--- a/relpipe-data/examples.xml Fri Jan 11 22:39:05 2019 +0100
+++ b/relpipe-data/examples.xml Fri Jan 11 22:39:30 2019 +0100
@@ -687,6 +687,67 @@
</p>
+ <h2>Read an Atom feed using XQuery and relpipe-in-xml</h2>
+
+ <p>
+ Atom Syndication Format is a standard for publishing web feeds a.k.a web syndication.
+ These feeds are usually consumed by a <em>feed reeder</em> that aggregates news from many websites and displays them in a uniform format.
+ The Atom feed is an XML with a list of recent news containing their titles, URLs and short annotations.
+ It also contains some metadata (website author, title etc.).
+ </p>
+ <p>
+ Using this simple XQuery<m:podČarou>see <a href="https://en.wikibooks.org/wiki/XQuery">XQuery</a> at Wikibooks</m:podČarou>
+ <em>FLWOR Expression</em>
+ we convert the Atom feed into the XML serialization of relational data:
+ </p>
+
+ <m:pre jazyk="xq" src="examples/atom.xq" odkaz="ano"/>
+
+ <p>
+ This is similar operation to <a href="https://www.postgresql.org/docs/current/functions-xml.html">xmltable</a> used in SQL databases.
+ It converts an XML tree structure to the relational form.
+ In our case, the output is still XML, but in a format that can be read by <code>relpipe-in-xml</code>.
+ All put together in a single shell script:
+ </p>
+
+ <m:pre jazyk="bash" src="examples/atom.sh"/>
+
+ <p>Will generate a table with web news:</p>
+
+ <m:pre jazyk="text" src="examples/atom.txt"/>
+
+ <p>
+ For frequent usage we can create a script or funcrion called <code>relpipe-in-atom</code>
+ that reads Atom XML on STDIN and generates relational data on STDOUT.
+ And then do any of these:
+ </p>
+
+ <m:pre jazyk="bash"><![CDATA[wget … | relpipe-in-atom | relpipe-out-tabular
+wget … | relpipe-in-atom | relpipe-out-csv
+wget … | relpipe-in-atom | relpipe-out-gui
+wget … | relpipe-in-atom | relpipe-out-nullbyte | while read_nullbyte published title url; do echo "$title"; done
+wget … | relpipe-in-atom | relpipe-out-csv | csv2rec | …
+]]></m:pre>
+
+ <p>
+ There are several implementations of XQuery.
+ <a href="http://galax.sourceforge.net/">Galax</a> is one of them.
+ <a href="http://xqilla.sourceforge.net/">XQilla</a> or
+ <a href="http://basex.org/basex/xquery/">BaseX</a> are another ones (and support newer versions of the standard).
+ There are also XSLT processors like <a href="http://xmlsoft.org/XSLT/xsltproc2.html">xsltproc</a>.
+ BaseX can be used instead of Galax – we just replace
+ <code>galax-run -context-item /dev/stdin</code> with <code>basex -i /dev/stdin</code>.
+ </p>
+
+ <p>
+ Reading Atom feeds in a terminal might not be the best way to get news from a website,
+ but this simple example learns us how to convert arbitrary XML to relational data.
+ And of course, we can generate multiple relations from a single XML using a single XQuery script.
+ XQuery can be also used for operations like JOIN or UNION and for filtering and other transformations
+ as will be shown in further examples.
+ </p>
+
+
</text>
</stránka>