690 		<h2>Read an Atom feed using XQuery and relpipe-in-xml</h2>

   691 		

   692 		<p>

   693 			Atom Syndication Format is a standard for publishing web feeds a.k.a web syndication. 

   694 			These feeds are usually consumed by a <em>feed reeder</em> that aggregates news from many websites and displays them in a uniform format.

   695 			The Atom feed is an XML with a list of recent news containing their titles, URLs and short annotations.

   696 			It also contains some metadata (website author, title etc.).

   697 		</p>

   698 		<p>

   699 			Using this simple XQuery<m:podČarou>see <a href="https://en.wikibooks.org/wiki/XQuery">XQuery</a> at Wikibooks</m:podČarou>

   700 			<em>FLWOR Expression</em>

   701 			we convert the Atom feed into the XML serialization of relational data:

   702 		</p>

   703 		

   704 		<m:pre jazyk="xq" src="examples/atom.xq" odkaz="ano"/>

   705 		

   706 		<p>

   707 			This is similar operation to <a href="https://www.postgresql.org/docs/current/functions-xml.html">xmltable</a> used in SQL databases.

   708 			It converts an XML tree structure to the relational form.

   709 			In our case, the output is still XML, but in a format that can be read by <code>relpipe-in-xml</code>.

   710 			All put together in a single shell script:

   711 		</p>

   712 		

   713 		<m:pre jazyk="bash" src="examples/atom.sh"/>

   714 		

   715 		<p>Will generate a table with web news:</p>

   716 		

   717 		<m:pre jazyk="text" src="examples/atom.txt"/>

   718 		

   719 		<p>

   720 			For frequent usage we can create a script or funcrion called <code>relpipe-in-atom</code>

   721 			that reads Atom XML on STDIN and generates relational data on STDOUT.

   722 			And then do any of these:

   723 		</p>

   724 		

   725 		<m:pre jazyk="bash"><![CDATA[wget … | relpipe-in-atom | relpipe-out-tabular

   726 wget … | relpipe-in-atom | relpipe-out-csv

   727 wget … | relpipe-in-atom | relpipe-out-gui

   728 wget … | relpipe-in-atom | relpipe-out-nullbyte | while read_nullbyte published title url; do echo "$title"; done

   729 wget … | relpipe-in-atom | relpipe-out-csv | csv2rec | …

   730 ]]></m:pre>

   731 

   732 		<p>

   733 			There are several implementations of XQuery.

   734 			<a href="http://galax.sourceforge.net/">Galax</a> is one of them. 

   735 			<a href="http://xqilla.sourceforge.net/">XQilla</a> or

   736 			<a href="http://basex.org/basex/xquery/">BaseX</a> are another ones (and support newer versions of the standard).

   737 			There are also XSLT processors like <a href="http://xmlsoft.org/XSLT/xsltproc2.html">xsltproc</a>.

   738 			BaseX can be used instead of Galax – we just replace

   739 			<code>galax-run -context-item /dev/stdin</code> with <code>basex -i /dev/stdin</code>.

   740 		</p>

   741 		

   742 		<p>

   743 			Reading Atom feeds in a terminal might not be the best way to get news from a website,

   744 			but this simple example learns us how to convert arbitrary XML to relational data.

   745 			And of course, we can generate multiple relations from a single XML using a single XQuery script.

   746 			XQuery can be also used for operations like JOIN or UNION and for filtering and other transformations

   747 			as will be shown in further examples.

   748 		</p>

   749 		

   750