CS 253: Topics in Database Systems: C2

Dr. Alexandra I. Cristea

http://www.dcs.warwick.ac.uk/~acristea/

CS 253: Topics in Database Systems: C2

• Previously we looked at:– XML– XSL– XSLT

• Next:– XPath– XQuery

XPath

XPath• XPath is a syntax for defining parts of an XML

document • XPath uses path expressions to navigate in

XML documents • XPath contains a library of standard functions • XPath is a major element in XSLT • XPath is a W3C recommendation, thus a

Standard (16. November 1999 )

XPath Path Expressions

• Uses path expressions to select nodes or node-sets in an XML document. – These path expressions look very much

like the expressions you see when you work with a traditional computer file system.

XPath Standard Functions

• over 100 built-in functions. – string values, – numeric values, – date and time comparison, – node and QName manipulation, – sequence manipulation, – Boolean values, – and more.

XPath Terminology• Nodes

• Atomic values

• Items (atomic values or nodes)

• Relationships of nodes– Parent– Children– Siblings– Ancestors– Descendants

XPath Nodes• 7 kinds of nodes:

– element, – attribute, – text, – namespace, – processing-instruction, – comment, and – document (root) nodes.

• XML documents are treated as trees of nodes. The root of the tree is called the document node (or root node).

Nodes Examples<?xml version="1.0" encoding="ISO-8859-1"?> <bookstore> <book> <title lang="en">Harry Potter</title><author>J K. Rowling</author> <year>2005</year> <price>29.99</price> </book> </bookstore>

Document node

Element node

Attribute node

Atomic values Examples*<?xml version="1.0" encoding="ISO-8859-1"?> <bookstore> <book> <title lang="en">Harry Potter</title><author>J K. Rowling</author> <year>2005</year> <price>29.99</price> </book> </bookstore>

*nodes with no children or parent

Selecting nodes

Expression Description

nodename Selects all child nodes of the node

/ Selects from the root node

// Selects nodes in the document from the current node that match the selection no matter where they are

. Selects the current node

.. Selects the parent of the current node

@ Selects attributes

Examples of selecting nodesPath Expression Result

bookstore Selects all the child nodes of the bookstore element

/bookstore Selects the root element bookstoreNote: If the path starts with a slash ( / ) it always represents an absolute path to an element!

bookstore/book Selects all book elements that are children of bookstore

//book Selects all book elements no matter where they are in the document

bookstore//book Selects all book elements that are descendant of the bookstore element, no matter where they are under the bookstore element

//@lang Selects all attributes that are named lang

Predicates

• Predicates are used to find a specific node or a node that contains a specific value.

• Predicates are always embedded in square brackets.

Example predicates

Path Expression Result

/bookstore/book[1] Selects the first book element that is the child of the bookstore element

/bookstore/book[last()] Selects the last book element that is the child of the bookstore element

/bookstore/book[last()-1] Selects the last but one book element thatis the child of the bookstore element

/bookstore/book[position()<3]

Selects the first two book elements that are children of the bookstore element

Example predicates – cont. Path Expression Result

//title[@lang] Selects all the title elements that have an attribute named lang

//title[@lang='eng'] Selects all the title elements that have an attribute named lang with a value of 'eng'

/bookstore/book[price>35.00]

/bookstore/book[price>35.00]/title

Selects all the book elements of the bookstore element that have a price element with a value greater than 35.00

Selects all the title elements of the book elements of the bookstore element that have a price element with a value greater than 35.00

Selecting Unknown Nodes

Wildcard Description

* Matches any element node

@* Matches any attribute node

node() Matches any node of any kind

Example: selecting several paths

Path Expression Result

//book/title | //book/price Selects all the title AND price elements of all book elements

//title | //price

/bookstore/book/title | //price

Selects all the title AND price elements in the documentSelects all the title elements of the book element of the bookstore element AND all the price elements in the document

Location Path Expression• A location path can be absolute or

relative.

• An absolute location path: /step/step/... • A relative location path: step/step/...

• Location step:axisname::nodetest[predicate]

XPath Axesself

child parent

ancestor descendant

ancestor-or-self descendant-or-self

preceding-sibling following-sibling

preceding following

attribute

namespace

AxisName Result

ancestor Selects all ancestors (parent, grandparent, etc.) of the current node

ancestor-or-self Selects all ancestors (parent, grandparent, etc.) of the current node and the current node itself

attribute Selects all attributes of the current node

child Selects all children of the current node

descendant Selects all descendants (children, grandchildren, etc.) of the current node

descendant-or-self

Selects all descendants (children, grandchildren, etc.) of the current node and the current node itself

following Selects everything in the document after the closing tag of the current node

following-sibling

Selects all siblings after the current node

namespace Selects all namespace nodes of the current node

parent Selects the parent of the current node

preceding Selects everything in the document that is before the start tag ofthe current node

preceding-sibling

Selects all siblings before the current node

self Selects the current node

axisname::nodetest[predicate]

