XQuery XML Example Mandatory statement Root element XML elements - - PDF document

XQuery

CMPT 354: Database I -- XQuery 2

XML Example

Root element Mandatory statement XML elements Element names Element content

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Hierarchical Structure

PersonList Student Title Contents Person Person Name: John Doe Id: 111111111 Address Number: 123 Street: Main St Name: Joe Public Id: 666666666 Address Number: 666 Street: Hollow Rd

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Querying XML Documents

• XPath

– Simple and efficient – Based on path expression

• XSLT

– A full-blown programming language with powerful query capabilities

• XQuery

– SQL-style query language – Has the most powerful and elegant query capabilities

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XPath Data Model

An attribute is not a child of its parent node That is, if A is an attribute of P, then P is a parent of A, but A is not a child of P

e-children: subelements a-children: attributes t-children: text in an element

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XPath

• An XPath expression takes a document tree and returns a

set of nodes in the tree

• Basic syntax – the UNIX file naming schema

– / – the root node – . – the current node – .. – the parent node – Absolute path expression – Relative path expression

/Students/Student/CrsTaken Suppose the current node is Name ./First and First return the same results

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Accessing Attributes and Text

• Attributes: use symbol @
• Text: use text()
• Comment: use comment()

/Students/Student/CrsTaken/@CrsCode returns {CS308, MAT123} /Students/Student/Name/First/text() returns {John, Bart} /comment()

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Advanced Navigation in XPath

• Select specific nodes

– /Students/Student[1]/CrsTaken[2]: the second course taken by the first student – /Students/Student/CrsTaken[last()]: the last course taken by the last student (not all students)

• Wildcard

– //: descendant or self

• //CrsTaken: select all CrsTaken elements in the tree
• .//CrsTaken: select all descendants of the current node to find the

CrsTaken elements

– *: collect all e-children of a node irrespective of type

• Student/*: select all e-children of the Student children of the current

node

• /*//*/: select all e-grandchildren of the root and their e-descendants

– @*: select all attributes

• CrsTaken/@*: select all attributes of the CrsTaken nodes that sit below

the current node

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XPath Queries (1)

• Select all student nodes where the student has taken a

course in Fall 1994

– //Student[CrsTaken/@Semester=“F1994”] – […]: selection condition

• Select elements based on the contents of an element

rather than of an attribute

– //Student[Status=“Undergraduate” and starts-with(.//Last, “P”) and not(.//Last=.//First)]

• Search for students who have van as part of their name

– //Student[contains(concat(Name//text()), “van”)]

• Select the students who have taken at least five courses

– //Student[count(CrsTaken) >= 5]

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XPath Queries (2)

• Select all the CrsTaken elements in the document,

that occur in Student elements with Status U4 and whose CrsCode attribute has the value CS305

– //Student[Status=“U4”]/CrsTaken[@CrsCode=“CS305”]

• Select all Student elements such that the student

took MAT123 in fall 1994

– //Student[CrsTaken/@CrsCode=“MAT123”][CrsTaken/S emester=“F1994”] – //Student[CrsTaken/@CrsCode=“MAT123” and CrsTaken/@Semester=“F1994”]

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XPath Queries (3)

• Suppose Grade is an optional attribute of
• CrsTaken. Select all students who have a

CrsTaken element with an explicitly specified Grade attribute (regardless of its value)

– //Student[CrsTaken/@Grade]

• A union of elements of different types: the

CrsTaken elements pertain to the fall 1994 semester, and the Class elements that describe fall 1994 course offerings

– //CrsTaken[@Semester=“F1994”] | //Class[Semester=“F1994”]

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XPointer – A Smarter URL

• Allow the user to concatenate a URL and a

path expression

• URL#xpointer(XPathExpr1)xpointer(XPathE

xpr2)…

– The document at URL is found – XPathExpr1 is evaluated against the document

• If a nonempty set of document tree nodes is

returned, done

– XPathExpr2 is tried, and so on

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XPointer – Example

http://www.foo.edu/report.xml#xpointer(//Student[2]) http://www.foo.edu/Report.xml# xpointer(//Student[CrsTaken/@CrsCode=“MAT123” and CrsTaken/@Semester=“F1994”])

