Xquery basics tutorial

XQuery Tutorial
Peter Fankhauser, Fraunhofer IPSI
Peter.Fankhauser@ipsi.fhg.de
Philip Wadler, Avaya Labs
wadler@avaya.com

Acknowledgements
This tutorial is joint work with:
Mary Fernandez (AT&T)
Gerald Huck (IPSI/Infonyte)
Ingo Macherius (IPSI/Infonyte)
Thomas Tesch (IPSI/Infonyte)
Jerome Simeon (Lucent)
The W3C XML Query Working Group
Disclaimer: This tutorial touches on open issues of XQuery.
Other members of the XML Query WG may disagree with our
view.

Goals
After this tutorial, you should understand
• Part I XQuery expressions, types, and laws
• Part II XQuery laws and XQuery core
• Part III XQuery processing model
• Part IV XQuery type system and XML Schema
• Part V Type inference and type checking
• Part VI Where to go for more information

“Where a mathematical reasoning can be had, it’s as
great folly to make use of any other, as to grope for a
thing in the dark, when you have a candle standing by
you.”
— Arbuthnot

XQuery by example
Titles of all books published before 2000
/BOOKS/BOOK[@YEAR < 2000]/TITLE
Year and title of all books published before 2000
for $book in /BOOKS/BOOK
where $book/@YEAR < 2000
return <BOOK>{ $book/@YEAR, $book/TITLE }</BOOK>
Books grouped by author
for $author in distinct(/BOOKS/BOOK/AUTHOR) return
<AUTHOR NAME="{ $author }">{
/BOOKS/BOOK[AUTHOR = $author]/TITLE
}</AUTHOR>

Some XML data
<BOOKS>
<BOOK YEAR="1999 2003">
<AUTHOR>Abiteboul</AUTHOR>
<AUTHOR>Buneman</AUTHOR>
<AUTHOR>Suciu</AUTHOR>
<TITLE>Data on the Web</TITLE>
<REVIEW>A fine book.</REVIEW>
</BOOK>
<BOOK YEAR="2002">
<TITLE>XML in Scotland</TITLE>
<REVIEW>The best ever!</REVIEW>
</BOOK>
</BOOKS>

Data model
XML
<BOOK YEAR="1999 2003">
</BOOK>
XQuery
element BOOK {
attribute YEAR { 1999, 2003 },
element AUTHOR { "Abiteboul" },
element AUTHOR { "Buneman" },
element AUTHOR { "Suciu" },
element TITLE { "Data on the Web" },
element REVIEW { "A", element EM { "fine" }, "book." }
}

DTD (Document Type Deﬁnition)
<!ELEMENT BOOKS (BOOK*)>
<!ELEMENT BOOK (AUTHOR+, TITLE, REVIEW?)>
<!ATTLIST BOOK YEAR CDATA #OPTIONAL>
<!ELEMENT AUTHOR (#PCDATA)>
<!ELEMENT TITLE (#PCDATA)>
<!ENTITY % INLINE "( #PCDATA | EM | BOLD )*">
<!ELEMENT REVIEW %INLINE;>
<!ELEMENT EM %INLINE;>
<!ELEMENT BOLD %INLINE;>

Schema
<xsd:schema targetns="http://www.example.com/books"
xmlns="http://www.example.com/books"
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
attributeFormDefault="qualified"
elementFormDefault="qualified">
<xsd:element name="BOOKS">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="BOOK"
minOccurs="0" maxOccurs="unbounded"/>
</xsd:sequence>
</xsd:complexType>
</xsd:element>

Schema, continued
<xsd:element name="BOOK">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="AUTHOR" type="xsd:string"
<xsd:element name="TITLE" type="xsd:string"/>
<xsd:element name="REVIEW" type="INLINE"
minOccurs="0" maxOccurs="1"/>
<xsd:sequence>
<xsd:attribute name="YEAR" type="NONEMPTY-INTEGER-LIST"
use="optional"/>
</xsd:complexType>
</xsd:element>

Schema, continued2
<xsd:complexType name="INLINE" mixed="true">
<xsd:choice minOccurs="0" maxOccurs="unbounded">
<xsd:element name="EM" type="INLINE"/>
<xsd:element name="BOLD" type="INLINE"/>
</xsd:choice>
</xsd:complexType>
<xsd:simpleType name="INTEGER-LIST">
<xsd:list itemType="xsd:integer"/>
</xsd:simpleType>
<xsd:simpleType name="NONEMPTY-INTEGER-LIST">
<xsd:restriction base="INTEGER-LIST">
<xsd:minLength value="1"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:schema>

XQuery types
define element BOOKS { BOOK* }
define element BOOK { @YEAR?, AUTHOR+, TITLE, REVIEW? }
define attribute YEAR { xsd:integer+ }
define element AUTHOR { xsd:string }
define element TITLE { xsd:string }
define type INLINE { ( xsd:string | EM | BOLD )* }
define element REVIEW { #INLINE }
define element EM { #INLINE }
define element BOLD { #INLINE }

XQuery and Schema
Authors and title of books published before 2000
schema "http://www.example.com/books"
namespace default = "http://www.example.com/books"
validate
<BOOKS>{
for $book in /BOOKS/BOOK[@YEAR < 2000] return
<BOOK>{ $book/AUTHOR, $book/TITLE }</BOOK>
}</BOOKS>
∈
element BOOKS {
element BOOK {
element AUTHOR { xsd:string } +,
element TITLE { xsd:string }
} *
}

Another Schema
<xsd:schema targetns="http://www.example.com/answer"
xmlns="http://www.example.com/answer"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
elementFormDefault="qualified">
<xsd:element name="ANSWER">
<xsd:complexType>
<xsd:sequence>
<xsd:element ref="BOOK"
<xsd:complexType>
<xsd:sequence>
<xsd:element name="TITLE" type="xsd:string"/>
<xsd:element name="AUTHOR" type="xsd:string"
</xsd:sequence>
</xsd:complexType>
</xsd:sequence>
</xsd:complexType>
</xsd:element>
</xsd:schema>

Another XQuery type
element ANSWER { BOOK* }
element BOOK { TITLE, AUTHOR+ }
element AUTHOR { xsd:string }
element TITLE { xsd:string }

