xmldb 04 queryi - tu braunschweig · 2008. 11. 18. · •querying xml documents – "querying xml...

XML Databases4. XML Query Languages, 17.11.08

Silke EcksteinAndreas KupferInstitut für InformationssystemeTechnische Universität Braunschweighttp://www.ifis.cs.tu-bs.de

4.1 Introduction

4.2 XPath

4.3 XPointer

4. XML Query Languages I

4.4 XLink

4.5 Overview

4.6 References

2XML Databases – Silke Eckstein – Institut für Informationssysteme – TU Braunschweig

• Querying XML Documents– "Querying XML data" essentially means

• to identify (or address) nodes,

• to test certain further properties of these nodes,

• then to operate on the matches,

• and finally, to construct result XML documents as answers.

4.1 Introduction

– In the XML context, the language XQuery plays the role that SQL has in relational databases.

– XQuery can express all of the above constituents of XML querying:• XPath, as an embedded sublanguage, expresses the locate and test

parts;

• XQuery can then iterate over selected parts, operate on and construct answers from these.

• There are more XML languages that make use of XPath as embedded sublanguages.

XML Databases – Silke Eckstein – Institut für Informationssysteme – TU Braunschweig 3[Scholl07]

• Xpath – Navigational access to XML documents

– In a sense, the traversal or navigation of trees of XML nodes lies at the core of every XML query language.

– To this end, XQuery embeds XPath as its tree navigation sub-language:

4.1 Introduction

sub-language:

• Every XPath expression also is a correct XQuery expression.

• XPath 2.0: http://www.w3.org/TR/xpath20/ .

– Since navigation expressions extract (potentially huge volumes of) nodes from input XML documents, the efficient implementation of the sub-language XPath is a prime concern when it comes to the construction of XQuery processors.

XML Databases – Silke Eckstein – Institut für Informationssysteme – TU Braunschweig 4

• XPath as an embedded sublanguage– XPath is a declarative, expression-based language to locateand test doc nodes (with lots of syntactic sugar to make querying sufficiently sweet).

– Addressing document nodes is a core task in the XML world.XPath occurs as an embedded sub-language in

4.1 Introduction

XPath occurs as an embedded sub-language in• XSLT (extract and transform XML document [fragments] into XML,

XHTML, PDF, . . . )

• XQuery (compute with XML document nodes and contents, compute new docs, . . . )

• XPointer (representation of the address of one or more doc nodes in a given XML document)

• XMLSchema (represent sets of elements as scopes for uniqueness or key concepts)


• XML Pointer Language

– Allows to use XPath expressions in URIs

– Used to identify ranges within XML documents

• XML Linking Language

– To define links, which can…

4.1 Introduction


• Be as easy as HTML links

• Have multiple end points

• Be stored independently of the referenced documents

– Allows to add metadata to links

– Uses XPointer and XPath


4.1 Introduction

4.2 XPath

4.3 XPointer


4.4 XLink

4.5 Overview

4.6 References


• Context node

– In XPath, a path traversal starts off from a sequence of context nodes.

• XPath navigation syntax is simple:

4.2 XPath

An XPath step

• It is a common error in XQuery expressions to try and start an XPath traversal without the context node sequence being actually defined.


An XPath step

cs0/step

• cs0 denotes the context node sequence, from which a navigation in direction step is taken.

• Multiple steps

– An XPath navigation may consist of multiple stepsstepi ,i ≥ 1 taken in succession.

– Step step1 starts o from the context node sequence cs0 and arrives at a sequence of new nodes cs1.

4.2 XPath

arrives at a sequence of new nodes cs1.

– cs1 is then used as the new context node sequence for step2, and so on.


Multi-step XPath path

cs0/step1/step2/…

≡

((cs0/step1)/step2)/…

cs1

• XPath location steps

– A step (or location step) stepi specifies

4.2 XPath

1 n

XPath step

ax :: nt [p1] … [pn]

A step (or location step) stepi specifies

1. the axis ax, i.e., the direction of navigation taken from the context nodes,

2. a node test nt, which can be used to navigate to nodes of certain kind (e.g., only attribute nodes) or name,

3. optional predicates pi which further filter the sequence of nodes we navigated to.


