XML Technologies and Applications

1
XML Technologies and Applications
Rajshekhar SunderramanDepartment of Computer
Science Georgia State University Atlanta, GA
30302raj_at_cs.gsu.edu V (c). XML Querying
XQuery December 2005
2
Outline

• Introduction
• XML Basics
• XML Structural Constraint Specification
• Document Type Definitions (DTDs)
• XML Schema
• XML/Database Mappings
• XML Parsing APIs
• Simple API for XML (SAX)
• Document Object Model (DOM)
• XML Querying and Transformation
• XPath
• XSLT
• XQuery
• XML Applications

3
XQuery XML Query Language

• Integrates XPath with earlier proposed query
  languages XQL, XML-QL
• SQL-style, not functional-style
• Much easier to use as a query language than XSLT
• Can do pretty much the same things as XSLT and
  more, but typically easier
• 2004 XQuery 1.0

4
transcript.xml

• ltTranscriptsgt
• ltTranscriptgt
• ltStudent StudId111111111 NameJohn Doe/gt
• ltCrsTaken CrsCodeCS308 SemesterF1997
  GradeB/gt
• ltCrsTaken CrsCodeMAT123 SemesterF1997
  GradeB/gt
• ltCrsTaken CrsCodeEE101 SemesterF1997
  GradeA/gt
• ltCrsTaken CrsCodeCS305 SemesterF1995
  GradeA/gt
• lt/Transcriptgt
• ltTranscriptgt
• ltStudent StudId987654321 NameBart
  Simpson /gt
• ltCrsTaken CrsCodeCS305 SemesterF1995
  GradeC/gt
• ltCrsTaken CrsCodeCS308 SemesterF1994
  GradeB/gt
• lt/Transcriptgt
• contd

5
transcript.xml (contd)

• ltTranscriptgt
• ltStudent StudId123454321 NameJoe Blow
  /gt
• ltCrsTaken CrsCodeCS315 SemesterS1997
  GradeA /gt
• ltCrsTaken CrsCodeCS305 SemesterS1996
  GradeA /gt
• ltCrsTaken CrsCodeMAT123 SemesterS1996
  GradeC /gt
• lt/Transcriptgt
• ltTranscriptgt
• ltStudent StudId023456789 NameHomer
  Simpson /gt
• ltCrsTaken CrsCodeEE101 SemesterF1995
  GradeB /gt
• ltCrsTaken CrsCodeCS305 SemesterS1996
  GradeA /gt
• lt/Transcriptgt
• lt/Transcriptsgt

6
XQuery Basics

• General structure (FLWR expressions)
• FOR variable declarations
• LET variable expression,
• variable expression,
• WHERE condition
• RETURN document
• Example
• ( students who took MAT123 )
• FOR t IN doc(http//xyz.edu/transcript.xml)/
  /Transcript
• WHERE t/CrsTaken/_at_CrsCode MAT123
• RETURN t/Student
• Result
• ltStudent StudId111111111 NameJohn Doe /gt
• ltStudent StudId123454321 NameJoe Blow /gt

XQuery expression
comment
7
XQuery Basics (contd)

• Previous query doesnt produce a well-formed XML
  document the following does
• ltStudentListgt
• FOR t IN doc(transcript.xml)//Transcript
• WHERE t/CrsTaken/_at_CrsCode MAT123
• RETURN t/Student
• lt/StudentListgt
• FOR binds t to Transcript elements one by one,
  filters using WHERE, then places Student-children
  as e-children of StudentList using RETURN

Query inside XML
8
FOR vs LET
For iteration
FOR x IN doc(transcript.xml) RETURN ltresultgt
x lt/resultgt
Returns ltresultgt lttranscriptgt...lt/transcriptgtlt/
resultgt ltresultgt lttranscriptgt...lt/transcriptgtlt/re
sultgt ltresultgt lttranscriptgt...lt/transcriptgtlt/resu
ltgt ...
LET x doc(transcript.xml) RETURN ltresultgt
x lt/resultgt
Let set value is assigned to variable.
Returns ltresultgt lttranscriptgt...lt/transcriptgt
lttranscriptgt...lt/transcriptgt
lttranscriptgt...lt/transcriptgt
... lt/resultgt
9
Document Restructuring with XQuery