XQuery with multiple Schemas
Title and authors of books published before 2000
schema "http://www.example.com/books"
schema "http://www.example.com/answer"
namespace B = "http://www.example.com/books"
namespace A = "http://www.example.com/answer"
validate
<A:ANSWER>{
for $book in /B:BOOKS/B:BOOK[@YEAR < 2000] return
<A:BOOK>{
<A:TITLE>{ $book/B:TITLE/text() }</A:TITLE>,
for $author in $book/B:AUTHOR return
<A:AUTHOR>{ $author/text() }</A:AUTHOR>
}<A:BOOK>
}</A:ANSWER>

Projection
Return all authors of all books
/BOOKS/BOOK/AUTHOR
⇒
<AUTHOR>Abiteboul</AUTHOR>,
<AUTHOR>Buneman</AUTHOR>,
<AUTHOR>Suciu</AUTHOR>,
∈
element AUTHOR { xsd:string } *

Laws — relating XPath to XQuery
Return all authors of all books
/BOOKS/BOOK/AUTHOR
=
for $dot1 in $root/BOOKS return
for $dot2 in $dot1/BOOK return
$dot2/AUTHOR

Laws — Associativity
Associativity in XPath
BOOKS/(BOOK/AUTHOR)
=
(BOOKS/BOOK)/AUTHOR
Associativity in XQuery
for $dot2 in $dot1/BOOK return
$dot2/AUTHOR
=
for $dot2 in (
$dot1/BOOK
) return
$dot2/AUTHOR

Selection
Return titles of all books published before 2000
⇒
∈
element TITLE { xsd:string } *

Laws — relating XPath to XQuery
Return titles of all books published before 2000
=
return $book/TITLE

Laws — mapping into XQuery core
Comparison defined by existential
$book/@YEAR < 2000
=
some $year in $book/@YEAR satisfies $year < 2000
Existential defined by iteration with selection
some $year in $book/@YEAR satisfies $year < 2000
=
not(empty(
for $year in $book/@YEAR where $year < 2000 returns $year
))
Selection defined by conditional
for $year in $book/@YEAR where $year < 2000 returns $year
=
for $year in $book/@YEAR returns
if $year < 2000 then $year else ()

Laws — mapping into XQuery core
=
for $book in /BOOKS/BOOK return
if (
not(empty(
for $year in $book/@YEAR returns
if $year < 2000 then $year else ()
))
) then
$book/TITLE
else
()

Selection — Type may be too broad
Return book with title ”Data on the Web”
/BOOKS/BOOK[TITLE = "Data on the Web"]
⇒
<BOOK YEAR="1999 2003">
</BOOK>
∈
BOOK*
How do we exploit keys and relative keys?

Selection — Type may be narrowed
Return book with title ”Data on the Web”
treat as element BOOK? (
/BOOKS/BOOK[TITLE = "Data on the Web"]
)
∈
BOOK?
Can exploit static type to reduce dynamic checking
Here, only need to check length of book sequence, not type

Iteration — Type may be too broad
Return all Amazon and Fatbrain books by Buneman
define element AMAZON-BOOK { TITLE, AUTHOR+ }
define element FATBRAIN-BOOK { AUTHOR+, TITLE }
define element BOOKS { AMAZON-BOOK*, FATBRAIN-BOOK* }
for $book in (/BOOKS/AMAZON-BOOK, /BOOKS/FATBRAIN-BOOK)
where $book/AUTHOR = "Buneman" return
$book
∈
( AMAZON-BOOK | FATBRAIN-BOOK )*
⊆
AMAZON-BOOK*, FATBRAIN-BOOK*
How best to trade oﬀ simplicity vs. accuracy?

Construction in XQuery
Return year and title of all books published before 2000
return
<BOOK>{ $book/@YEAR, $book/TITLE }</BOOK>
⇒
<BOOK YEAR="1999 2003">
</BOOK>
∈
element BOOK {
attribute YEAR { integer+ },
element TITLE { string }
} *

Construction — mapping into XQuery core
<BOOK YEAR="{ $book/@YEAR }">{ $book/TITLE }</BOOK>
=
element BOOK {
attribute YEAR { data($book/@YEAR) },
$book/TITLE
}

Grouping
Return titles for each author
}</AUTHOR>
⇒
<AUTHOR NAME="Abiteboul">
</AUTHOR>,
<AUTHOR NAME="Buneman">
</AUTHOR>,
<AUTHOR NAME="Suciu">
</AUTHOR>

Grouping — Type may be too broad
}</AUTHOR>
∈
element AUTHOR {
attribute NAME { string },
element TITLE { string } *
}
⊆
element AUTHOR {
element TITLE { string } +
}

Grouping — Type may be narrowed
define element TITLE { string }
treat as element TITLE+ (
)
}</AUTHOR>
∈
element AUTHOR {
element TITLE { string } +
}

Join
Books that cost more at Amazon than at Fatbrain
define element BOOK { TITLE, PRICE, ISBN }
let $amazon := document("http://www.amazon.com/books.xml"),
$fatbrain := document("http://www.fatbrain.com/books.xml")
for $am in $amazon/BOOKS/BOOK,
$fat in $fatbrain/BOOKS/BOOK
where $am/ISBN = $fat/ISBN
and $am/PRICE > $fat/PRICE
return <BOOK>{ $am/TITLE, $am/PRICE, $fat/PRICE }</BOOK>

Join — Unordered
Books that cost more at Amazon than at Fatbrain, in any order
unordered(
)
Reordering required for cost-eﬀective computation of joins

Join — Sorted
sortby TITLE

Join — Laws
sortby TITLE
=
unordered(
) sortby TITLE

Join — Laws
unordered(
) sortby TITLE
=
unordered(
for $am in unordered($amazon/BOOKS/BOOK),
$fat in unordered($fatbrain/BOOKS/BOOK)
) sortby TITLE

Left outer join
Books at Amazon and Fatbrain with both prices,
and all other books at Amazon with price
for $am in $amazon/BOOKS/BOOK, $fat in $fatbrain/BOOKS/BOOK
,
for $am in $amazon/BOOKS/BOOK
where not($am/ISBN = $fatbrain/BOOKS/BOOK/ISBN)
return <BOOK>{ $am/TITLE, $am/PRICE }</BOOK>
∈
element BOOK { TITLE, PRICE, PRICE } *
,
element BOOK { TITLE, PRICE } *

