XQuery 3.0

This article is part of the Query Portal. It summarizes features of the upcoming XQuery 3.0 and XPath 3.0 Recommendations that have already been implemented in BaseX.

EQName: Expanded QNames

Template:Mark A QName can now be directly prefixed with its namespace URI, which will be enclosed in quotes and followed by a colon.

Examples:

• "http://www.w3.org/2005/xpath-functions/math":pi() returns the number π
• "java:java.io.FileOutputStream":new("output") creates a new Java file output stream

Namespace Constructors

Template:Mark New namespaces can now be created via so-called 'Computed Namespace Constructors'.

 
element node { namespace pref { 'http://url.org/' } }

String Concatenations

Template:Mark Two vertical bars || (also names pipe characters) can be used to concatenate two strings. This operator is a shortcut for the ft:contains() function.

 
'Hello' || ' ' || 'Universe'

Group By

FLWOR expressions have been extended to include the group by clause, which is well-established among relational database systems. group by can be used to apply value-based partitioning to query results:

Example:

 
for $ppl in doc('xmark')//people/person  
let $ic := $ppl/profile/@income
let $income :=  if($ic < 30000) then
                   "challenge" 
                else if($ic >= 30000 and $ic < 100000) then 
                   "standard" 
                else if($ic >= 100000) then 
                   "preferred" 
                else 
                   "na"  
group by $income
order by $income
return element { $income } { count($ppl) }

This query is a rewrite of Query #20 contained in the XMark Benchmark Suite to use group by. The query partitions the customers based on their income.

Result:

<challenge>4731</challenge>
<na>12677</na>
<prefered>314</prefered>
<standard>7778</standard>

In contrast to the relational GROUP BY statement, the XQuery counterpart concatenates the values of all non-grouping variables that belong to a specific group. In the context of our example, all nodes in //people/person that belong to the "preferred" partition are concatenated in $ppl after grouping has finished. You can see this effect by changing the return statement to:

 
...
return element { $income } { $ppl }

Result:

<challenge>
  <person id="person0">
    <name>Kasidit Treweek</name>
    …
  <person id="personX">
    …
</challenge>

Try/Catch

The try/catch construct can be used to handle errors at runtime:

Example:

 try {
   1 + '2'
 } catch *($code, $desc) {
   concat('Error [', $code, ']: ', $desc)
 }

Result: Error [XPTY0004]: '+' operator: number expected, string found.

Switch

The switch statement is available in many other programming languages. It chooses one of several expressions to evaluate based on its input value.

Example:

for $fruit in ("Apple", "Pear", "Peach")
return switch ($fruit)
  case "Apple" return "red"
  case "Pear"  return "green"
  case "Peach" return "pink"
  default      return "unknown"

Result: red green pink

Function Items

One of the most distinguishing features added in XQuery 3.0 are function items, also known as lambdas or lambda functions. They make it possible to abstract over functions and thus writing more modular code.

Examples:

Function items can be obtained in three different ways:

• Declaring a new inline function:

  let $f := function($x, $y) { $x + $y }
  return $f(17, 25)

  Result: 42

• Getting the function item of an existing (built-in oder user-defined) XQuery function. The arity (number of arguments) has to be specified as there can be more than one function with the same name:

  let $f := math:pow#2
  return $f(5, 2)

  Result: 25

• Partially applying another function or function item. This is done by supplying only some of the required arguments, writing the placeholder ? in the positions of the arguments left out. The produced function item has one argument for every placeholder.

  let $f := fn:substring(?, 1, 3)
  return (
    $f('foo123'),
    $f('bar456')
  )

  Result: foo bar

Function items can also be passed as arguments to and returned as results from functions. These so called Higher-Order Functions like fn:map and fn:fold-left are discussed in more depth on their own Wiki page.

Serialization

Serialization parameters can now be defined within XQuery expressions. Parameters are placed in the query prolog and need to be specified as option declarations, using the output prefix.

Example:

declare option output:omit-xml-declaration "no";
declare option output:method "xhtml";
<html/>

Result: <?xml version="1.0" encoding="UTF-8"?><html></html>

Context Item

The context item can now be specified in the prolog of an XQuery expressions:

Example:

declare context item :=
  <xml>
    <text>Hello</text>
    <text>World</text>
  </xml>;

for $t in .//text()
return string-length($t)

Result: 5 5

Functions

BaseX supports all functions that have been added in Version 3.0 of the XQuery Functions and Operators Working Draft. The new functions are listed below:

• math:pi(), math:sin(), and many others (see Math Module)
• fn:analyze-string()
• fn:available-environment-variables()
• fn:element-with-id()
• fn:environment-variable()
• fn:filter()
• fn:fold-left()
• fn:fold-right()
• fn:format-date()
• fn:format-dateTime()
• fn:format-integer()
• fn:format-number()
• fn:format-time()
• fn:function-arity()
• fn:function-lookup()
• fn:function-name()
• fn:generate-id()
• fn:has-children()
• fn:head()
• fn:innermost()
• fn:map()
• fn:map-pairs()
• fn:outermost()
• fn:parse-xml()
• fn:path()
• fn:serialize()
• fn:tail()
• fn:unparsed-text()
• fn:unparsed-text-available()
• fn:unparsed-text-lines()
• fn:uri-collection()

New signatures have been added for the following functions:

• fn:document-uri() with 0 arguments
• fn:string-join() with 1 argument
• fn:node-name() with 0 arguments
• fn:round() with 2 arguments
• fn:data() with 0 arguments