1. XML path expressions
CSE 350 Fall 2003

2. Xpath: XML Path language
(11/99)
Building block for other W3C standards:
XSL Transformations (XSLT)
XML Link (XLink)
XML Pointer (XPointer)
XML Query
Input and output are XML

3. Example for XPath Queries
<bib> <book> <publisher> Addison-Wesley </publisher> <author> Serge Abiteboul </author> <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>

4. XPath: Simple Expressions
Example 1: /bib/book/year
Result: <year> 1995 </year>
<year> 1998 </year>
Example 2: /bib/paper/year
Result: empty (there were no papers)
Given an OEM instance, the value of a path expression p is a set of objects

5. XPath: Restricted Kleene Closure
Example 3: //author
Result:<author> Serge Abiteboul </author>
<author> <first-name> Rick </first-name>
<last-name> Hull </last-name>
</author>
<author> Victor Vianu </author>
<author> Jeffrey D. Ullman </author>
Example 4: /bib//first-name
Result: <first-name> Rick </first-name>

6. Xpath Functions Functions in XPath: Example 5: /bib/book/author/text()
text() = matches the text value
node() = matches any node (= * or text())
name() = returns the name of the current tag
Example 5: /bib/book/author/text()
Result: Serge Abiteboul
Jeffrey D. Ullman
(Rick Hull doesn’t appear because he has firstname, lastname)

7. More Xpath syntax Wildcard: Attribute:
* Matches any element
Example 6: //author/*
Result: <first-name> Rick </first-name>
<last-name> Hull </last-name>
Attribute:
@price means that price is has to be an attribute
Example 7:
Result: “55”

8. Xpath: Qualifiers More examples (9,10,11): /bib/book[@price < “60”]
Example 8: /bib/book/author[firstname]
Result: <author> <first-name> Rick </first-name>
<last-name> Hull </last-name>
</author>
More examples (9,10,11):
< “60”]
< “25”]
/bib/book[author/text()]

9. Path Expressions Example
&96
&243
&206
&25
“Serge”
“Abiteboul”
1997
“Victor”
“Vianu”
122
133
paper
book
references
author
title
year
http
publisher
page
firstname
lastname
first
last
Bib
&o44
&o45
&o46
&o47
&o48
&o49
&o50
&o51
&o52
Example:
DB =
Bib.paper={&o12,&o29}
Bib.book.publisher= ?
Bib.paper.author.lastname= ?

10. Regular Path Expressions
R ::= label | _ | R.R | (R|R) | R* | R+ | R?
Examples:
Bib.(paper|book).author
Bib.book.author.lastname?
Bib.book.(references)*.author
Bib.(_)*.zip
Answer is:
U{eval(root(DB), P) | P  lang(R)} where lang(R) = the set of strings generated by R

11. 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
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
matches price attribute in book, in bib
matches…

12. Definition Intuition:
find all objects x in DB s.t. the path from DB root to object x matches the path expression P
Path expression P over DB evaluates to eval(root(DB), P)
eval(x,e) = {x}
eval(x,L.P) = U {eval(y,P) | (x, L, y) edges(DB)}

13. Applications of Path Expressions
Navigating uncertain structure:
Bib.book.author.lastname?
Syntactic substitute for inheritance:
Bib.(paper|book).author
Bib.publication.author
better, but we don’t have inheritance
Computing transitive closure:
Bib.(_)*.zip
all accessible zipcodes
Bib.book.(references)*.author
all authors accessible via book references