• Reconstruct lists of students taking each class
  using the Transcript records
• FOR c IN distinct values(doc(transcript.xml)//
  CrsTaken)
• RETURN
• ltClassRoster CrsCodec/_at_CrsCode
  Semesterc/_at_Semestergt
• FOR t IN doc(transcript.xml)//Transcript
• WHERE t/CrsTaken/_at_CrsCode c/_at_CrsCode and
• _at_Semester c/_at_Semester
• RETURN t/Student
• ORDER BY t/Student/_at_StudId
• lt/ClassRostergt
• ORDER BY c/_at_CrsCode

Query inside RETURN similar to query inside
SELECT in OQL
10
Document Restructuring (contd)

• Output elements have the form
• ltClassRoster CrsCodeCS305 SemesterF1995gt
• ltStudent StudId111111111 NameJohn
  Doe/gt
• ltStudent StudId987654321 NameBart
  Simpson/gt
• lt/ClassRostergt
• Problem the above element will be output twice
  for each of the following two bindings of c
• ltCrsTaken CrsCodeCS305 SemesterF1995
  GradeC/gt
• ltCrsTaken CrsCodeCS305 SemesterF1995
  GradeA/gt
• Note grades are different distinct-values( )
  wont eliminate transcript records that refer to
  same class!

Bart Simpsons
John Does
11
Document Restructuring (contd)

• Solution instead of
• FOR c IN distinct-values(doc(transcript.xml)
  //CrsTaken)
• use
• FOR c IN doc(classes.xml)//Class
• where classes.xml lists course offerings
  (course code/semester)
• explicitly (no need to extract them from
  transcript records) shown on
• next slide
• Then c is bound to each class exactly once, so
  each class roster
• will be output exactly once

12
http//xyz.edu/classes.xml

• ltClassesgt
• ltClass CrsCodeCS308 SemesterF1997 gt
• ltCrsNamegtSElt/CrsNamegt ltInstructorgtAdrian
  Joneslt/Instructorgt
• lt/Classgt
• ltClass CrsCodeEE101 SemesterF1995 gt
• ltCrsNamegtCircuitslt/CrsNamegt ltInstructorgtDavid
  Joneslt/Instructorgt
• lt/Classgt
• ltClass CrsCodeCS305 SemesterF1995 gt
• ltCrsNamegtDatabaseslt/CrsNamegt ltInstructorgtMary
  Doelt/Instructorgt
• lt/Classgt
• ltClass CrsCodeCS315 SemesterS1997 gt
• ltCrsNamegtTPlt/CrsNamegt ltInstructorgtJohn
  Smythlt/Instructorgt
• lt/Classgt
• ltClass CrsCodeMAR123 SemesterF1997 gt
• ltCrsNamegtAlgebralt/CrsNamegt ltInstructorgtAnn
  Whitelt/Instructorgt
• lt/Classgt
• lt/Classesgt

13
Document Restructuring (contd)

• More problems the above query will list classes
  with no students. Reformulation that avoids this
• FOR c IN doc(classes.xml)//Class
• WHERE
• doc(transcripts.xml)//CrsTaken_at_CrsCode
  c/_at_CrsCode
• and _at_Semester
  c/_at_Semester
• RETURN
• ltClassRoster CrsCodec/_at_CrsCode
  Semesterc/_at_Semestergt
• FOR t IN doc(transcript.xml)//Transcript
  
• WHERE t/CrsTaken_at_CrsCode c/_at_CrsCode and
• _at_Semester c/_at_Semester
• RETURN t/Student
• ORDER BY t/Student/_at_StudId
• lt/ClassRostergt
• ORDER BY c/_at_CrsCode