Why type closure is important
Closure problems for Schema
• Deterministic content model
• Consistent element restriction
element BOOK { TITLE, PRICE, PRICE } *
,
element BOOK { TITLE, PRICE } *
⊆
element BOOK { TITLE, PRICE+ } *
The ﬁrst type is not a legal Schema type
The second type is a legal Schema type
Both are legal XQuery types

Part I.9
Nulls and three-valued logic

Books with price and optional shipping price
define element BOOK { TITLE, PRICE, SHIPPING? }
define element PRICE { xsd:decimal }
define element SHIPPING { xsd:decimal }
<BOOKS>
<BOOK>
<PRICE>40.00</PRICE>
<SHIPPING>10.00</PRICE>
</BOOK>
<BOOK>
<PRICE>45.00</PRICE>
</BOOK>
</BOOKS>

Approaches to missing data
Books costing $50.00, where default shipping is $5.00
where $book/PRICE + if_absent($book/SHIPPING, 5.00) = 50.00
return $book/TITLE
⇒
<TITLE>Data on the Web</TITLE>,
Books costing $50.00, where missing shipping is unknown
where $book/PRICE + $book/SHIPPING = 50.00
return $book/TITLE
⇒

Arithmetic, Truth tables
+ () 0 1
() () () ()
0 () 0 1
1 () 1 2
* () 0 1
() () () ()
0 () 0 0
1 () 0 1
OR3 () false true
() () () true
false () false true
true true true true
AND3 () false true
() () false ()
false false false false
true () false true
NOT3
() ()
false true
true false

Type error 1: Missing or misspelled element
Return TITLE and ISBN of each book
define element BOOK { TITLE, PRICE }
<ANSWER>{ $book/TITLE, $book/ISBN }</ANSWER>
∈
element ANSWER { TITLE } *

Finding an error by omission
Return title and ISBN of each book
Report an error any sub-expression of type (), other than the
expression () itself

Finding an error by assertion
Return title and ISBN of each book
define element ANSWER { TITLE, ISBN }
define element ISBN { xsd:string }
assert as element ANSWER (
)
Assertions might be added automatically, e.g. when there is a
global element declaration and no conﬂicting local declarations

Type Error 2: Improper type
define element BOOK { TITLE, PRICE, SHIPPING, SHIPCOST? }
define element SHIPPING { xsd:boolean }
define element SHIPCOST { xsd:decimal }
<ANSWER>{
$book/TITLE,
<TOTAL>{ $book/PRICE + $book/SHIPPING }</TOTAL>
}</ANSWER>
Type error: decimal + boolean

Type Error 3: Unhandled null
define element ANSWER { TITLE, TOTAL }
define element TOTAL { xsd:decimal }
assert as element ANSWER (
<ANSWER>{
$book/TITLE,
<TOTAL>{ $book/PRICE + $book/SHIPPING }</TOTAL>
}</ANSWER>
)
Type error: xsd : decimal? ⊆ xsd : decimal

Functions
Simplify book by dropping optional year
define element BOOK { @YEAR?, AUTHOR, TITLE }
define attribute YEAR { xsd:integer }
define element AUTHOR { xsd:string }
define function simple (element BOOK $b) returns element BOOK {
<BOOK> $b/AUTHOR, $b/TITLE </BOOK>
}
Compute total cost of book
define function cost (element BOOK $b) returns xsd:integer? {
$b/PRICE + $b/SHIPPING
}

A part hierarchy
define type PART { COMPLEX | SIMPLE }
define type COST { @ASSEMBLE | @TOTAL }
define element COMPLEX { @NAME & #COST, #PART* }
define element SIMPLE { @NAME & @TOTAL }
define attribute NAME { xsd:string }
define attribute ASSEMBLE { xsd:decimal }
define attribute TOTAL { xsd:decimal }
<COMPLEX NAME="system" ASSEMBLE="500.00">
<SIMPLE NAME="monitor" TOTAL="1000.00"/>
<SIMPLE NAME="keyboard" TOTAL="500.00"/>
<COMPLEX NAME="pc" ASSEMBLE="500.00">
<SIMPLE NAME="processor" TOTAL="2000.00"/>
<SIMPLE NAME="dvd" TOTAL="1000.00"/>
</COMPLEX>
</COMPLEX>

A recursive function
define function total (#PART $part) returns #PART {
if ($part instance of SIMPLE) then $part else
let $parts := $part/(COMPLEX | SIMPLE)/total(.)
return
<COMPLEX NAME="$part/@NAME" TOTAL="
$part/@ASSEMBLE + sum($parts/@TOTAL)">{
$parts
}</COMPLEX>
}
⇒
<COMPLEX NAME="system" TOTAL="5000.00">
<SIMPLE NAME="monitor" TOTAL="1000.00"/>
<SIMPLE NAME="keyboard" TOTAL="500.00"/>
<COMPLEX NAME="pc" TOTAL="3500.00">
<SIMPLE NAME="processor" TOTAL="2000.00"/>
<SIMPLE NAME="dvd" TOTAL="1000.00"/>
</COMPLEX>
</COMPLEX>

Wildcards types and computed names
Turn all attributes into elements, and vice versa
define function swizzle (element $x) returns element {
element {name($x)} {
for $a in $x/@* return element {name($a)} {data($a)},
for $e in $x/* return attribute {name($e)} {data($e)}
}
}
swizzle(<TEST A="a" B="b">
<C>c</C>
<D>d</D>
</TEST>)
⇒
<TEST C="c" D="D">
<A>a</A>
b
</TEST>
∈
element

Templates
Convert book listings to HTML format
<HTML><H1>My favorite books</H1>
<UL>{
<LI>
{ data($book/TITLE) },
{ data($book/@YEAR)[position()=last()] }.
</LI>
}</UL>
</HTML>
⇒
<HTML><H1>My favorite books</H1>
<UL>
<LI>Data on the Web, 2003.</LI>
<LI>XML in Scotland, 2002.</LI>
</UL>
</HTML>