• XPath axes

– XPath defines a family of 12 axes allowing for flexible navigation within the node hierarchy of an XML tree.

– XPath axes semantics

• ○○○○ marks the context node

4.2 XPath

• ○○○○ marks the context node

• @ marks attribute nodes,

• ● represents any other node kind (inner ● nodes are element nodes).


@@ @@

@@

@@

• XPath axes: child, parent, attribute

– The child axis does not navigate to the attribute nodes below ○○○○.

4.2 XPath

– The only way to access attributes is to use the attribute axis explicitly.


@@ @@

@@

@@

• XPath axes: descendant, ancestor, self

– In a sense, descendant and ancestor represent the transitive closures of child and parent,

4.2 XPath

closures of child and parent, respectively.


@@ @@

@@

@@

• XPath axes: preceding, following, ancestor-or-self

– Note: In the serialized XML document, nodes in the preceding (following) axis appear completely

4.2 XPath

axis appear completelybefore (after) the context node.


@@ @@

@@

@@

• XPath axes: preceding-sibling, following-sibling, descendant-or-self

4.2 XPath


@@ @@

@@

@@

• XPath axes: Examples (1)

– In these first examples, there is a single initial context node, i.e., a context node sequence of length 1: the root element a.

• Here, we set the node test nt to simply node() which

4.2 XPath

• Here, we set the node test nt to simply node() which

means to not filter any nodes selected by the axis.


XPath example

(

f

)/child::node()

(

f

,

)

⇒⇒⇒⇒


4.2 XPath

XPath example

(

f

)/attribute::node()

attribute b { "0" }⇒⇒⇒⇒


)/attribute::node()

XPath example

(

f

)/descendant::node()

(f,

f,

text { "f" },

),

)

⇒⇒⇒⇒


4.2 XPath

XPath example

(

f

)/child::node()/child::node()

(f,

)⇒⇒⇒⇒

– Notes:

• If an extracted node has no suitable XML representation by itself, XQuery serializes the result using the XQuery node constructor syntax, e.g.,

attribute b {"0" } or text { "f" } .

• Nodes are serialized showing their content. This does not imply that all of the content nodes have been selected by the XPath expression!


)/child::node()/child::node()

• XPath results: Order & duplicates

– XPath Semantics: The result node sequence of any XPath navigation is returned in document order with no duplicate nodes (remember: node identity).

– Examples:

4.2 XPath

– Examples:


Duplicate nodes are removed in XPath results . . .

(

f

)/child::node()/parent::node()

...

⇒⇒⇒⇒

(

)/child::node()/following-sibling::node()

(,

)⇒⇒⇒⇒

• XPath: Results in document order

– Note:

4.2 XPath

XPath: context node sequence of length > 1

(,

)/child::node()(,,

,)⇒⇒⇒⇒

– Note:

• The XPath document order semantics require to occur before and to occur before .

– The result (,,,) would have been OK as well.

– In contrast, the result (,,,) is inconsistent with respect to the order of nodes from separate trees!


• XPath: Node test

– Once an XPath step arrives at a sequence of nodes, we may apply a node test to filter nodes based on kind and name.

4.2 XPath

Kind Test Semantics

node() let any node pass


node() let any node pass

text() preserve text nodes only

attribute() preserve attribute nodes only

comment() preserve comment nodes only

processing-instruction() preserve processing instructions

processing-instruction(p) preserve processing instructions of the form

document-node() preserve the (invisible) document root node

• XPath: Name test

– A node test may also be a name test, preserving only those element or attribute nodes with matching names.