Test that classes arent empty
14
XQuery Semantics

• So far the discussion was informal
• XQuery semantics defines what the expected result
  of a query is
• Defined analogously to the semantics of SQL

15
XQuery Semantics (contd)

• Step 1 Produce a list of bindings for variables
• The FOR clause binds each variable to a list of
  nodes specified by an XQuery expression.
• The expression can be
• An XPath expression
• An XQuery query
• A function that returns a list of nodes
• End result of a FOR clause
• Ordered list of tuples of document nodes
• Each tuple is a binding for the variables in the
  FOR clause

16
XQuery Semantics (contd)

• Example (bindings)
• Let FOR declare A and B
• Bind A to document nodes v,w B to x,y,z
• Then FOR clause produces the following list of
  bindings for A and B
• A/v, B/x
• A/v, B/y
• A/v, B/z
• A/w, B/x
• A/w, B/y
• A/w, B/z

17
XQuery Semantics (contd)

• Step 2 filter the bindings via the WHERE clause
• Use each tuple binding to substitute its
  components for variables retain those bindings
  that make WHERE true
• Example WHERE A/CrsTaken/_at_CrsCode
  B/Class/_at_CrsCode
• Binding A/w, where w ltCrsTaken
  CrsCodeCS308 /gt
• B/x, where x ltClass CrsCodeCS308
  /gt
• Then w/CrsTaken/_at_CrsCode x/Class/_at_CrsCode, so
  the WHERE condition is satisfied binding
  retained

18
XQuery Semantics (contd)

• Step 3 Construct result
• For each retained tuple of bindings, instantiate
  the RETURN clause
• This creates a fragment of the output document
• Do this for each retained tuple of bindings in
  sequence

19
Grouping and Aggregation

• Does not use separate grouping operator
• OQL does not need one either (XML data model is
  object-oriented and hence similarities with OQL)
• Subqueries inside the RETURN clause obviate this
  need (like subqueries inside SELECT did so in
  OQL)
• Uses built-in aggregate functions count, avg,
  sum, etc. (some borrowed from XPath)

20
Aggregation Example

• Produce a list of students along with the number
  of courses each student took
• FOR t IN fndoc(transcripts.xml)//Transc
  ript,
• s IN t/Student
• LET c t/CrsTaken
• RETURN
• ltStudentSummary
• StudId s/_at_StudId
• Name s/_at_Name
• TotalCourses fncount(fndistinct-valu
  es(c)) /gt
• ORDER BY StudentSummary/_at_TotalCourses
• The grouping effect is achieved because c is
  bound to a new set of nodes for each binding of t

21
Quantification in XQuery

• XQuery supports explicit quantification
• SOME (?) and EVERY (?)
• Example Find students who have taken MAT123.
• FOR t IN fndoc(transcript.xml)//Transcript
• WHERE SOME ct IN t/CrsTaken
• SATISFIES ct/_at_CrsCode MAT123
• RETURN t/Student

22
Quantification (contd)

• Retrieve all classes (from classes.xml) where
  each student took the class.
• FOR c IN fndoc(classes.xml)//Class
• LET g
• ( Transcript records that correspond to class
  c )
• FOR t IN fndoc(transcript.xml)//Transcript
  
• WHERE t/CrsTaken/_at_Semester c/_at_Semester AND
• t/CrsTaken/_at_CrsCode c/_at_CrsCode
• RETURN t
• h FOR s in fndoc(transcript.xml)//Transc
  ript
• RETURN s ( all transcript records )
• WHERE EVERY tr IN h SATISFIES
• tr IN g
• RETURN c ORDER BY c/_at_CrsCode

23
XQuery Summary

• FOR-LET-WHERE-RETURN FLWR

FOR/LET Clauses
List of tuples
WHERE Clause
List of tuples
RETURN Clause
Instance of Xquery data model