XQueryX
A query in XQuery:
for $b in document("bib.xml")//book
where $b/publisher = "Morgan Kaufmann" and $b/year = "1998"
return $b/title
The same query in XQueryX:
<q:query xmlns:q="http://www.w3.org/2001/06/xqueryx">
<q:flwr>
<q:forAssignment variable="$b">
<q:step axis="SLASHSLASH">
<q:function name="document">
<q:constant datatype="CHARSTRING">bib.xml</q:constant>
</q:function>
<q:identifier>book</q:identifier>
</q:step>
</q:forAssignment>

XQueryX, continued
<q:where>
<q:function name="AND">
<q:function name="EQUALS">
<q:step axis="CHILD">
<q:variable>$b</q:variable>
<q:identifier>publisher</q:identifier>
</q:step>
<q:constant datatype="CHARSTRING">Morgan Kaufmann</q:consta
</q:function>
<q:function name="EQUALS">
<q:identifier>year</q:identifier>
</q:step>
<q:constant datatype="CHARSTRING">1998</q:constant>
</q:function>
</q:function>
</q:where>

XQueryX, continued2
<q:return>
<q:identifier>title</q:identifier>
</q:step>
</q:return>
</q:flwr>
</q:query>

Part II
XQuery laws and XQuery core

“I never come across one of Laplace’s ‘Thus it plainly
appears’ without feeling sure that I have hours of hard
work in front of me.”
— Bowditch

XPath and XQuery
Converting XPath into XQuery core
e/a
=
sidoaed(for $dot in e return $dot/a)
sidoaed = sort in document order and eliminate duplicates

Why sidoaed is needed
<WARNING>

Do not press button,
computer will explode!

</WARNING>
Select all nodes inside warning
/WARNING//*
⇒

Do not press button,
computer will explode!
,
not,
explode!

Why sidoaed is needed, continued
Select text in all emphasis nodes (list order)
for $x in /WARNING//* return $x/text()
⇒
"Do ",
" press button, computer will ",
"not",
"explode!"
Select text in all emphasis nodes (document order)
/WARNING//*/text()
=
sidoaed(for $x in /WARNING//* return $x/text())
⇒
"Do ",
"not",
" press button, computer will ",
"explode!"

Some laws
for $v in () return e
= (empty)
()
for $v in (e1 , e2) return e3
= (sequence)
(for $v in e1 return e3) , (for $v in e2 return e3)
data(element a { d })
= (data)
d

More laws
for $v in e return $v
= (left unit)
e
for $v in e1 return e2
= (right unit), if e1 is a singleton
let $v := e1 return e2
for $v1 in e1 return (for $v2 in e2 return e3)
= (associative)
for $v2 in (for $v1 in e1 return e2) return e3

Using the laws — evaluation
for $x in (<A>1</A>,<A>2</A>) return {data($x)}
= (sequence)
for $x in <A>1</A> return {data($x)} ,
for $x in <A>2</A> return {data($x)}
= (right unit)
let $x := <A>1</A> return {data($x)} ,
let $x := <A>2</A> return {data($x)}
= (let)
{data(<A>1</A>)} ,
{data(<A>2</A>)}
= (data)
1,2

Using the laws — loop fusion
let $b := for $x in $a return { data($x) }
return for $y in $b return <C>{ data($y) }</C>
= (let)
for $y in (
for $x in $a return { data($x) }
) return <C>{ data($y) }</C>
= (associative)
for $x in $a return
(for $y in { data($x) } return <C>{ data($y) }</C>)
= (right unit)
for $x in $a return <C>{ data({ data($x) }) }</C>
= (data)
for $x in $a return <C>{ data($x) }</C>

An example in XQuery
Join books and review by title
for $b in /BOOKS/BOOK, $r in /REVIEWS/BOOK
where $b/TITLE = $r/TITLE
return
<BOOK>{
$b/TITLE,
$b/AUTHOR,
$r/REVIEW
}</BOOK>

The same example in XQuery core
for $b in (
for $dot in $root return
for $dot in $dot/child::BOOKS return $dot/child::BOOK
) return
for $r in (
for $dot in $root return
for $dot in $dot/child::REVIEWS return $dot/child::BOOK
) return
if (
not(empty(
for $v1 in (
for $dot in $b return $dot/child::TITLE
) return
for $v2 in (
for $dot in $r return $dot/child::TITLE
) return
if (eq($v1,$v2)) then $v1 else ()
))
) then (
element BOOK {
for $dot in $b return $dot/child::TITLE ,
for $dot in $b return $dot/child::AUTHOR ,
for $dot in $r return $dot/child::REVIEW
}
)
else ()

XQuery core: a syntactic subset of XQuery
• only one variable per iteration by for
• no where clause
• only simple path expressions iteratorVariable/Axis::NodeTest
• only simple element and attribute constructors
• sort by
• function calls

The 4 C’s of XQuery core
• Closure:
input: XML node sequence
output: XML node sequence
• Compositionality:
expressions composed of expressions
no side-eﬀects
• Correctness:
dynamic semantics (query evaluation time)
static semantics (query compilation time)
• Completeness:
XQuery surface syntax can be expressed completely
relationally complete (at least)

“Besides it is an error to believe that rigor in the proof is
the enemy of simplicity. On the contrary we find it con-
firmed by numerous examples that the rigorous method
is at the same time the simpler and the more easily com-
prehended. The very effort for rigor forces us to find out
simpler methods of proof.”
— Hilbert

Part III
XQuery Processing Model

Analysis Step 1: Map to XQuery Core
Query Analysis Step 1:
Mapping to XQuery Core
XQuery
Parser
XML Schema
Parser
XQuery
Normalizer
XQuery
Operator Tree
Schema Type
Tree
XQuery
Expression
XML Schema
Description
XML
Document
XQuery Core
Operator Tree

Analysis Step 2: Infer and Check Type
Type Inference & Check
XQuery
Parser
XML Schema
Parser
Type Inference &
Type Check
XQuery
Normalizer
Result Type
Tree
Static
Error
XQuery
Operator Tree
Schema Type
Tree
XQuery
Expression
XML Schema
Description
XML
Document
XQuery Core
Operator Tree

