XML,%20XML%20Schema,%20XPath%20and%20XQuery%20Query%20Languages

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XML, XML Schema, XPath and XQuery Query Languages

• CS561

Slides collated from several sources, including
D. Suciu at Univ. of Washington
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XML Data
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XML

• W3C standard to complement HTML
• origins structured text SGML
• motivation
• HTML describes presentation
• XML describes content
• HTML e XML subset SGML

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From HTML to XML
HTML describes the presentation
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HTML

• lth1gt Bibliography lt/h1gt
• ltpgt ltigt Foundations of Databases lt/igt
• Abiteboul, Hull, Vianu
• ltbrgt Addison Wesley, 1995
• ltpgt ltigt Data on the Web lt/igt
• Abiteboul, Buneman, Suciu
• ltbrgt Morgan Kaufmann, 1999

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XML

• ltbibliographygt
• ltbookgt lttitlegt Foundations lt/titlegt
• ltauthorgt Abiteboul lt/authorgt
• ltauthorgt Hull lt/authorgt
• ltauthorgt Vianu lt/authorgt
• ltpublishergt Addison Wesley
  lt/publishergt
• ltyeargt 1995 lt/yeargt
• lt/bookgt
• lt/bibliographygt

XML describes the content
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XML Terminology

• tags book, title, author,
• start tag ltbookgt, end tag lt/bookgt
• elements ltbookgtltbookgt,ltauthorgtlt/authorgt
• elements are nested
• empty element ltredgtlt/redgt abbrv. ltred/gt
• an XML document single root element

well formed XML document if it has matching tags
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XML Attributes

• ltbook price 55 currency USDgt
• lttitlegt Foundations of Databases lt/titlegt
• ltauthorgt Abiteboul lt/authorgt
• ltyeargt 1995 lt/yeargt
• lt/bookgt

attributes are alternative ways to represent data
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More XML Oids and References

• ltperson ido555gt ltnamegt Jane lt/namegt lt/persongt
• ltperson ido456gt ltnamegt Mary lt/namegt
• ltchildren
  idrefo123 o555/gt
• lt/persongt
• ltperson ido123 mothero456gtltnamegtJohnlt/namegt
• lt/persongt

oids and references in XML are just syntax
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So Far

• Differences between xml data versus relational
  data ?
• Data model?
• Typed?
• Homogeneity?
• Correctness?
• Usage/Purpose ?

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XML Data Model

• Numerous competing models
• Document Object Model (DOM)
• class hierarchy (node, element, attribute,)
• defines API to inspect/modify the document
• XML query data model (formal)

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XML Namespaces

• http//www.w3.org/TR/REC-xml-names
• name prefixlocalpart

ltbook xmlnsisbnwww.isbn-org.org/defgt
lttitlegt lt/titlegt ltnumbergt 15 lt/numbergt
ltisbnnumbergt . lt/isbnnumbergt lt/bookgt
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XML Namespaces

• syntactic ltnumbergt , ltisbnnumbergt
• semantic provide URL for shared schema

lttag xmlnsmystyle http//gt
ltmystyletitlegt
lt/mystyletitlegt ltmystylenumbergt
lt/taggt
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So Far

• What are namespaces good for ?
• Are they typically available for relational
  databases?

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Schemas for XML
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DTD - Element Type Definitions
lt!ELEMENT paper (title,author, year,
(journalconference) )gt
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XML Schemas

• generalizes DTDs (SGML derivative)
• now, instead uses XML syntax
• two main documents structure and data types
• XML Schema more powerful but more complex

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XML Schema

• ltxsdelement namepaper typepapertype/gt
• ltxsdcomplexType namepapertypegt
• ltxsdsequencegt
• ltxsdelement nametitle
  typexsdstring/gt
• ltxsdelement nameauthor
  minOccurs0/gt
• ltxsdelement nameyear/gt
• ltxsd choicegt lt xsdelement
  namejournal/gt
• ltxsdelement
  nameconference/gt
• lt/xsdchoicegt
• lt/xsdsequencegt
• lt/xsdcomplexType
• lt/xsdelementgt

DTD lt!ELEMENT paper (title,author,year,
(journalconference))gt
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So Far

• Differences between xml schema versus
  relational schema ?
• Purpose ? Do we need it ?
• Definition time?
• Strictness of typing ?
• Underlying model ?

