XQuery and XML Querying: A University Course by Farnoush Banaei-Kashani - Spring 2008, Study notes of Deductive Database Systems

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USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Querying XML

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

XQuery References

http://www.w3.org/TR/xquery/XQuery 1.0: An XML Query Language

http://www.w3.org/TR/xmlquery-use-casesXML Query Use Cases

http://www.gnu.org/software/qexo/Qexo: The GNU Kawa implementation of XQuery

XQuery Tutorial by M

ø

ller & Schwartzbach

http://www.brics.dk/~amoeller/XML/querying/

tutorial/xquery-tutorial.pdfhttp://homepages.inf.ed.ac.uk/wadler/papers/xquery-Xquery Tutorial by Fankhauser & Wadler

http://www.galaxquery.org/Galax: an open-source XQuery implementation

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Why a new query language?

Semi-structured: XML data is not rigidly structured

Self-describing: schema exists with data

Can naturally model irregularities

Missing elements (e.g., bestseller?)

Multiple occurrences of the same element (reviews*)

values in othersElements w/ atomic values in some data items and structured

Collections of elements with heterogeneous structure

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

XQuery 1.

XPath, XQL, XML-QL, Lorel, YATL, SQL, and OQL.Based on XML query language Quilt, with borrowed features from

same expressions will generate the same results.XQuery 1.0 relies on XPath 2.0 and XML Schema datatypes. The

Basic definitions: 

It is a functional language and strongly typed

Basic building block:

(^) expression

The value of an expression is always a

(^) sequence

: a collection of zero or

more

(^) items

An item is either an

(^) atomic value

(^) or a

(^) node

A node conforms to one of

(^) 7 node types:

(root) nodeElement, attribute, namespace, text, comment, processing-instruction, and document

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Example XML Document

A fine book.SuciuBunemanAbiteboul

The best ever!Buneman

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

XQuery Examples

Titles of all books published before 2000:

/BOOKS/BOOK[@YEAR < 2000]/TITLE

Year and title of all books published before 2000:

return { $book/@YEAR, $book/TITLE }where $book/@YEAR < 2000for $book in /BOOKS/BOOK

Books grouped by author:

for $author in distinct(/BOOKS/BOOK/AUTHOR) return { /BOOKS/BOOK[AUTHOR = $author]/TITLE

}

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Path Expressions

The simplest kind of query is just an

XPath 2.

doc("recipes.xml")//recipe[title="Ricotta Pie"]//ingredient[@amount] expression. A simple path expression looks like:

doc("recipes.xml") The initial context for the path expression is given by

// means any child node (not just the immediate child node)

recipe collectionThe result is all simple ingredients used to prepare Ricotta Pie in the

(^) (http://www.brics.dk/~amoeller/XML/xml/recipes.xml)

ingredient The result is given as a list of XML fragments, each rooted with an

(^) element

The

(^) order

(^) of the fragments respects the document order (order matters!

• as opposed to SQL)

Only return ingredients with

amount

attribute

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

FLWOR Expressions

The

main engine

of XQuery is the FLWOR expression:

F

or-

L

et-

W

here-

O

rder-

R

eturn

Pronounced "flower"

returnorder by avg($e/salary) descendingwhere count($e) >= 10let $e := doc("emps.xml")//employee[deptno = $d] for $d in doc("depts.xml")//deptno example:Generalizes SELECT-FROM-HAVING-WHERE from SQL;

<big-dept> { $d, {avg($e/salary)}{count($e)},

</big-dept>

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

FLWOR Expressions

(cont

d)

Note the difference between

for

and

let

for $x in /company/employee

Generates a list of bindings of

$x

to each

employee

element in

the

company

, but:

let $x := /company/employee

Generates a single binding of

$x

to the list of

employee

elements

in the

company

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Projection

Return all authors of all books:

/BOOKS/BOOK/AUTHOR

BunemanSuciu,Buneman,Abiteboul,Returns:

The same query can also be written as a for loop:

for $dot1 in $root/BOOKS return $dot1/BOOK/AUTHOR

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Selection

(cont

d)

Return book with title

Data on the Web

/BOOKS/BOOK[TITLE = "Data on the Web"]

Returns: A fine book.SuciuBunemanAbiteboul

Return the review of the book with the title

Data on the

Web

/BOOKS/BOOK[TITLE = "Data on the Web"]/REVIEW

A fine book.Returns:

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Construction

Return year and title of all books published before 2000

returnwhere $book/@YEAR < 2000for $book in /BOOKS/BOOK { $book/@YEAR, $book/TITLE }

Returns:

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

Join

for $am in $amazon/BOOKS/BOOK,$fatbrain := doc("http://www.fatbrain.com/books.xml")let $amazon := doc("http://www.amazon.com/books.xml"),Fatbrain:Return the books that cost more at Amazon than at

$fat in $fatbrain/BOOKS/BOOK

where $am/ISBN = $fat/ISBN and $am/PRICE > $fat/PRICE

return { $am/TITLE, $am/PRICE, $fat/PRICE }

$p/income } return { $p/ssn } { $n/name } {for $n in doc("neighbors.xml")//neighbor[ssn = $p/ssn]for $p in doc("www.irs.gov/taxpayers.xml")//personReturn the name and income of your neighbors:

USC - CSCI585 - Spring 2008 - Farnoush Banaei-Kashani

^ Other XML Query Languages

XML-QL

Lore (Lightweight Object Repository) & Lorel

XSL, easy to express recursive processing:

All

(^) author

elements, regardless of how deep they occur in the data:

<xsl:template>

<xsl:apply-templates/>

</xsl:template>

<xsl:template

match="author">

<xsl:value-of/>

</xsl:template>

constructing resultsXQL: XSL match patterns+some concise syntax for

with a GUIXML-GL: similar in expressiveness power to XML-QL but

WebL: markup algebra + service combinators