• //DDD/parent::*

<AAA>           <BBB>               <DDD>

               </DDD>           </BBB>

</AAA>

axisname::nodetest[predicate]

• //BBB/child::*

<AAA>           <BBB>               <DDD>

               </DDD>           </BBB>

</AAA>

Note: /AAA is equivalent to /child::AAA

More examples• http://www.zvon.org/xxl/XPathTutorial/Genera

l/examples.html– Check basics, //, *, predicates, attributes, functions

(new ones: count, name, normalize-space, starts-with, contains, string-length, floor, ceiling), axes, operators (mod)

– Note: The ancestor, descendant, following, preceding and self axes partition a document (ignoring attribute and namespace nodes): they do not overlap and together they contain all the nodes in the document. (see example)

XPath Conclusion• We have learned:

– XPath definition– Path expressions– Standard functions– Terminology– Predicates– Location paths– Axes– Some operators

• Before we go on, one more thing about XML:

• XML Namespaces

Naming ambiguity

The Idea to Solve it

• Assign a URI (~ URL) to every sub-language:– E.g., for XHTML 1.0:

http://www.w3.org/1999/xhtml

• Qualify element names with URIs:– {http://www.w3.org/1999/xhtml}head

The actual solution

• Namespace declarations bind URIs to prefixes:

• Default namespace (no prefix) declared with: xmlns=“…”

• Lexical Scope

• Attribute names can also be prefixed

Applying namespaces

• Next we look at how to query XML

• This can be done, to some extent, as we have seen, within XSLT,

• but the main language developed for this purpose is …

XQuery

What is XQuery?• XQuery is the language for querying XML data • XQuery for XML is like SQL for databases • XQuery is built on XPath expressions • XQuery is defined by the W3C • XQuery is supported by all the major database

engines (IBM, Oracle, Microsoft, etc.) • XQuery is a W3C recommendation (Jan 2007)

thus a standard

XQuery and XPath

• XQuery 1.0 and XPath 2.0 share the same data model and support the same functions and operators.

XQuery - Examples of Use

• Extract information to use in a Web Service

• Generate summary reports

• Transform XML data to XHTML

• Search Web documents for relevant information

Usage Scenario: Document-Oriented

• Queries could be used– To retrieve parts of documents– To provide dynamic indexes– To perform context-sensitive searching– To generate new documents as

combinations of existing ones

Usage Scenario: Programming

• Queries could be used to automatically generate documentation

Usage Scenario: Hybrid

• Queries could be used to data mine hybrid data, such as patient records

XQuery compared to XPath

• XQuery 1.0 is a strict superset of XPath 2.0 XPath 2.0 expression is directly an XQuery

1.0 expression (a query)

• The extra expressive power is the ability to:– Join information from different sources and– Generate new XML fragments

Relationship to XSLT

• XQuery, XSLT: both domain-specific languages for combining and transforming data from multiple sources

• different in design - historical reasons– XQuery: designed from scratch– XSLT: intellectual descendant of CSS

• technically, they may emulate each other

XQuery query

• Prolog– Like XPath, XQuery expressions are

evaluated relatively to a context– explicitly provided by a prolog (header)

~ header with definitions

• Body– The actual query

XQuery Prolog (i.e., header(s))

• Settings define various parameters for the XQuery processor language, such as:

xquery version “1.0”;module "http://www.w3.org/2003/05/xpath-functions"default element namespace=

"http://www.w3.org/1999/xhtml"declare namespace xs= "http://www.w3.org/2001/XMLSchema"import module "http://www.w3.org/2003/05/xpath-

functions" at "logo.xq"define function addLogo($root as node()) as node()* { }(: etc :)

XQuery capabilities

• Generate

• Join

• Select

Generate: constructors• XQuery expressions may compute new XML nodes

• Expressions may denote:– element, character data, comment and processing

instruction nodes node is created with a unique node identity

• Constructors may be either – direct or – computed

Direct constructors in XQuery

<XMLfragment>my fragment </XMLfragment>

• Evaluates to the given XML fragment

Explicit, computed constructors

Variable bindings

<employee empid="{$id}"> <name>{$name}</name>

{$job} <deptno>{$deptno}</deptno> <salary>{$SGMLspecialist+100000}</salary>

</employee>

How to Select Nodes with XQuery?

• Functions– XQuery uses functions to extract data from

XML documents.

• (X)Path Expressions– XQuery uses path expressions to navigate

through elements in an XML document.

• Predicates– XQuery uses predicates to limit the extracted

data from XML documents.

Functions

• doc() – function to open a file

• Example:– doc("books.xml")

• Note: A call to a function can appear where an expression may appear.

