CSE 636 Data Integration XML Query Languages XPath

CSE 636 Data Integration XML Query Languages XPath

XPath • http: //www. w 3. org/TR/xpath (11/99) • Building block for other W 3 C standards: – – • XSL Transformations (XSLT) XML Link (XLink) XML Pointer (XPointer) XQuery Was originally part of XSL 2

Example for XPath Queries <bib> <book> <publisher> Addison-Wesley </publisher> <author> Serge Abiteboul </author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author> <author> Victor Vianu </author> <title> Foundations of Databases </title> <year> 1995 </year> </book> <book price=“ 55”> <publisher> Freeman </publisher> <author> Jeffrey D. Ullman </author> <title> Principles of Database and Knowledge Base Systems </title> <year> 1998 </year> </book> </bib> 3

Data Model for XPath / Document XML PI The root Element bib Comment Element book The root element Element book Element publisher Element author Text Addison-Wesley Text Serge Abiteboul … … 4

XPath: Simple Expressions /bib/book/year Result: <year> 1995 </year> <year> 1998 </year> /bib/paper/year Result: empty (there were no papers) 5

XPath: Restricted Kleene Closure //author Result: <author> Serge Abiteboul </author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author> <author> Victor Vianu </author> <author> Jeffrey D. Ullman </author> /bib//first-name Result: <first-name> Rick </first-name> 6

XPath: Functions /bib/book/author/text() Result: Serge Abiteboul Jeffrey D. Ullman Rick Hull doesn’t appear because he has firstname, lastname Functions in XPath: – text() = matches the text value – node() = matches any node (= * or @* or text()) – name()= returns the name of the current tag 7

XPath: Wildcard //author/* Result: <first-name> Rick </first-name> <last-name> Hull </last-name> * Matches any element 8

XPath: Attribute Nodes /bib/book/@price Result: “ 55” @price means that price is has to be an attribute 9

XPath: Qualifiers /bib/book/author[first-name] Result: <author> <first-name> Rick </first-name> <last-name> Hull </last-name> </author> 10

XPath: More Qualifiers /bib/book/author[firstname][address[//zip][city]]/lastname Result: <lastname> … </lastname> 11

XPath: More Qualifiers /bib/book[@price < “ 60”] /bib/book[author/@age < “ 25”] /bib/book[author/text()] 12

XPath: Summary bib matches a bib element * matches any element / matches the root element /bib matches a bib element under root bib/paper matches a paper in bib//paper matches a paper in bib, at any depth //paper matches a paper at any depth paper|book matches a paper or a book @price matches a price attribute bib/book/@price matches price attribute in book, in bib/book[@price<“ 55”]/author/lastname matches… 13

XPath: More Details • An XPath expression, p, establishes a relation between: – A context node, and – A node in the answer set • In other words, p denotes a function: – S[p] : Nodes {Nodes} • Examples: – – author/firstname. = self. . = parent part/*/*/subpart/. . /name = part/*/*[subpart]/name 14

The Root and the Root <bib> <paper> 1 </paper> <paper> 2 </paper> </bib> • • bib is the “document element ” The “root” is above bib • • /bib = returns the document element / = returns the root • • • Why? Because we may have comments before and after <bib> They become siblings of <bib> 15

XPath: More Details • We can navigate along 13 axes: ancestor-or-self parent attribute We’ve only seen these, so far child descendant-or-self descendant following-sibling namespace preceding-sibling self 16

XPath: More Details • Examples: – child: : author/child: lastname = author/lastname – child: : author/descendant-or-self: : node()/child: : zip = author//zip – child: : author/parent: : * = author/. . – child: : author/attribute: : age = author/@age • What does this mean ? – /bib/book/publisher/parent: : */author – /bib//address[ancestor: : book] – /bib//author/ancestor: : *//zip 17