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XQuery

• Integration of the best features of XQL and

XML-QL

– XQL: an extension of XPath – XML-QL: an SQL-style query language

• XQuery uses XPath as a syntax for its path

expressions

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Selection

FOR variable declarations WHERE condition RETURN result

• FOR clause plays the same role as the

FROM clause in SQL

• WHERE clause is borrowed from SQL with

the same functionality

• RETURN clause is analogous to SELECT

and specifies the templates for the result document

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Example Document

Suppose the document is stored at http://xyz.edu/transcripts.xml

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A Simple Query

• Find all students who have ever taken MAT123

FOR $t IN document(“http://xyz.edu/transcripts.xml”)//Transcript WHERE $t/CrsTaken/@CrsCode=“MAT123” RETURN $t/Student – $t ranges over all Transcript nodes in the document – Output: <Student StudId=“111111111” Name=“John Doe”/> <Student StudId=“123454321” Name=“Joe Blow”/>

• Yield a well-formed XML document by query

<StudentList> ( FOR $t IN document(“http://xyz.edu/transcripts.xml”)//Transcript WHERE $t/CrsTaken/@CrsCode=“MAT123” RETURN $t/Student ) </StudentList>

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Reconstructing Using XQuery

• Create a class roster

<CrsTaken CrsCode=“CS305” Semester=“F1995” Grade=“A”/> <CrsTaken CrsCode=“CS305” Semester=“F1995” Grade=“C”/>

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Joining Two Documents

http://xyz.edu/classes.xml http://xyz.edu/transcrips.xml

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Outer Join

Another way to generate a class roster

FOR $c IN document(“http://xyz.edu/classes.xml)//Class RETURN <ClassRoster CrsCode=$c/@CrsCode Semester=$c/@Semester> $c/CrsName $c/Instructor ( FOR $t IN document(“http://xyz.edu/transcripts.xml”)//Transcript WHERE $t/CrsTaken/@CrsCode=$c/@CrsCode AND $t/CrsTaken/@Semester=$c/@Semester RETURN $t/Student SORTBY($t/Student/@StudId) ) </ClassRoster> SORTBY($c/@CrsCode)

An empty class may also be listed!

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Equijoin

FOR $c IN document(“http://xyz.edu/classes.xml)//Class WHERE document(“http://xyz.edu/transcripts.xml”)//CrsTaken[@CrsCode=$c/@CrsCode and @Semester=$c/@Semester RETURN <ClassRoster CrsCode=$c/@CrsCode Semester=$c/@Semester> $c/CrsName $c/Instructor ( FOR $t IN document(“http://xyz.edu/transcripts.xml”)//Transcript WHERE $t/CrsTaken/@CrsCode=$c/@CrsCode AND $t/CrsTaken/@Semester=$c/@Semester RETURN $t/Student SORTBY($t/Student/@StudId) ) </ClassRoster> SORTBY($c/@CrsCode)

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Semantics – FOR Clause

• Specify the documents to be used in the query
• Declare variables
• Bind each variable to its range

– The range is an ordered set of document nodes specified by an XQuery expression – An XPath expression, a query or a function that returns a list of nodes – If variables $a and $b bind with nodes <v, w> and <x, y, z>, respectively, then the ordered list of tuples will be produced: <v, x>, <v, y>, <v, z>, <w, x>, <w, y>, <w, z>

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Semantics – WHERE and RETURN

• WHERE clause: select an ordered sublist

from the original list of tuples by filtering the tuples of bindings

• RETURN clause: apply to every surviving

tuple of bindings, and create fragments for the output document

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Summary and To-Do-List

• XPath
• XQuery
• Assignment 3