4.2 XPath

NameTest Semantics

name preserve element nodes with tag name only

– Note:

• In general we will have cs/ax::* ⊆⊆⊆⊆ cs/ax::node().


name preserve element nodes with tag name only(for attribute axis: preserve attributes)

* preserve element nodes with arbitrary tag names(for attribute axis: preserve attributes)

• XPath: Node test example

4.2 XPath

Collect and concatenate all text nodes of a tree

string-join

(AB

C

/descendant-or-self::node()/child::text()

– The XQuery builtin function string-join has signature string-join(string*, string) as string .


, "")

Equivalent: compute the string value of node a

string(AB

C

)

"ABC"⇒⇒⇒⇒

• XPath: Ensuring order is not for free– The strict XPath requirement to construct a result in

document order may imply sorting effort depending on the actual XPath implementation strategy used by the processor.

4.2 XPath

(

(,

⇒⇒⇒⇒

• In many implementations, the descendant-or-self::x step will yield the context node sequence (...,... ) for the child::y step.

• Such implementations thus will typically extract before from the input document.


)/descendant-or-self::x/child::y

(,

)⇒⇒⇒⇒

• XPath: Predicates– The optional third component of a step formulates a

list of predicates [p1]...[pn] against the nodes selected by an axis.

– XPath predicate evaluation:

4.2 XPath

– XPath predicate evaluation:• Predicates have higher precedence than the XPath step

operator /,i.e.:cs/step[p1][p2] ≡ cs/((step [p1])[p2])

• The pi are evaluated left-to-right for each node in turn. In pi, the current context item is available as '.'.

– Context item: predicates may be applied to sequences of arbitrary items (not only nodes)


• XPath: Predicates– An XPath predicate pi, may be any XQuery expression evaluating

to some value v. To finally evaluate the predicate, XQuery computes the effective Boolean value ebv(v).

4.2 XPath

Value v ebv(v)

() false()

• Item x ∉ {0,""; NaN; false()}, items xi arbitrary. Builtin function boolean(item*) as boolean also computes the eective Boolean value.


0, NaN false()

"" false()

false() false()

x true()

(x1, x2,..., xn) true()

• XPath: Predicate example

4.2 XPath

Select all elements with an id attribute

(

e

(

...

,

,

e

⇒⇒⇒⇒

– Note: Existential semantics of path predicates.


)/descendant-or-self::*[./attribute::id]

e

)

Select all elements with a "b" grandchild element(

e

)/descendant-or-self::*[./child::*/child::b]

⇒⇒⇒⇒

• XPath: Predicates and atomization

– In XQuery, if any item x – atomic value or node – is used in a context where a value is required, atomization is applied to convert x into an atomic value.

• Nodes in value contexts commonly appear in XPath predicates. Consider:

4.2 XPath

Consider:


Value comparison in a predicate

(

42

42

43

)/descendant-or-self::*[. eq 42]

(42,

42,

42

)⇒⇒⇒⇒

• Atomization– Atomization turns a sequence (x1,...,xn) of

items into a sequence of atomic values (v1,...,vn):1. If xi is an atomic value, vi ≡ xi2. if xi is a node, vi is the typed value of xi .

4.2 XPath

2. if xi is a node, vi is the typed value of xi .

– Remember: the typed value is equal to the string value if xi has not been validated. In this case, vi has type untypedAtomic.

– The XQuery builtin function data(item*) as anyAtomicType*may be used to perform atomization explicitly (rarely necessary).


• XPath: Predicates and atomization–

4.2 XPath

Atomization (and casting) made explizit

(

42

42

43

– Note: the value comparison operator eq is witness to the value context in which '.' is used in this query.

– For the context item 42 (a non-validated node), data(.) returns "42" of type untypedAtomic.


43

)/descendant-or-self::*[data(.) cast as double

eq

42 cast as double]

• Atomization and subtree traversals– Since atomization of nodes is pervasive in XQuery

expression evaluation, e.g., during evaluation of• arithmetic and comparison expressions,• function call and return,• explicit sorting (order by),

4.2 XPath

• explicit sorting (order by),

– efficient subtree traversals are of prime importance for any implementation of the language:


Applying data() to a node and its subtree

data(

foo

bar

)

≡ data

a

ed

cb

"fo"

"o"

"b" "ar"

• XPath: Positional access

– Inside a predicate [p] the current context item is '.'.

