XQuery 3.0

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This article is part of the XQuery Portal. It summarizes the most interesting features of the XQuery 3.0 Recommendations. XQuery 3.0 if fully supported by BaseX.

Contents

[edit] Enhanced FLWOR Expressions

Most clauses of FLWOR expressions can now be specified in an arbitrary order: additional let and for clauses can be put after a where clause, and multiple where, order by and group by statements can be used. This means that many nested loops can now be rewritten to a single FLWOR expression.

Example:

for $country in db:open('factbook')//country
where $country/@population > 100000000
let $name := $country/name[1]
for $city in $country//city[population > 1000000]
group by $name
return <country name='{ $name }'>{ $city/name }</country>

A new count clause enhances the FLWOR expression with a variable that enumerates the iterated tuples.

for $n in (1 to 10)[. mod 2 = 1]
count $c
return <number count="{ $c }" number="{ $n }"/>

The allowing empty provides functionality similar to outer joins in SQL:

for $n allowing empty in ()
return 'empty? ' || empty($n)

Window clauses provide a rich set of variable declarations to process sub-sequences of iterated tuples. An example:

for tumbling window $w in (2, 4, 6, 8, 10, 12, 14)
    start at $s when fn:true()
    only end at $e when $e - $s eq 2
return <window>{ $w }</window>

More information on window clauses, and all other enhancements, can be found in the specification.

[edit] Simple Map Operator

The simple map operator ! provides a compact notation for applying the results of a first to a second expression: the resulting items of the first expression are bound to the context item one by one, and the second expression is evaluated for each item. The map operator may be used as replacement for FLWOR expressions:

Example:

(: Simple map notation :)
(1 to 10) ! element node { . },
(: FLWOR notation :)
for $i in 1 to 10
return element node { $i }

A map operator is defined to be part of a path expression, which may now be mixed of path and map operators. In contrast to the map operator, the results of the map operator will not be made duplicate-free and returned in document order.

[edit] 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>

[edit] Try/Catch

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

Example:

try {
  1 + '2'
} catch err:XPTY0004 {
  'Typing error: ' || $err:description
} catch * {
  'Error [' || $err:code || ']: ' || $err:description
}

Result: Typing error: '+' operator: number expected, xs:string found.

Within the scope of the catch clause, a number of variables are implicitly declared, giving information about the error that occurred:

[edit] 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

[edit] 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 write more modular code.

Examples:

Function items can be obtained in three different ways:

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.

[edit] Expanded QNames

A QName can now be directly prefixed with the letter "Q" and a namespace URI in the Clark Notation.

Examples:

The syntax differed in older versions of the XQuery 3.0 specification, in which the prefixed namespace URI was quoted:

[edit] Namespace Constructors

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

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

[edit] String Concatenations

Two vertical bars || (also names pipe characters) can be used to concatenate strings. This operator is a shortcut for the fn:concat() function.

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

[edit] External Variables

Default values can now be attached to external variable declarations. This way, an expression can also be evaluated if its external variables have not been bound to a new value.

 
declare variable $user external := "admin";
"User:", $user

[edit] 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 namespace output = "http://www.w3.org/2010/xslt-xquery-serialization";
declare option output:omit-xml-declaration "no";
declare option output:method "xhtml";
<html/>

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

In BaseX, the output prefix is statically bound and can thus be omitted. Note that all namespaces need to be specified when using external APIs, such as XQJ.

[edit] Context Item

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

Example:

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

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

Result: 5 5

[edit] Annotations

XQuery 3.0 introduces annotations to declare properties associated with functions and variables. For instance, a function may be declared %public, %private, or %updating.

Example:

declare %private function local:max($x1, $x2) {
  if($x1 > $x2) then $x1 else $x2
};

local:max(2, 3)

[edit] 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:

New signatures have been added for the following functions:

[edit] Changelog

Version 7.7
Version 7.3
Version 7.2
Version 7.1
Version 7.0
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