Analysis Step 3: Generate DM Accessors
XQuery Compilation
XQuery
Parser
XML Schema
Parser
Type Inference &
Type Check
XQuery
Normalizer
Result Type
Tree
Static
Error
XQuery
Operator Tree
Schema Type
Tree
XQuery
Compiler
DM Accessors
Functions & Ops
XQuery
Expression
XML Schema
Description
XML
Document
XQuery Core
Operator Tree

Eval Step 1: Generate DM Instance
Query
Analysis
XQuery
Parser
Wellformed XML
Parser
XML Schema
Parser
Type Inference &
Type Check
XQuery
Normalizer
Result Type
Tree
Static
Error
XQuery
Operator Tree
Data Model
Instance
Schema Type
Tree
XQuery
Compiler
DM Accessors
Functions & Ops
XQuery
Expression
XML Schema
Description
XML
Document
XQuery Core
Operator Tree
Query Evaluation Step 1:
Instantiating the Data Model

Eval Step 2: Validate and Assign Types
Query
Analysis
XQuery
Parser
Wellformed XML
Parser
XML Schema
Parser
Type Inference &
Type Check
XML Schema
Validator
XQuery
Normalizer
Data Model
Instance + Types
Result Type
Tree
Static
Error
XQuery
Operator Tree
Data Model
Instance
Schema Type
Tree
XQuery
Compiler
DM Accessors
Functions & Ops
XQuery
Expression
XML Schema
Description
XML
Document
Validation
Error
XQuery Core
Operator Tree
Validation and Type Assignment

Eval Step 3: Query Evaluation
Query
Analysis
XQuery
Processor
XQuery
Parser
Wellformed XML
Parser
XML Schema
Parser
Type Inference &
Type Check
XML Schema
Validator
XQuery
Normalizer
Data Model
Instance + Types
Result Type
Tree
Static
Error
Result
Instance (+ Types)
XQuery
Operator Tree
Data Model
Instance
Schema Type
Tree
XQuery
Compiler
DM Accessors
Functions & Ops
XQuery
Expression
XML Schema
Description
XML
Document
Validation
Error
Dynamic
Error
XQuery Core
Operator Tree
Query Evaluation

XQuery Processing Model
Query
Analysis
XQuery
Processor
XQuery
Parser
Wellformed XML
Parser
XML Schema
Parser
Type Inference &
Type Check
XML Schema
Validator
XQuery
Normalizer
Data Model
Instance + Types
Result Type
Tree
Static
Error
Result
Instance (+ Types)
XQuery
Operator Tree
Data Model
Instance
Schema Type
Tree
XQuery
Compiler
DM Accessors
Functions & Ops
XQuery
Expression
XML Schema
Description
XML
Document
Validation
Error
Dynamic
Error
XQuery Core
Operator Tree
Query
Evaluation

XQuery Processing Model: Idealizations
• Query normalization and compilation:
static type information is useful for logical optimization.
a real implementation translates to and optimizes further on
the basis of a physical algebra.
• Loading and validating XML documents:
a real implementation can operate on typed datamodel in-
stances directly.
• Representing data model instances:
a real implementation is free to choose native, relational, or
object-oriented representation.

XQuery et al. Speciﬁcations
query-datamodel
xquery-operators
xquery
(xpath 2.0)
XML 1.0
xmlschema-
formal
query-semantics
static sem.
xmlschema-1
xmlschema-2
query-semantics
mapping to core
XPath/XQuery
Datamodel
Result Type
Tree
Static
Error
Result
Instance (+ Types)
XQueryX
(e.g)
XPath/XQuery
Datamodel
Schema
Components
query-semantics
dynamic sem. query-datamodel +
xquery-operators
XQuery
Syntax
XML
Document
Validation
Error
Dynamic
Error
XQuery Core
Syntax
XML Schema
XML Query
WG
XSLT
WG
XML Schema
WG

XQuery et al. Speciﬁcations: Legend
• XQuery 1.0: An XML Query Language (WD)
http://www.w3.org/TR/xquery/
• XML Syntax for XQuery 1.0 (WD)
http://www.w3.org/TR/xqueryx/
• XQuery 1.0 Formal Semantics (WD)
http://www.w3.org/TR/query-semantics/
xquery core syntax, mapping to core,
static semantics, dynamic semantics
• XQuery 1.0 and XPath 2.0 Data Model (WD)
http://www.w3.org/TR/query-datamodel/
node-constructors, value-constructors, accessors
• XQuery 1.0 and XPath 2.0 Functions and Operators (WD)
http://www.w3.org/TR/xquery-operators/
• XML Schema: Formal Description (WD)
http://www.w3.org/TR/xmlschema-formal/
• XML Schema Parts (1,2) (Recs)
http://www.w3.org/TR/xmlschema-1/
http://www.w3.org/TR/xmlschema-2/

Without Schema (1) Map to XQuery Core
...
AnyType
FOR $v IN $d/au
RETURN {$v}
<au>Paul</au>
<au>Mary</au>
FOR $v IN
(FOR $dot IN $d RETURN child::““:au)
RETURN ELEMENT ““:p {$v}
XQuery
Parser
XQuery
Normalizer

Without Schema (2) Infer Type
ELEMENT p {
ELEMENT au {AnyType}*
}*
...
AnyType
FOR $v IN $d/au
RETURN {$v}
<au>Paul</au>
<au>Mary</au>
FOR $v IN
XQuery
Parser
Type Inference &
Type Check
XQuery
Normalizer

Without Schema (3) Evaluate Query
ELEMENT p {
ELEMENT au {AnyType}*
}*
<au>Paul<au>
<au>Mary<au>
... ...
AnyType
append(
map($v, element-node(“p“,(),(),$v,“Any“)),
append(map ($dot,children($dot)),$d)
)
)
FOR $v IN $d/au
RETURN {$v}
<au>Paul</au>
<au>Mary</au>
FOR $v IN
XQuery
Processor
XQuery
Parser
Wellformed XML
Parser
Type Inference &
Type Check
XQuery
Normalizer
XQuery
Compiler

Without Schema (4) Dynamic Error
ELEMENT p {
ELEMENT au {double}*
}*
... ...
AnyType
append(
map($v, element-node(“p“,(),(),number($v)+1,“p“)),
)
)
FOR $v IN $d/au
RETURN {$v+1}
<au>Paul</au>
<au>Mary</au>
FOR $v IN
RETURN ELEMENT ““:p {
number($v)+1}
XQuery
Processor
XQuery
Parser
Wellformed XML
Parser
Type Inference &
Type Check
XQuery
Normalizer
XQuery
Compiler
Dynamic
Error

