XPath

Laks V.S. Lakshmanan

UBC

CPSC 534B

Overview

• data model recap

• XPath examples

• some advanced features

• summary

XPath in the beginning • used to be part of XSLT

– A formal semantics of patterns in XSLT (Phil Wadler)

• also influenced XLink, XPointer, • resources:

– www.w3c.org/TR/xpath

– Galax (complete reference impl. of XQuery) http://db.bell-labs.com/galax/

• (w3c.org – major resource many XML and other web related stuff, incl. XQuery, semantic web, etc.)

Example XML DB <bib> <book price=15> <title> What is the name of this book?</title> <author nationality=american> <first> Raymond</first> <last>Smullyan</last> </author> <publisher>Penguin</publisher> <year>1970</year> </book> <book><publisher><name>Bentam Books</name><address>New

York</address></publisher> <author><first>Douglas</first><mi>R</mi><last>Hofstadter</last></author> <author>D.C. Dennett</author> <title>The Mind’s I: Reflections on Self and Soul</title> <year>1981</year> </book> </bib>

Corresponding Tree root

bib

comments

processing instructions

root doc. element

book book

title author publisher

priceooo ooo

attribute

element

unorderedusually, single valued. Exception: IDREFS.

ordered. no apriori car-dinality constraint.

note distinction between the two roots.

Simple Examples

• /bib/book/publisher • answer: <publisher>Penguin</publisher>

<publisher><name>Bentam Books</name>

<address>New York</address></publisher>

• /bib/book/author/name

• what’s the answer?

• / -- returns root element, while

• /bib -- returns doc. root element, i.e., the bib element under the root.

Descendants• /bib/book//address• answer: <address>New York</address></publisher>

• /bib/book//mi• answer: <mi>R</mi>

• //title • answer: <title> What is the name of this book?</title>

<title>The Mind’s I: Reflections on Self and Soul</title>

Note: results ordered as per i/p document order.

Wildcard

• //author/*• answer: <first> Raymond</first> <last>Smullyan</last>

<first>Douglas</first><mi>R</mi><last>Hofstadter</last>

why only two authors(’ info.) returned?

Note: * matches any element.

• what does //* return?

• is the answer identical to that for /bib?

Attributes

• XML data model – diff. kinds of nodes: element, attribute, text, comment, processing instruction, ...

• /bib/book/@price

• answer: ``15”

contrast with answer for previous queries.

• /bib/book/@*

what do you think it should return?

Branching & Qualifiers/Predicates

• /bib/book/author[mi]

• returns only second book.

• /bib/book[author/@nationality=american]

• returns only first book. • /bib/book[publisher[address][name]][price<20]//title

• returns the titles of books with a publisher who has a name & an address and with a price < 20.

Reaching out at other nodes • XPath has the functions text(), node(),

name(). Meanings illustrated below. • /bib/book/publisher/text() • answer: Penguin

– why first pub doesn’t appear?

• /bib/book/node() • returns all child nodes of book, regardless

of type (attr, text, element). • /bib/*/name() – returns tag of current

element.

Mixing it all

• /bib/book[author[hobby=tennis]][title/text()]//year

• what does it say?

• Features of XPath seen so far tree pattern query.

$x

$y

$w

$z

$z$x.tag=bib & $y.tag=comment &$z.tag=publisher ...

distinguished node

XPath – Summary • / -- matches the root.

• /bib – matches bib element under root.

• bib – matches any bib element.

• * -- matches any element.

• bib/book – matches any book element under a bib element.

• bib//book – ditto, but at any depth.

• //book – matches any book element at any depth in the document.

• author|editor – matches any author or editor element.

• @hobby – matches any hobby attribute.

• //author/@hobby -- matches any price attribute of an author at any depth of the doc.

• /bib/book[author[@hobby]][@price<20]//publisher – what does it match?

XPath – The 13 axes • child • descendant • attribute • descendant-or-self

• following

• following-sibling

• ancestor

• ancestor-or-self

• parent

• preceding

• preceding-sibling

• self

• namespace

Some Abbreviations• child::book/child::author book/author• child::book/descendant::mi book//mi • child::first/parent::* first/..• child::book/attribute::price book/@price• child::book/child::author/parent::*/

child::year book[author]/year• /bib//mi[ancestor::book] ?• /bib//mi/ancestor::book//publisher ?• /bib//mi/ancestor::*//publisher ?

More examples

• /bib/descendant::*[name()=address] /bib//address

• /bib//book//first/parent::*[name()=author]

• /bib//book//mi[ancestor:*[name()=author or name()=editor]]

• navigation axes increase expressive power

• BUT, when schema is known, can often simplify XPath expressions

Simplifying XPE with schema

• /bib//book//first/parent::*[name()=author] /bib//book//author[first]

• /bib//book//mi[ancestor:*[name()=author or name()=editor] S /bib//book//*[name()=author or name()]//mi

bib

book

author title publisher year

first mi last name address

example schema graph S

editor

*

*+ 1 ?

?

1 ??1 1

XPath, formally speaking • XPE a binary relation over document

nodes: p(context node, answer node). • basic cases:

– “.” is, i.e., self (x,x)– “..” is parent, i.e., (current node, its parent) – publisher/address is (current node, address

node reachable from current node via publisher child)

– book/*/mi/../name() book/*[mi]/name() what is the relationship captured by this XPE?

XPath, formally speaking • Relative path expressions – every XPE E we

have seen so far, except E may be used as a predicate or as an extension to an absolute XPE.

• Absolute XPE – how you get a (unary) query out of an XPE.

• E.g.: author/mi and publisher/address are relative XPEs.

//author/mi, /bib/book[author/mi], /bib/book[author/mi][publisher/address]//year are all absolute XPEs.

• More details: see resources and stay tuned for homework.