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What Is XQuery
by Per Bothner | Pages: 1, 2

Path Expressions and Relationship to XPath

XQuery borrows path expressions from XPath. XQuery can be viewed as a generalization of XPath. Except for some obscure forms (mostly unusual "axis specifiers"), all XPath expressions are also XQuery expressions. For this reason the XPath specification is also being revised by the XQuery committee, with the plan that XQuery 1.0 and XPath 2.0 will be released about the same time.

The following simple example assumes an XML file "mybook.xml" whose root element is a <book>, containing some <chapter> children:

let $book := document("mybook.xml")/book
return $book/chapter

The document function returns the root node of a document. The /book expression selects the child elements of the root that are named book, so $book gets set to the single root element.

The $book/chapter selects the child elements of the top-level book elements, which results in a sequence of the second-level chapter nodes in document order.

The next example includes a predicate:

$book//para[@class="warning"]

The double slash is a convenience syntax to select all descendants (rather than just children) of $book, selecting only <para> element nodes that have an attribute node named class whose value is "warning"

One difference to note between XPath and XQuery is that XPath expressions may return a node set, whereas the same XQuery expression will return a node sequence. For compatibility these sequences will be in document order and with duplicates removed, which makes them equivalent to sets.

XSLT is very useful for expressing very simple transformations, but more complicated stylesheets (especially anything with non-trivial logic or programming) can often be written more concisely using XQuery.

Iterating Over Sequences

A for expression lets you "loop" over the elements of a sequence:

for $x in (1 to 3) return ($x,10+$x)

The for expression first evaluates the expression following the in. Then for each value of the resulting sequence, the variable (in this case $x) is bound to the value, and the return expression evaluated using that variable binding. The value of the entire for expression is the concatenation of all values of the return expression, in order. So the example evaluates to this 6-element sequence: 1,11,2,12,3,13.

Here is a more useful example. Assume again that mybook.xml is a <book> that contains some <chapter> elements. Each <chapter> has a <title>. The following will create a simple page that lists the titles:

<html>{
  let $book := document("mybook.xml")/book
  for $ch in $book/chapter
    return <h2>{$ch/title)</h2>
}</html>

The term "FLWR expressions" includes both for and let expressions. The acronym FLWR refers to the fact that it consists of one or more for and/or let clauses, an optional where clause, and a result clause. A where clause causes the result clause to be evaluated only when the where where expression is true.

The next example has a nested loop, allowing us to combine two sequences: one of customer elements and the other of order elements. We want to find the name(s) of customers who have ordered the part whose part_id is "xx".

for $c in customers
for $o in orders
where $c.cust_id=$o.cust_id and $o.part_id="xx"
return $c.name

This is essentially a join of two tables as commonly performed using relational databases. An important goal for XQuery is that it should be usable as a query language for XML databases. Compare the corresponding SQL statement,

select customers.name
from customers, orders
where customers.cust_id=orders.cust_id
  and orders.part_id="xx"

Functions

XQuery wouldn't be much of a programming language without user-defined functions. Such function definitions appear in the query prologue of an XQuery program. It's worth noting that function parameters and function results can be primitive values, nodes, or sequences of either.

The following is a recursive utility function. It returns all the descendant nodes of the argument, including the argument node itself. It does a depth-first traversal of the argument, returning the argument, and then looping over the argument node's children, recursively calling itself for each child.

define function descendant-or-self ($x)
{
  $x,
  for $y in children($x)
    return descendant-or-self($y)
}
descendant-or-self(<a>X<b>Y</b></a>)

Which evaluates to this sequence of length 4:

<a>X<b>Y</b></a>; "X"; <b>Y</b>; "Y"

Sorting and Context

If you want to sort a sequence you can use a sortby expression. To sort a sequence of books in order of author name you can do:

$books sortby (author/name)

The sortby takes an input sequence (in this case $books) and one or more ordering expressions. During sorting the implementation needs to compare two values from the input sequence to determine which comes first. It does that by evaluating the ordering expression(s) in the context of a value from the input sequence. So the path expression author/name is evaluated many times, each time relative to a different book as the context (or current) item.

Path expressions also use and set the context. In author/name the name children that are returned are those of the context item, which is an author item.

Type Specification

XQuery is a strongly typed programming language. Like Java and C#, for example, it's a mix of static typing (type consistency checked at compile-time) and dynamic typing (run-time type tests). However, the types in XQuery are different from the classes familiar from object-oriented programming. Instead, it has types to match XQuery's data model, and it allows you to import types form XML Schema.

if ($child instance of element section)
then process-section($child)
else ( ) {--nothing--}

This invokes the process-section function if the value of $child is an element whose tag name is section. XQuery has a convenient typeswitch shorthand for matching a value against a number of types. The following converts a set of tag names to a different set.

define function convert($x) {
  typeswitch ($x)
    case element para return <p>{process-children($x)}</p>
    case element emph  return <em>{process-children($x)}</em>
    default return process-children($x)
}
define function process-children($x) {
  for $ch in children($x) return convert($ch)
}

Resources

The primary XQuery resource is www.w3.org/XML/Query. This has links to the draft standards, mailing lists, and implementations. The main documents are

The actual XQuery specification (www.w3.org/TR/xquery/) is not difficult to read, and is probably where you should go next.
The Data Model specification (www.w3.org/TR/query-datamodel/) goes into nodes and the functions you can use to manipulate them.
The Functions and Operators specification (www.w3.org/TR/xquery-operators/) defines the other (non-node) functions, including string and date/time functions.
The Use Cases document (www.w3.org/TR/xmlquery-use-cases) contains lots of useful examples of XQuery programs to solve specific problems.
The Formal Semantics specification (www.w3.org/TR/query-semantics/) uses formal mathematical notation and is not for the faint of heart. Most people should skip it.

There's only one XQuery book so far, mainly because there are significant loose ends in the specification: Early Adopter XQuery from Wrox. I am co-authoring (with James McGovern, Kurt Cagle, James Linn and Vaidyanathan Nagarjan) XQuery Kick Start for Sams Publishing, due to be released in 2003. There are no complete standards-conforming implementations either, but the XQuery site lists known implementations, some of which have executable demos. The only open-source implementation currently available seems to be my Qexo. (The Qexo implementation is interesting in that it compiles XQuery programs on-the-fly directly to Java bytecodes.) I recommend considering XQuery when you need a powerful and convenient tool for analyzing or generating XML.

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