CS 253: Topics in Database Systems: XPath, NameSpaces

Dr. Alexandra I. Cristea

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

CS 253: Topics in Database Systems: XPath, NameSpaces

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• Previously we looked at:– XML

• Next:– XPath– Namespaces

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XPath

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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 )

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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.

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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.

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XPath Terminology• Nodes

• Atomic values

• Items (atomic values or nodes)

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

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XPath Nodes• 7 kinds of nodes:

1. element,

2. attribute,

3. text,

4. namespace,

5. processing-instruction,

6. comment, and

7. document (root) nodes. • XML documents are treated as trees of nodes. The root

of the tree is called the document node (or root node).

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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 (root) node

Element node

Attribute node

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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

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Selecting nodes

Expression Description

nodename Selects all child nodes with this name

/ Selects from the root node

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

. Selects the current node

.. Selects the parent of the current node

@ Selects attributes

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Examples of selecting nodesPath Expression Result

bookstore Selects all the bookstore elements

/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

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Predicates

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

• Predicates are always embedded in square brackets.

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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

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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

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Selecting Unknown Nodes

Wildcard Description

* Matches any element node

@* Matches any attribute node

node() Matches any node of any kind

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Example: selecting several paths

Path Expression Result

//book/title | //book/price Selects all the title as well as price elements of all book elements

//title | //price

/bookstore/book/title | //price

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

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XPath Axesself

child parent

ancestor descendant

ancestor-or-self descendant-or-self

preceding-sibling following-sibling

preceding following

attribute

namespace

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axisname::nodetest[predicate]

• //DDD/parent::*

<AAA>           <BBB>               <DDD>

               </DDD>           </BBB>

</AAA>

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axisname::nodetest[predicate]

• //BBB/child::*

<AAA>           <BBB>               <DDD>

               </DDD>           </BBB>

</AAA>

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

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

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XPath Conclusion• We have learned:

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

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

• XML Namespaces

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Naming ambiguity

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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

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

Web Naming and Addressing Overview (URIs, URLs, ...)

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The actual solution

• Namespace declarations bind URIs to prefixes:

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

• Lexical Scope

• Attribute names can also be prefixed

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Applying namespaces

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• 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 …