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Elements versus Types in XML Schema
DTD lt!ELEMENT person (name, address) gt
ltxsdelement namepersongt ltxsdcomplexTypegt
ltxsdsequencegt ltxsdelement namename
typexsdstring/gt
ltxsdelement nameaddress
typexsdstring/gt lt/xsdsequencegt
lt/xsdcomplexTypegtlt/xsdelementgt
ltxsdelement nameperson
typettt /gtltxsdcomplexType nametttgt
ltxsdsequencegt ltxsdelement namename
typexsdstring/gt
ltxsdelement nameaddress
typexsdstring/gt lt/xsdsequencegtlt/xsdco
mplexTypegt
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Elements versus Types in XML Schema

• Types
• Simple types (integers, strings, ...)
• Complex types (regular expressions, like in DTDs)
• Element-type-element alternation
• Root element has a complex type
• Complex type is a regular expression of elements
• Those elements have their complex types ...
• ...
• Leaves have simple types

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Local and Global Types in XML Schema

• Local type
• ltxsdelement namepersongt
  define locally the persons type
  lt/xsdelementgt
• Global type ltxsdelement nameperson
  typettt/gt ltxsdcomplexType nametttgt
  define here the type ttt
  lt/xsdcomplexTypegt

Global types can be reused in other elements
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Local v.s. Global Elements inXML Schema

• Local element
• ltxsdcomplexType nametttgt
  ltxsdsequencegt ltxsdelement
  nameaddress type.../gt...
  lt/xsdsequencegt lt/xsdcomplexTypegt
• Global element ltxsdelement nameaddress
  type.../gt ltxsdcomplexType nametttgt
  ltxsdsequencegt ltxsdelement
  refaddress/gt ... lt/xsdsequencegt
  lt/xsdcomplexTypegt

Global elements like in DTDs
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Regular Expressions in XML Schema

• Recall the element-type-element alternation
• ltxsdcomplexType name....gt
  regular expression on
  elements lt/xsdcomplexTypegt
• Regular expressions
• ltxsdsequencegt A B C lt/...gt
• ltxsdchoicegt A B C lt/...gt
• ltxsdgroupgt A B C lt/...gt
  
• ltxsd... minOccurs0 maxOccursunboundedgt
  ..lt/...gt
• ltxsd... minOccurs0 maxOccurs1gt ..lt/...gt

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Regular Expressions in XML Schema

• Regular expressions
• ltxsdsequencegt A B C lt/...gt
  A B C
• ltxsdchoicegt A B C lt/...gt
  A B C
• ltxsdgroupgt A B C lt/...gt
  (A B C)
• ltxsd... minOccurs0 maxOccursunboundedgt
  ..lt/...gt (...)
• ltxsd... minOccurs0 maxOccurs1gt ..lt/...gt
  (...)?

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Regular Expressions in XML Schema

• Recall the element-type-element alternation
• ltxsdcomplexType name....gt
  regular expression on
  elements lt/xsdcomplexTypegt
• Regular expressions
• ltxsdsequencegt A B C lt/...gt
  A B C
• ltxsdchoicegt A B C lt/...gt
  A B C
• ltxsdgroupgt A B C lt/...gt
  (A B C)
• ltxsd... minOccurs0 maxOccursunboundedgt
  ..lt/...gt (...)
• ltxsd... minOccurs0 maxOccurs1gt ..lt/...gt
  (...)?

