XQuery

XQuery, short for XML Query Language, is a functional, declarative language designed to query, transform, and construct XML data. It is widely used in web services, document databases, content management systems, and enterprise integration pipelines where XML is a primary data format. Developers can use XQuery through standards-compliant processors such as BaseX, eXist-db, MarkLogic, or Saxon, and integrate it into applications via command-line tools, database engines, or language bindings in environments like Java and Python.

XQuery exists to provide a powerful and expressive way to work with XML beyond simple transformations. Its design philosophy emphasizes declarative querying, composability, and predictable results, allowing developers to describe what data they want rather than how to retrieve it procedurally. By building on XPath and functional concepts, XQuery enables concise querying, restructuring, and aggregation of complex XML documents while remaining readable and maintainable.

XQuery: Basic Queries

The most fundamental use of XQuery is selecting nodes from XML documents using XPath expressions and returning them in a structured form.

for $book in doc("library.xml")/library/book
return $book/title

This query iterates over each book element in an XML document and returns its title. The for expression establishes a sequence, and the return clause defines the output. This approach resembles selection and projection in SQL, but operates on hierarchical XML structures rather than tables.

XQuery: FLWOR Expressions

XQuery centers around FLWOR expressions, which stand for for, let, where, order by, and return. These constructs enable complex querying and transformation logic.

for $book in doc("library.xml")/library/book
where $book/price > 20
order by $book/title
return <expensive>{$book/title}</expensive>

This example filters books by price, sorts them by title, and wraps each result in a new XML element. FLWOR expressions provide a clear and expressive way to combine filtering, ordering, and construction, making XQuery well suited for document-oriented data processing.

XQuery: Constructing XML

Beyond querying, XQuery excels at constructing new XML documents from existing data. Element constructors allow precise control over output structure.

<catalog>
{
  for $book in doc("library.xml")/library/book
  return
    <item>
      <name>{$book/title}</name>
      <cost>{$book/price}</cost>
    </item>
}
</catalog>

This query builds a new catalog document from an existing library source. XML construction is explicit and readable, enabling transformations similar in spirit to template-based approaches in XSLT while retaining query-oriented flexibility.

XQuery: Functions and Modularity

XQuery supports user-defined functions, allowing reusable logic and modular query design across large codebases.

declare function local:discount($price as xs:decimal) as xs:decimal {
  $price * 0.9
};

for $book in doc("library.xml")/library/book
return local:discount($book/price)

This example defines a local function to apply a discount to book prices. Functions promote reuse and clarity, making XQuery suitable for large-scale XML processing tasks. This functional style aligns with concepts found in Scala and other declarative languages.

XQuery: Working with Non-XML Data

Modern XQuery implementations often support working with JSON and other data formats, enabling hybrid document processing workflows.

for $item in json-doc("data.json")?items?*
return <value>{$item?name}</value>

This snippet demonstrates querying JSON data and constructing XML output. Such capabilities allow XQuery to act as a bridge between XML-centric systems and JSON-based APIs, complementing data handling patterns found in JSON and scripting logic in JavaScript.

Overall, XQuery provides a powerful, declarative language for querying and transforming structured data, particularly XML. When used alongside XSLT, SQL, JSON, JavaScript, or Java-based systems, it enables developers to process complex document structures with clarity and precision. Its FLWOR expressions, XML construction capabilities, functional design, and support for multiple data formats make XQuery a dependable foundation for modern document-oriented and enterprise data workflows.