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: – –Galax (complete reference impl. of XQuery) (w3c.org – major resource many XML and other web related stuff, incl. XQuery, semantic web, etc.)

Example XML DB What is the name of this book? Raymond Smullyan Penguin 1970 Bentam Books New York Douglas R Hofstadter D.C. Dennett The Mind’s I: Reflections on Self and Soul 1981

Corresponding Tree root bib comments processing instructions root doc. element book titleauthorpublisher price ooo attribute element unordered usually, single valued. Exception: IDREFS. ordered. no apriori car- dinality constraint. note distinction between the two roots.

Simple Examples /bib/book/publisher answer: Penguin Bentam Books New York /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: New York /bib/book//mi answer: R //title answer: What is the name of this book? The Mind’s I: Reflections on Self and Soul Note: results ordered as per i/p document order.

Wildcard //author/* answer: Raymond Smullyan Douglas R Hofstadter 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,... answer: ``15” contrast with answer for previous queries. what do you think it should return?

Branching & Qualifiers/Predicates /bib/book/author[mi] returns only second book. 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 $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 – matches any hobby attribute. -- matches any price attribute of an author at any depth of the doc. – 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  child::book/child::author/parent::*/child::y ear  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 authortitlepublisheryear firstmilastnameaddress 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.