Path Expressions

• Example:select all the title elements in the "books.xml"

file:

doc("books.xml")/bookstore/book/title

Predicates

• Example:select all the book elements under the

bookstore element that have a price element with a value that is less than 30 :

doc("books.xml")/bookstore/book[price<30]

At a glance: function, path, predicate

FLWOR

• For, Let, Where, Order by, Return

= main engine

~ SQL syntax (SFHW)

~ programs and function calls

FLWOR by comparison with Path expressions

• select all the title elements under the book elements that are under the bookstore element that have a price element with

a value that is higher than 30.

• Path expression:doc("books.xml")/bookstore/book[price>30]/title

• FLWOR expression: for $x in doc("books.xml")/bookstore/book where $x/price>30 return $x/title

Sorting in FLWOR• for $x in doc("books.xml")/bookstore/book

where $x/price>30

order by $x/title

return $x/title

Present the Result In an HTML List

<ul>

{

for $x in doc("books.xml")/bookstore/book/title

order by $x

return <li>{$x}</li>

}

</ul>

Result HTML List

<ul> <li><title lang="en">Everyday

Italian</title></li> <li><title lang="en">Harry Potter</title></li> <li><title lang="en">Learning XML</title></li> <li><title lang="en">XQuery Kick

Start</title></li> </ul>

Eliminate element (here: title)

<ul>

{

for $x in doc("books.xml")/bookstore/book/title

order by $x

return <li>data($x)</li> (: also text() :)

}

</ul>

New result HTML List

<ul>

<li>Everyday Italian</li>

<li>Harry Potter</li>

<li>Learning XML</li>

<li>XQuery Kick Start</li>

</ul>

Another FLWOR Expression

The Difference between for and let

The Difference between for and let

The Difference between for and let

The Difference between for and let

FLWOR Basic Building Blocks

General rules

• for and let may be used many times in any order

• only one where is allowed

• many different sorting criteria can be specified (descending, ascending, etc.)

Joining documentsfor $p IN doc("www.irs.gov/taxpayers.xml")//person

for $n IN doc("neighbors.xml")//neighbor[ssn = $p/ssn]

return

<person>

<ssn> { $p/ssn } </ssn>

{ $n/name }

<income> { $p/income } </income>

</person>

ConditionalsFOR $b IN doc(“bib.xml”)/book

RETURN <short>   {$b/title}   <author>    {IF count($b/author) < 3      $b/author     ELSE      $b/author[1], <author>and others</author>    }   </author> </short>

Functions• DEFINE FUNCTION depth($e) RETURNS

xsd:integer{ IF (empty($e/*) THEN 1 ELSE max(FOR $c in $e/* RETURN depth($c)) ) +1}

• FOR $b in doc(“bib.xml”)/bookRETURN depth($b)

Existential and Universal Quantifiers

• FOR $b in doc(“bib.xml”)/bookWHERE SOME $author IN $b/author   SATISFIES $author/text() = “Ullman”RETURN $b

• FOR $b in doc(“bib.xml”)/bookWHERE EVERY $author IN $b/author           SATISFIES $author/text() = “Ullman”RETURN $b

Return books where all authors are “Ullman”

Return books where at least one author is “Ullman”

XQuery on Distributed Sources

XQuery Syntax• Declarative, functional language

~ SQL

• Nested expressions• Case sensitive• White spaces:

– Tabs, space, CR, LF– Ignored between language constructs– Significant in quoted strings

• No special EOL character

Keywords and names• Keywords and operators

– Case-sensitive, generally lower case– May have several meanings depending on the

context• E.g. “*” or “in”

– No reserved words

• All names must be valid XML names – For variables, functions, elements, attributes– Can be associated with a namespace

Comments

Comparisons• Value comparisons

Eq, ne, lt, le, gt, ge

Used to compare individual values

Each operand must be a single atomic value (or a node containing a single atomic value)

• General comparisons=, !=, <, <=, >, >=

Can be used with sequences of multiple items

Example

Query Prolog

XQuery gives you a choice:

• Path Expressions:– If you just want to copy certain elements

and attributes as is

• FLWOR Expressions:– Allow sorting– Allow adding elements/attributes– Verbose, but can be clearer

XQuery tools

• XStylus Studio 2007 http://www.stylusstudio.com/xml_download.html (free trial version)– See also short its XQuery intro at:

http://www.stylusstudio.com/xquery_primer.html

XML and programming

• XSLT, XPath and XQuery provide tools for specialized tasks.

• But many applications are not covered: – domain-specific tools for concrete XML

languages – general tools that nobody has thought of yet

XML in general-purpose programming languages

• parse XML documents into XML trees

• navigate through XML trees

• construct XML trees

• output XML trees as XML documents

• DOM and SAX are corresponding APIs that are language independent and supported by numerous languages. JDOM is an API that is tailored to Java.

XQuery Conclusion• We have learned:

– XQuery definition– Usage scenarios– Comparison w. XSLT and XPath– Capabilities– Functions, path expressions and predicates– FLWOR– Extensions for generic programming with XML