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Attributes in XML Schema
ltxsdelement namepaper typepapertype/gt ltxsd
complexType namepapertypegt
ltxsdsequencegt ltxsdelement
nametitle typexsdstring/gt . .
. . . . lt/xsdsequencegt ltxsdattribute
namelanguage" type"xsdNMTOKEN"
fixedEnglish"/gt lt/xsdcomplexTypegt
Attributes are associated with the type, not the
element Only to complex types more trouble if
we want to add attributes to simple types.
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Derived Types by Extensions
ltcomplexType name"Address"gt ltsequencegt
ltelement name"street" type"string"/gt
ltelement name"city"
type"string"/gt lt/sequencegt lt/complexTypegt
ltcomplexType name"USAddress"gt
ltcomplexContentgt ltextension base
"ipoAddress"gt ltsequencegt ltelement
name"state" type"ipoUSState"/gt
ltelement name"zip"
type"positiveInteger"/gt lt/sequencegt
lt/extensiongt lt/complexContentgt lt/complexTypegt
Corresponds to inheritance
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Key Constraints in XML
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Keys in XML Schema
XML
ltpurchaseReportgt ltregionsgt ltzip code"95819"gt
ltpart number"872-AA" quantity"1"/gt ltpart
number"926-AA" quantity"1"/gt ltpart
number"833-AA" quantity"1"/gt ltpart
number"455-BX" quantity"1"/gt lt/zipgt ltzip
code"63143"gt ltpart number"455-BX"
quantity"4"/gt lt/zipgt lt/regionsgt ltpartsgt
ltpart number"872-AA"gtLawnmowerlt/partgt ltpart
number"926-AA"gtBaby Monitorlt/partgt ltpart
number"833-AA"gtLapis Necklacelt/partgt ltpart
number"455-BX"gtSturdy Shelveslt/partgt
lt/partsgt lt/purchaseReportgt
XML Schema for Key
ltkey name"NumKey"gt ltselector
xpath"parts/part"/gt ltfield xpath"_at_number"/gt lt
/keygt
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Keys in XML Schema

• In general, syntax is

ltkey namesomeDummyNameHere"gt ltselector
xpathp"/gt ltfield xpathp1"/gt ltfield
xpathp2"/gt . . . ltfield
xpathpk"/gt lt/keygt
Notes All XPath expressions start at the
element currently being defined The fields must
identify a single node.
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Keys in XML Schema

• Unique guarantees uniqueness
• Key guarantees uniqueness and existence
• All XPath expressions are restricted
• /a/b /a/c OK for selector
• //a/b//c OK for field
• Note better than DTDs ID mechanism

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Examples of Keys in XML Schema

• Examples

ltkey name"fullName"gt ltselector
xpath".//person"/gt ltfield xpath"firstname"/gt
ltfield xpath"surname"/gt lt/keygt ltunique
name"nearlyID"gt ltselector xpath".//"/gt
ltfield xpath"_at_id"/gt lt/uniquegt
Note Must have single firstname, Single surname
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Foreign Keys in XML Schema

• Example

ltkeyref name"personRef" refer"fullName"gt
ltselector xpath".//personPointer"/gt ltfield
xpath"_at_first"/gt ltfield xpath"_at_last"/gt lt/keyrefgt
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So Far

• Differences between keys/foreign-keysin xml
  versus relational model?
• Purpose ?
• Underlying model ?

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XPath

• The Basic Building Block

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XPath

• Goal Permit access some nodes from document
• XPath main construct Axis navigation
• Navigation step axis node-test predicates
• Examples
• descendantnode()
• childauthor
• attributebooktitle XML
• XPath path consists of one or more navigation
  steps, separated by /
• Navigation step axis node-test predicates
• Examples
• /descendantnode()/childauthor
• /descendantnode()/childauthorparent/attribute
  booktitle XML2

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XPath

• Goal Permit access some nodes from document
• XPath main construct Axis navigation
• Navigation step axis node-test predicates
• Examples
• descendantnode()
• childauthor
• attributebooktitle XML

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XPath

• XPath path consists of one or more navigation
  steps, separated by /
• Navigation step axis node-test predicates
• Examples
• /descendantnode() /childauthor
• /descendantnode() /childauthor parent
  /attributebooktitle XML2
• XPath offers shortcuts
• no axis means child
• // º /descendant-or-selfnode()/

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XPath- Child Axis Navigation

• author is shorthand for childauthor.
• Examples
• aaa -- all the children nodes labeled aaa
• aaa/bbb -- all the bbb grandchildren of aaa
  children
• /bbb all the bbb grandchildren of any child
• Notes
• . -- the context node
• / -- the root node

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XPath- Child Axis Navigation