With Schema (1) Generate Types
GROUP d {ELEMENT au*}
ELEMENT au {string}
FOR $v IN $d/au
RETURN {$v}
<au>Paul</au>
<au>Mary</au>
XML Schema
Parser
<element name= “au“
type= “string“/>
<group name= “d“>
<element ref= “au“
minOccurs= “0“
maxOccurs=“unbounded“/>
</group>

With Schema (2) Infer Type
ELEMENT p {
ELEMENT au {string}
}*
...
ELEMENT au {string}
FOR $v IN $d/au
RETURN {$v}
<au>Paul</au>
<au>Mary</au>
FOR $v IN
XQuery
Parser
XML Schema
Parser
Type Inference &
Type Check
XQuery
Normalizer
minOccurs= “0“
</group>

With Schema (3) Validate and Evaluate
<au>“Paul“</au>
<au>“Mary“</au>
ELEMENT p {
ELEMENT au {string}
}*
<au>“Paul“<au>
<au>“Mary“<au>
... ...
ELEMENT au {string}
append(
map($v, element-node(“p“,(),(),$v,“p“)),
)
)
FOR $v IN $d/au
RETURN {$v}
<au>Paul</au>
<au>Mary</au>
FOR $v IN
XQuery
Processor
XQuery
Parser
Wellformed XML
Parser
XML Schema
Parser
Type Inference &
Type Check
XML Schema
Validator
XQuery
Normalizer
XQuery
Compiler
minOccurs= “0“
</group>

With Schema (4) Static Error
ELEMENT p {ELEMENT au}* ⊄
ELEMENT p {ELEMENT au}+
Static
Error
...
ELEMENT au {string}
ELEMENT p {ELEMENT au}+
ASSERT AS ELEMENT p
FOR $v IN $d/au
RETURN {$v}
<au>Paul</au>
<au>Mary</au>
FOR $v IN
XQuery
Parser
XML Schema
Parser
Type Inference &
Type Check
XQuery
Normalizer
<element name= “p“>
<complexType>
minOccurs= “1“
</complexType>
</element>
minOccurs= “0“
</group>

Part IV
From XML Schema
to XQuery Types

XML Schema vs. XQuery Types
• XML Schema:
structural constraints on types
name constraints on types
range and identity constraints on values
type assignment and determinism constraint
• XQuery Types as a subset:
structural constraints on types
local and global elements
derivation hierarchies, substitution groups by union
name constraints are an open issue
no costly range and identity constraints
• XQuery Types as a superset:
XQuery needs closure for inferred types, thus no determinism
constraint and no consistent element restriction.

XQuery Types
unit type u ::= string string
| integer integer
| attribute a { t } attribute
| attribute * { t } wildcard attribute
| element a { t } element
| element * { t } wildcard element
type t ::= u unit type
| () empty sequence
| t , t sequence
| t | t choice
| t? optional
| t+ one or more
| t* zero or more
| x type reference

Expressive power of XQuery types
Tree grammars and tree automata
deterministic non-deterministic
top-down Class 1 Class 2
bottom-up Class 2 Class 2
Tree grammar Class 0: DTD (global elements only)
Tree automata Class 1: Schema (determinism constraint)
Tree automata Class 2: XQuery, XDuce, Relax
Class 0 < Class 1 < Class 2
Class 0 and Class 2 have good closure properties.
Class 1 does not.

Importing schemas and using types
• SCHEMA targetNamespace
SCHEMA targetNamespace AT schemaLocation
import schemas
• VALIDATE expr
validate and assign types to the results of expr
(a loaded document or a query)
• ASSERT AS type (expr)
check statically whether the type of (expr) matches type.
• TREAT AS type (expr)
check dynamically whether the type of (expr) matches type
• CAST AS type (expr)
convert simple types according to conversion table
open issue: converting complex types.

Primitive and simple types
Schema
<xsd:simpleType name="myInteger">
<xsd:restriction base="xsd:integer">
<xsd:minInclusive value="10000"/>
<xsd:maxInclusive value="99999"/>
</xsd:restriction>
</xsd:simpleType>
<xsd:simpleType name="listOfMyIntType">
<xsd:list itemType="myInteger"/>
</xsd:simpleType>
XQuery type
DEFINE TYPE myInteger { xsd:integer }
DEFINE TYPE listOfMyIntType { myInteger* }

Local simple types
Schema
<xsd:element name="quantity">
<xsd:simpleType>
<xsd:restriction base="xsd:positiveInteger">
<xsd:maxExclusive value="100"/>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
XQuery type
DEFINE ELEMENT quantity { xsd:positiveInteger }
Ignore: id, ﬁnal, annotation, minExclusive, minInclusive, max-
Exclusive, maxInclusive, totalDigits, fractionDigits, length, min-
Length, maxLength, enumeration, whiteSpace, pattern at-
tributes.

Complex-type declarations (1)
Schema
<xsd:element name="purchaseOrder" type="PurchaseOrderType"/>
<xsd:element name="comment" type="xsd:string"/>
<xsd:complexType name="PurchaseOrderType">
<xsd:sequence>
<xsd:element name="shipTo" type="USAddress"/>
<xsd:element name="billTo" type="USAddress"/>
<xsd:element ref="comment" minOccurs="0"/>
<xsd:element name="items" type="Items"/>
</xsd:sequence>
<xsd:attribute name="orderDate" type="xsd:date"/>
</xsd:complexType>

Complex-type declarations (2)
XQuery type
DEFINE ELEMENT purchaseOrder { PurchaseOrderType }
DEFINE ELEMENT comment { xsd:string }
DEFINE TYPE PurchaseOrderType {
ATTRIBUTE orderDate { xsd:date }?,
ELEMENT shipTo { USAddress },
ELEMENT billTo { USAddress },
ELEMENT comment?,
ELEMENT items { Items },
}
<sequence> ⇒ ’,’
<choice> ⇒ ’|’
<all> ⇒ ’&’
Open issue: name of group PurchaseOrderType is insigniﬁcant.

