generates relational data on STDOUT.
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			And then do any of these:
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		</p>
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		<m:pre jazyk="bash"><![CDATA[wget … | relpipe-in-atom | relpipe-out-tabular
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wget … | relpipe-in-atom | relpipe-out-csv
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wget … | relpipe-in-atom | relpipe-out-gui
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wget … | relpipe-in-atom | relpipe-out-nullbyte | while read_nullbyte published title url; do echo "$title"; done
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wget … | relpipe-in-atom | relpipe-out-recfile
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]]></m:pre>
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		<p>
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			There are several implementations of XQuery.
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			<a href="http://galax.sourceforge.net/">Galax</a> is one of them. 
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			<a href="http://xqilla.sourceforge.net/">XQilla</a> or
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			<a href="http://basex.org/basex/xquery/">BaseX</a> are another ones (and support newer versions of the standard).
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			There are also XSLT processors like <a href="http://xmlsoft.org/XSLT/xsltproc2.html">xsltproc</a>.
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			BaseX can be used instead of Galax – we just replace
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			<code>galax-run -context-item /dev/stdin</code> with <code>basex -i /dev/stdin</code>.
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			Or for XQilla we use: <code>xqilla -i /dev/stdin</code>.
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		</p>
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		<p>
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			Reading Atom feeds in a terminal might not be the best way to get news from a website,
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			but this simple example learns us how to convert arbitrary XML to relational data.
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			And of course, we can generate multiple relations from a single XML using a single XQuery script.
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			XQuery can be also used for operations like JOIN or UNION and for filtering and other transformations
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			as will be shown in further examples.
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		</p>
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25dec6931f18 Lepší odsazení, tabulátory.
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	</text>
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