• author is shorthand for childauthor.
• Examples
• aaa -- all the children nodes labeled aaa (1,3)
• aaa/bbb -- all the bbb grandchildren of aaa
  children (4)
• /bbb all the bbb grandchildren of any child
  (4,6)
• Notes
• . -- the context node
• / -- the root node

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XPath- Child Axis Navigation

• /doc -- all doc children of the root
• ./aaa -- all aaa children of the context node
  
  (equivalent to aaa)
• text() -- all text children of context node
• node() -- all children of the context node
  (includes text and attribute nodes)
• .. -- parent of the context node
• .// -- the context node and all its descendants
• // -- the root node and all its descendants
• //text() -- all the text nodes in the document

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Predicates

• 2 -- the second child node of the context node
• chapter5 -- the fifth chapter child of context
  node
• last() -- the last child node of the context
  node
• chaptertitleintroduction -- the chapter
  children of the context node that have one or
  more title children whose string-value is
  introduction (string-value is concatenation of
  all text on descendant text nodes)
• person.//firstname joe -- the person
  children of the context node that have in their
  descendants a firstname element with string-value
  Joe

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Axis navigation

• So far, our expressions have moved us down by
  moving to children nodes.
• Exceptions are
• . stay where you are
• / go to the root
• // all descendants of the root
• .// all descendants of the context node

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Axis navigation

• XPath has several axes ancestor,
  ancestor-or-self, attribute, child, descendant,
  descendant-or-self, following, following-sibling,
  namespace, parent, preceding, preceding-sibling,
  self
• Some of these describe single nodes
• self, parent
• Some describe sequences of nodes
• All others

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XPath Navigation Axes
ancestor
following-sibling
preceding-sibling
self
child
attribute
following
preceding
namespace
descendant
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XPath Abbreviated Syntax
(nothing) child _at_ attribute // /descendan
t-or-selfnode() . selfnode() .// descendan
t-or-selfnode .. parentnode() / (document
root)
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XPath

• Widely adopted -- in XML-Schema and in many query
  languages.
• About as expressive as regular path expressions

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So Far

• Differences between SQL and XPATH?
• What are similar query capabilities?
• What features does SQL have, but not XPATH?
• What features does XPATH support, but not SQL?
• Is XPath a full-fledged query language?

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Query Languages - XQuery
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Summary of XQuery

• FLWR expressions
• FOR and LET expressions
• Collections and sorting
• Resources
• XQuery A Query Language for XML Chamberlin,
  Florescu, et al.
• W3C recommendation www.w3.org/TR/xquery/

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XQuery

• Designed based on Quilt (which is based on
  XML-QL)
• http//www.w3.org/TR/xquery/2/2001
• XML Query data model (ordered)

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FLWR (Flower) Expressions

• FOR ... LET... FOR... LET...
• WHERE...
• RETURN...

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XQuery

• Find the titles of all books published after 1995

FOR x IN document("bib.xml")/bib/book WHERE
x/year gt 1995 RETURN x/title
How does result look like?
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XQuery

• Find all book titles published after 1995

FOR x IN document("bib.xml")/bib/book WHERE
x/year gt 1995 RETURN x/title
Result lttitlegt abc lt/titlegt lttitlegt def
lt/titlegt lttitlegt ghi lt/titlegt
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XQuery Example
FOR a IN (document("bib.xml")
/bib/bookpublisherMorgan
Kaufmann/author) RETURN ltresultgt
a, FOR t IN
/bib/bookauthora/title
RETURN t lt/resultgt
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XQuery Example
For each author of a book by Morgan Kaufmann,
list all books she published
FOR a IN (document("bib.xml")
/bib/bookpublisherMorgan
Kaufmann/author) RETURN ltresultgt
a, FOR t IN
/bib/bookauthora/title
RETURN t lt/resultgt
What is query result ?
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XQuery

• Result
• ltresultgt
• ltauthorgtJoneslt/authorgt
• lttitlegt abc lt/titlegt
• lttitlegt def lt/titlegt
• lt/resultgt
• ltresultgt
• ltauthorgtJoneslt/authorgt
• lttitlegt abc lt/titlegt
• lttitlegt def lt/titlegt
• lt/resultgt
• ltresultgt
• ltauthorgt Smith lt/authorgt
• lttitlegt ghi lt/titlegt
• lt/resultgt

