XQuery¶

XQuery is a full query and transformation language for XML. Where XPath only addresses a document and XSLT transforms it with template rules, XQuery lets you query one or many documents the way SQL queries tables — selecting, filtering, joining, grouping, and building new XML from the result.

It is best understood as XPath's bigger sibling. XQuery 3.1 and XSLT 3.0 are built on the same foundation:

• the same expression language — XPath 3.1 is a strict subset of XQuery,
• the same data model — the XDM (sequences of nodes and atomic values),
• the same function library — fn: functions, map:/array:, higher-order functions, JSON support.

So almost everything you learned about XPath carries over unchanged. XQuery adds the surrounding query machinery: FLWOR expressions, element constructors, and a module system.

The shape of a query¶

A FLWOR expression (For, Let, Where, Order by, Return — pronounced "flower") is the heart of the language. It reads top to bottom like a loop, and like SQL it ends in a projection:

xquery version "3.1";

<report>{
  for $cd in /catalog/cd
  let $price := xs:decimal($cd/price)
  where $price > 9.90
  order by $cd/title
  return <cd title="{ $cd/title }" price="{ $price }"/>
}</report>

Run against the running CD catalog, this produces a brand-new document:

<report>
  <cd title="Empire Burlesque" price="10.9"/>
</report>

Notice the two things XPath cannot do on its own: the for…return loops and filters, and the literal <report>…</report> with { … } holes is a direct element constructor — XQuery builds output XML inline, no xsl:element needed.

XQuery is not Saxon-only¶

A common worry is that XQuery lives or dies with Saxon. The opposite is true — and it is the reverse of XSLT 3.0, where Saxon is effectively the only complete implementation. XQuery has a healthy multi-vendor story, because the native-XML-database world adopted it as its query language:

The catch is only that most engines are databases: "trying XQuery" usually means installing BaseX rather than piping a file through a CLI — though both BaseX and Saxon run a standalone .xq file perfectly well. See XQuery in the real world for what each is used for.

XQuery versions¶

• 1.0 (2007) established FLWOR, constructors, and the XPath 2.0 type system.
• 3.0 (2014) added group by, count, the => arrow operator, inline functions, and try/catch.
• 3.1 (2017) is the current version: maps and arrays, native JSON (fn:parse-json, fn:json-doc), and lockstep parity with XSLT 3.0's function library.

This section teaches 3.1, the version every current engine implements.

The running example¶

Like the XPath and XSLT sections, every page queries the same small CD catalog, so you can focus on the XQuery rather than re-learning the data:

<catalog>
  <cd genre="rock"><title>Empire Burlesque</title><artist>Bob Dylan</artist><price>10.90</price><year>1985</year></cd>
  <cd genre="pop"><title>Hide your heart</title><artist>Bonnie Tyler</artist><price>9.90</price></cd>
  <cd genre="country"><title>Greatest Hits</title><artist>Dolly Parton</artist><price>9.90</price></cd>
</catalog>

Where to go next¶

1. FLWOR expressions — for/let/where/order by/return, group by, and building XML output.
2. XQuery vs XSLT — two languages, one data model: when to reach for which.
3. XQuery in the real world — native XML databases, RESTXQ web apps, the Update Facility, and embedded SQL XQuery.