XPath: Even More Details • name() = the name of the current node /bib//*[name()=book] same as /bib//book • What does this mean? /bib//*[ancestor: : *[name()!=book]] Is it equivalent to the following? /bib//*[name()!=book]//* • Navigation axis gives us strictly more power! 18

XPath: Example How do we evaluate this XPath expression? /bib//*[name()!=book]//* Let’s take it one step at a time bib A B book C D 19

XPath: Example /bib returns the following list of one node: Node bib A B book C D 20

XPath: Example /bib//* when executed on the previous node list, returns the following new list of nodes: Node A B book C D C D 21

XPath: Example /bib//*[name()!=book] when executed on the previous node list, it eliminates one node: Node A B book C D 22

XPath: Example /bib//*[name()!=book]//* gives us the resulting node list of the XPath expression: Node book C D 23

Keys in XML Schema • We forgot something about XML Schema – Keys – Key References • Why? • XPath is used for keys and key references 24

Keys in XML Schema XML: <purchase. Report> <regions> <zip code="95819"> <part number="872 -AA" quantity="1"/> <part number="926 -AA" quantity="1"/> <part number="833 -AA" quantity="1"/> <part number="455 -BX" quantity="1"/> </zip> <zip code="63143"> <part number="455 -BX" quantity="4"/> </zip> </regions> <part number="872 -AA">Lawnmower</part> <part number="926 -AA">Baby Monitor</part> <part number="833 -AA">Lapis Necklace</part> <part number="455 -BX">Sturdy Shelves</part> </parts> </purchase. Report> XML Schema: <key name="Num. Key"> <selector xpath="parts/part"/> <field xpath="@number"/> </key> 25

Keys in XML Schema: <xs: element name="purchase. Report"> <xs: complex. Type> <xs: sequence> <xs: element name="regions"> … </xs: element> <xs: element name="parts"> … </xs: element> </xs: sequence> </xs: complex. Type> <xs: key name="num. Key"> <xs: selector xpath="parts/part" /> <xs: field xpath="@number" /> </xs: key> <keyref name="num. Key. Ref" refer="num. Key"> <selector xpath="regions/zip/part" /> <field xpath="@number" /> </keyref> </xs: element> 26

Keys in XML Schema • In general, two flavors: <key name=“some. Name. Here"> <selector xpath=“p"/> <field xpath=“p 1"/> <field xpath=“p 2"/> … <field xpath=“pk"/> </key> <unique name=“some. Name. Here"> <selector xpath=“p"/> <field xpath=“p 1"/> <field xpath=“p 2"/> … <field xpath=“pk"/> </key> Note • All XPath expressions “start” at the element currently being defined • The fields must identify a single node 27

Keys in XML Schema • Unique = guarantees uniqueness • Key = guarantees uniqueness and existence • All XPath expressions are “restricted”: – /a/b | /a/c – //a/b/*/c OK for selector OK for field • Note: better than DTD’s ID mechanism 28

Keys in XML Schema • Examples Recall: must have a single forename, surname <key name="full. Name"> <selector xpath=". //person"/> <field xpath="forename"/> <field xpath="surname"/> </key> <unique name="nearly. ID"> <selector xpath=". //*"/> <field xpath="@id"/> </unique> 29

Foreign Keys in XML Schema • Examples <keyref name="person. Ref" refer="full. Name"> <selector xpath=". //person. Pointer"/> <field xpath="@first"/> <field xpath="@last"/> </keyref> 30

References • Lecture Slides – Dan Suciu – http: //www. cs. washington. edu/homes/suciu/COURSES/590 DS/0 6 xpath. htm – http: //www. cs. washington. edu/homes/suciu/COURSES/590 DS/1 4 constraintkeys. htm • BRICS XML Tutorial – A. Moeller, M. Schwartzbach – http: //www. brics. dk/~amoeller/XML/index. html • W 3 C's XPath homepage – http: //www. w 3. org/TR/xpath • W 3 C's XML Schema homepage – http: //www. w 3. org/XML/Schema • XML School – http: //www. w 3 schools. com 31