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XQuery Example Duplicates

• For each author of a book by Morgan Kaufmann,
  list all books she published

FOR a IN distinct(document("bib.xml")
/bib/bookpublisherMorgan
Kaufmann/author) RETURN ltresultgt
a, FOR t IN
/bib/bookauthora/title
RETURN t lt/resultgt
distinct a function that eliminates duplicates
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Example XQuery Result

• Result
• ltresultgt
• ltauthorgtJoneslt/authorgt
• lttitlegt abc lt/titlegt
• lttitlegt def lt/titlegt
• lt/resultgt
• ltresultgt
• ltauthorgt Smith lt/authorgt
• lttitlegt ghi lt/titlegt
• lt/resultgt

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XQuery

• FOR x in expr
• binds x to each element in the list expr
• Useful for iteration over some input list
• LET x expr
• binds x to the entire list expr
• Useful for common subexpressions and for grouping
  and aggregations

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XQuery with LET Clause
ltbig_publishersgt FOR p IN
distinct(document("bib.xml")//publisher)
LET b document("bib.xml")/bookpublisher
p WHERE count(b) gt 100 RETURN
p lt/big_publishersgt
count a (aggregate) function that returns
number of elements
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XQuery

• Find books whose price is larger than average

LET a avg(document("bib.xml")/bib/book/_at_price)
FOR b in document("bib.xml")/bib/book WHERE
b/_at_price gt a RETURN b
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FOR versus LET

• FOR
• Binds node variables ? iteration
• LET
• Binds collection variables ? one value

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FOR v.s. LET
Returns ltresultgt ltbookgt...lt/bookgtlt/resultgt
ltresultgt ltbookgt...lt/bookgtlt/resultgt ltresultgt
ltbookgt...lt/bookgtlt/resultgt ...
FOR x IN document("bib.xml")/bib/book RETURN
ltresultgt x lt/resultgt
Returns ltresultgt ltbookgt...lt/bookgt
ltbookgt...lt/bookgt
ltbookgt...lt/bookgt ... lt/resultgt
LET x document("bib.xml")/bib/book RETURN
ltresultgt x lt/resultgt
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Collections in XQuery

• Ordered and unordered collections
• /bib/book/author an ordered collection
• distinct(/bib/book/author) an unordered
  collection
• LET a /bib/book ? a is a collection
• b/author ? a collection (several authors...)

Returns ltresultgt ltauthorgt...lt/authorgt
ltauthorgt...lt/authorgt
ltauthorgt...lt/authorgt
... lt/resultgt
RETURN ltresultgt b/author lt/resultgt
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XQuery Summary

• FOR-LET-WHERE-RETURN FLWR

FOR/LET Clauses
List of tuples
WHERE Clause
List of tuples
RETURN Clause
Instances of XQuery data model
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XQuery

• Some more query features

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Sorting in XQuery
ltpublisher_listgt FOR p IN distinct(document("
bib.xml")//publisher) RETURN ltpublishergt
ltnamegt p/text() lt/namegt ,
FOR b IN document("bib.xml")//bookpublisher
p RETURN ltbookgt

b/title ,
b/_at_price
lt/bookgt SORTBY (price DESCENDING)
lt/publishergt SORTBY (name)
lt/publisher_listgt
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Sorting in XQuery

• Sorting arguments refer to name space of RETURN
  clause, not of FOR clause
• TIP To sort on an element you dont want to
  display, first return it, then remove it with an
  additional query.

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If-Then-Else
FOR h IN //holding RETURN ltholdinggt
h/title, IF
h/_at_type "Journal"
THEN h/editor ELSE
h/author lt/holdinggt SORTBY
(title)
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Existential Quantifiers
FOR b IN //book WHERE SOME p IN b//para
SATISFIES contains(p, "sailing") AND
contains(p, "windsurfing") RETURN b/title
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Universal Quantifiers
FOR b IN //book WHERE EVERY p IN b//para
SATISFIES contains(p, "sailing") RETURN
b/title
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So Far

• Similarities between SQL and XQuery?
• Differences between SQL and XQuery?

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XML, XML Data ModelXML Schema, XPath XQuery