Local elements and anonymous types (1)
Schema
<xsd:complexType name="Items"
<xsd:sequence>
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:simpleType>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="partNum" type="SKU" use="required"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
</xsd:complexType>

Local elements and anonymous types (2)
XQuery type
DEFINE TYPE Items {
ELEMENT item {
ELEMENT productName { xsd:string },
ELEMENT quantity { xsd:positiveInteger },
ELEMENT USPrice { xsd:decimal },
ELEMENT comment?,
ELEMENT shipDate { xsd:date }?,
ATTRIBUTE partNum { SKU }
}*
}
Local elements are supported by nested declarations

Occurrence constraints
Schema
<xsd:simpleType name="SomeUSStates">
<xsd:restriction base="USStateList">
<xsd:length value="3"/>
</xsd:restriction>
</xsd:simpleType>
XQuery type
DEFINE TYPE SomeUSStates { USState+ }
Only ? for {0,1}, * for {0,unbounded}, + for {1, unbounded}
More speciﬁc occurrence constraints only by explicit enumera-
tion.

Derivation by restriction (1)
Schema
<complexType name="ConfirmedItems">
<complexContent>
<restriction base="Items">
<xsd:sequence>
<element name="item" minOccurs="1" maxOccurs="unbounded">
<xsd:complexType>
<xsd:sequence>
<xsd:element name="productName" type="xsd:string"/>
<xsd:simpleType>
</xsd:restriction>
</xsd:simpleType>
</xsd:element>
<xsd:element name="USPrice" type="xsd:decimal"/>
<xsd:element name="shipDate" type="xsd:date" minOccurs="0"/>
</xsd:sequence>
<xsd:attribute name="partNum" type="SKU" use="required"/>
</xsd:complexType>
</xsd:element>
</xsd:sequence>
...

Derivation by restriction (2)
XQuery type
An instance of type ConﬁrmedItems is also of type Items.
DEFINE TYPE ConfirmedItems {
ELEMENT item {
ELEMENT productName { xsd:string },
ELEMENT quantity { xsd:positiveInteger },
ELEMENT USPrice { decimal },
ELEMENT ipo:comment?,
ELEMENT shipDate { xsd:date }?,
ATTRIBUTE partNum { SKU }
}+
}
Only structural part is preserved, complex type name Con-
ﬁrmedItem is not preserved (open issue).

Derivation by extension (1)
Schema
<complexType name="Address">
<element name="street" type="string"/>
<element name="city" type="string"/>
</complexType>
<complexType name="USAddress">
<complexContent>
<extension base="Address">
<element name="state" type="USState"/>
<element name="zip" type="positiveInteger"/>
</extension>
</complexContent>
</complexType>
<complexType name="UKAddress">
<complexContent>
<extension base="Address">
<element name="postcode" type="UKPostcode"/>
<attribute name="exportCode" type="positiveInteger" fixed="1"/>
</extension>
</complexContent>
</complexType>

Derivation by extension (2)
XQuery type
DEFINE TYPE Address {
ELEMENT street { xsd:string },
ELEMENT city { xsd:string }
( () {!-- possibly empty, except if Address is abstract --}
{!-- extensions from USAddress --}
| (ELEMENT state { USState },
ELEMENT zip { xsd:positiveInteger })
!-- extensions from UKAddress --
| (ELEMENT postcode { UKPostcode },
ATTRIBUTE exportCode { xsd:positiveInteger })
)
}
Group contains base type and all types derived from it.
Thereby USAddress and UKAddress are substitutable for Ad-
dress.

Substitution groups (1)
Schema
<element name="shipTo" address="ipo:Address">
<element name="shipToUS" type="ipo:USAddress"
substitutionGroup="ipo:shipTo"/>
<element name="order">
<complexType>
<sequence>
<element name="item" type="integer"/>
<element ref="shipTo"/>
</sequence>
</complexType>
<element>

Substitution groups (2)
XQuery types
DEFINE ELEMENT shipTo { Address }
DEFINE ELEMENT shipToUS { USAddress }
DEFINE TYPE shipTo_group {
shipTo | shipToUS
}
DEFINE ELEMENT order {
ELEMENT item { integer },
shipTo_group
}
Union semantics: group contains ’representative’ element & all
elements in its substitution group

XML Schema vs. XQuery Types - summary
XQuery types are aware of
• Global and local elements
• Sequence, choice, and simple repetition
• Derivation hierarchies and substitution groups
• Mixed content
• Built-in simple types
XQuery types are not aware of
• complex type names
open issue
• value constraints
check with VALIDATE

Part V
Type Inference and Subsumption

What is a type system?
• Validation: Value has type
v ∈ t
• Static semantics: Expression has type
e : t
• Dynamic semantics: Expression has value
e ⇒ v
• Soundness theorem: Values, expressions, and types match
if e : t and e ⇒ v then v ∈ t

What is a type system? (with variables)
• Validation: Value has type
v ∈ t
• Static semantics: Expression has type
¯x : ¯t e : t
• Dynamic semantics: Expression has value
¯x ⇒ ¯v e ⇒ v
• Soundness theorem: Values, expressions, and types match
if ¯v ∈ ¯t and ¯x : ¯t e : t and ¯x ⇒ ¯v e ⇒ v then v ∈ t

Documents
string s ::= "" , "a", "b", ..., "aa", ...
integer i ::= ..., -1, 0, 1, ...
document d ::= s string
| i integer
| attribute a { d } attribute
| element a { d } element
| () empty sequence
| d , d sequence

Type of a document
• Overall Approach:
Walk down the document tree
Prove the type of d by proving the types of its con-
stituent nodes.
• Example:
d ∈ t
element a { d } ∈ element a { t }
(element)
Read: the type of element a { d } is element a { t } if the
type of d is t.

