XQuery

Query and construct XML with XQuery — SQL for hierarchical data

#XQuery

XQuery is a full programming language for querying and constructing XML. XPath selects nodes from a document. XQuery builds entirely new documents from multiple sources.

For C# developers: XQuery resembles SQL for hierarchical data. If XPath is SELECT column FROM table WHERE condition, XQuery is the full SQL with JOIN, subqueries, and INSERT INTO.

#What's Here

  • FLWOR Expressions — The core of XQuery: For, Let, Where, Order by, Return. Like LINQ query syntax but for XML.

  • Advanced FLWOR — Window clauses, count, group by, while, otherwise — advanced FLWOR patterns.

  • Constructors — Building new XML elements, attributes, and documents from query results.

  • Conditionals and Error Handling — if/then/else, switch, typeswitch, try/catch, and the otherwise operator.

  • Operators — Comparison, arithmetic, sequence, and navigation operators.

  • Type System — XQuery types, sequence types, records, enums, union types, and type testing.

  • Update Facility — XQuery Update — insert, delete, replace, rename, and transform expressions.

  • Functions and Modules — Writing reusable XQuery functions and organizing code into modules.

  • XQuery vs XSLT — When to use which. They overlap significantly — here's how to choose.

  • Full-Text Search — Linguistic search with stemming, wildcards, proximity, and scoring.

  • Prolog and Declarations — Module declarations, namespace imports, options, and configuration.

  • Extensibility — Custom functions, modules, external functions, and .NET integration.

#Key Features

#Direct Element Constructors

XQuery builds XML inline using XML-literal syntax with embedded expressions in curly braces {}. The query output looks like the XML it produces:

xquery 
<root>
  <item name="test">{current-dateTime()}</item>
  <nested><child>text</child></nested>
</root>

See Constructors for full details on direct and computed constructors.

#String Constructors

XQuery 3.1+ supports string constructors using backtick syntax for creating strings that contain curly braces or other characters that are awkward to escape:

xquery 
``[This string contains {$variable} interpolation and literal { braces }]``

The content between [ and ] is treated as a string template. Expressions inside { } are evaluated and interpolated. To include a literal curly brace, double it: {{ or }}.

#Annotations

Function and variable declarations can carry annotations that control visibility and behavior:

xquery 
(: Public function — accessible to importers :)
declare %public function local:format($x) { ... };
(: Private function — internal to this module :)
declare %private function local:helper($x) { ... };
(: Annotations can also carry application-specific metadata :)
declare %rest:path("/api/users") %rest:GET
function local:get-users() { ... };

Annotations are declared with %name or %name("value") before the function or variable keyword. The built-in annotations %public and %private control visibility in library modules.

#The FLWOR Expression

XQuery's FLWOR expression selects, filters, sorts, and returns data in one pipeline:

xquery 
for $book in /catalog/book
where $book/price > 30
order by $book/title
return <result>{ $book/title, $book/price }</result>

The FLWOR version handles namespaces natively and runs on any XQuery processor.

For C# developers: FLWOR maps almost directly to LINQ query syntax.

csharp 
from book in catalog.Elements("book")
where (decimal)book.Element("price") > 30
orderby (string)book.Element("title")
select new XElement("result", book.Element("title"), book.Element("price"))