Type of a document — d ∈ t
s ∈ string
(string)
i ∈ integer
(integer)
d ∈ t
element a { d } ∈ element a { t }
(element)
d ∈ t
element a { d } ∈ element * { t }
(any element)
d ∈ t
attribute a { d } ∈ element a { t }
(attribute)
d ∈ t
attribute a { d } ∈ element * { t }
(any attribute)
d ∈ t define group x { t }
d ∈ x
(group)

Type of a document, continued
() ∈ ()
(empty)
d1 ∈ t1 d2 ∈ t2
d1 , d2 ∈ t1 , t2
(sequence)
d1 ∈ t1
d1 ∈ t1 | t2
(choice 1)
d2 ∈ t2
d2 ∈ t1 | t2
(choice 2)
d ∈ t+?
d ∈ t*
(star)
d ∈ t , t*
d ∈ t+
(plus)
d ∈ () | t
d ∈ t?
(option)

Type of an expression
• Overall Approach:
Walk down the operator tree
Compute the type of expr from the types of its con-
stituent expressions.
• Example:
e1 ∈ t1 e2 ∈ t2
e1 , e2 ∈ t1 , t2
(sequence)
Read: the type of e1 , e2 is a sequence of the type of e1 and
the type of e2

Type of an expression — E e ∈ t
environment E ::= $v1 ∈ t1, . . . , $vn ∈ tn
E contains $v ∈ t
E $v ∈ t
(variable)
E e1 ∈ t1 E, $v ∈ t1 e2 ∈ t2
E let $v := e1 return e2 ∈ t2
(let)
E () ∈ ()
(empty)
E e1 ∈ t1 E e2 ∈ t2
E e1 , e2 ∈ t1 , t2
(sequence)
E e ∈ t1 t1 ∩ t2 = ∅
E treat as t2 (e) ∈ t2
(treat as)
E e ∈ t1 t1 ⊆ t2
E assert as t2 (e) ∈ t2
(assert as)

Typing FOR loops
Return all Amazon and Fatbrain books by Buneman
define element AMAZON-BOOK { TITLE, AUTHOR+ }
define element FATBRAIN-BOOK { AUTHOR+, TITLE }
define element BOOKS { AMAZON-BOOK*, FATBRAIN-BOOK* }
for $book in (/BOOKS/AMAZON-BOOK, /BOOKS/FATBRAIN-BOOK)
where $book/AUTHOR = "Buneman" return
$book
∈
( AMAZON-BOOK | FATBRAIN-BOOK )*
E e1 ∈ t1
E, $x ∈ P(t1) e2 ∈ t2
E for $x in e1 return e2 ∈ t2 · Q(t1)
(for)
P(AMAZON-BOOK*,FATBRAIN-BOOK*) = AMAZON-BOOK | FATBRAIN-BOOK
Q(AMAZON-BOOK*,FATBRAIN-BOOK*) = *

Prime types
unit type u ::= string string
| integer integer
| attribute a { t } attribute
| attribute * { t } any attribute
| element a { t } element
| element * { t } any element
prime type p ::= u unit type
| p | p choice

Quantiﬁers
quantiﬁer q ::= () exactly zero
| - exactly one
| ? zero or one
| + one or more
| * zero or more
t · () = ()
t · - = t
t · ? = t?
t · + = t+
t · * = t*
, () - ? + *
() () - ? + *
- - + + + +
? ? + * + *
+ + + + + +
* * + * + *
| () - ? + *
() () ? ? * *
- ? - ? + *
? ? ? ? * *
+ * + * + *
* * * * * *
· () - ? + *
() () () () () ()
- () - ? + *
? () ? ? * *
+ () + * + *
* () * * * *
≤ () - ? + *
() ≤ ≤ ≤
- ≤ ≤ ≤ ≤
? ≤ ≤
+ ≤ ≤
* ≤

Factoring
P (u) = {u}
P (()) = {}
P (t1 , t2) = P (t1) ∪ P (t2)
P (t1 | t2) = P (t1) ∪ P (t2)
P (t?) = P (t)
P (t+) = P (t)
P (t*) = P (t)
Q(u) = -
Q(()) = ()
Q(t1 , t2) = Q(t1) , Q(t2)
Q(t1 | t2) = Q(t1) | Q(t2)
Q(t?) = Q(t) · ?
Q(t+) = Q(t) · +
Q(t*) = Q(t) · *
P(t) = () if P (t) = {}
= u1 | · · · | un if P (t) = {u1, . . . , un}
Factoring theorem. For every type t, prime type p, and quanti-
ﬁer q, we have t ⊆ p · q iﬀ P(t) ⊆ p? and Q(t) ≤ q.
Corollary. For every type t, we have t ⊆ P(t) · Q(t).

Uses of factoring
E e1 ∈ t1
E, $x ∈ P(t1) e2 ∈ t2
E for $x in e1 return e2 ∈ t2 · Q(t1)
(for)
E e ∈ t
E unordered(e) ∈ P(t) · Q(t)
(unordered)
E e ∈ t
E distinct(e) ∈ P(t) · Q(t)
(distinct)
E e1 ∈ integer · q1 q1 ≤ ?
E e2 ∈ integer · q2 q2 ≤ ?
E e1 + e2 ∈ integer · q1 · q2
(arithmetic)

Subtyping and type equivalence
Definition. Write t1 ⊆ t2 iff for all d, if d ∈ t1 then d ∈ t2.
Definition. Write t1 = t2 iff t1 ⊆ t2 and t2 ⊆ t1.
Examples
t ⊆ t? ⊆ t*
t ⊆ t+ ⊆ t*
t1 ⊆ t1 | t2
t , () = t = () , t
t1 , (t2 | t3) = (t1 , t2) | (t1 , t3)
element a { t1 | t2 } = element a { t1 } | element a { t2 }
Can decide whether t1 ⊆ t2 using tree automata:
Language(t1) ⊆ Language(t2) iff
Language(t1) ∩ Language(Complement(t2)) = ∅.

Part VI
Further reading and experimenting

Links
Phil’s XML page
http://www.research.avayalabs.com/~wadler/xml/
W3C XML Query page
http://www.w3.org/XML/Query.html
XML Query demonstrations
Galax - AT&T, Lucent, and Avaya
http://www-db.research.bell-labs.com/galax/
Quip - Software AG
http://www.softwareag.com/developer/quip/
XQuery demo - Microsoft
http://131.107.228.20/xquerydemo/
Fraunhofer IPSI XQuery Prototype
http://xml.ipsi.fhg.de/xquerydemo/
XQengine - Fatdog
http://www.fatdog.com/
X-Hive
http://217.77.130.189/xquery/index.html
OpenLink
http://demo.openlinksw.com:8391/xquery/demo.vsp

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