• An expression may also access the position of '.' in the context sequence via position(). The first item is located at position 1.

• Furthermore, the position of the last context item is available via

4.2 XPath

• Furthermore, the position of the last context item is available via last().

• A predicate of the form [position() eq i] with i being any

XQuery expression of numeric type, may be abbreviated by [i].


Positional access

(x1,x2,...,xn) [position() eq i] ⇒⇒⇒⇒ xi(x1,x2,...,xn)[position() eq last()] ⇒⇒⇒⇒ xn

• XPath: The context item '.'– As a useful generalization, XPath makes the current

context item '.' available in each step (not only in predicates)

– In the expression

4.2 XPath

– In the expressioncs/e

expression e will be evaluated with '.' set to each item in the context sequence cs (in order). The resulting sequence is returned.

• Remember: if e returns nodes (e has type node*), the resulting sequence is sorted in document order with duplicates removed.


4.2 XPath

Accessing '.'

(1,2,3)/(. + 42)

⇒⇒⇒⇒(43.0,44.0,45.0)

(1,2,3)/name(.)

⇒⇒⇒⇒


⇒⇒⇒⇒

(1,2,3)/name(.)

⇒⇒⇒⇒ ("a","b","c")

(1,2,3)/position()

⇒⇒⇒⇒ (1,2,3)

()/(./child::b, .)

⇒⇒⇒⇒ (,)

• Combining node sequences– Sequences of nodes (e.g., the results of XPath location step)

may be combined via• |, union (used synonymously), intersect, except

– These operators remove duplicate nodes based on identity and return their result in document order.

– Note: Introduced in the XPath context because a number of

4.2 XPath

– Note: Introduced in the XPath context because a number of useful navigation idioms are based on these operators:


Selecting all x children and attributes of context nodecs/(./child::x | ./attribute::x)

Select all siblings of context nodecs/(./preceding-sibling::node()

| ./following sibling::node())

orcs/(./parent::node()/child::node() except .)

• XPath: Abbreviations

– Since XPath expressions are pervasive in XQuery, query authors commonly use the succinct abbreviated XPath syntax to specify location steps.

4.2 XPath

Abbreviation Expansion

1(At the beginning of a path expression.)



nt child::nt

@ attribute::

.. parent::node()

// /descendant-or-self::node()/

/1 root(.)/

step1 ./step

• XPath abbrevation examples

4.2 XPath


a/b/c ./child::a/child::b/child::c

a//@id ./child::a/descendant-or-self::node()/attribute::id

//a root(.)/descendant-or-self::node()/child::a


//a root(.)/descendant-or-self::node()/child::a

a/text() ./child::a/child::text()

4.2 XPath


• PurchaseReport– regions [RegionsType]

• {keyref pNumKeyselector: zip/partfield: @number}

– parts [PartType]– @period

• RegionsType

– zip*

• part*

– @number

– @quantity

4.2 XPath

– @period– @periodEnding– {unique

selector: regions/zipfield: @code}

– {key pNumKeyselector: parts/partfield: @number}

– @quantity

• @code

• PartsType

– part*

• @number


4.1 Introduction

4.2 XPath

4.3 XPointer


4.4 XLink

4.5 Overview

4.6 References


• XML Pointer Language– Allows to use XPath expressions in URIs

– Used to identify ranges within XML documents

– URI + XPath == XPointer

– [XPointer] 4 specifications:

4.3 XPointer

– [XPointer] 4 specifications:• http://www.w3.org/TR/xptr-framework

XPointer Framework (Recommendation 25 March 2003)

• http://www.w3.org/TR/xptr-xmlnsxmlns() Scheme (Recommendation 25 March 2003)

• http://www.w3.org/TR/xptr-elementelement() Scheme (Recommendation 25 March 2003)

• http://www.w3.org/TR/xptr-xpointer(Working Draft 19 December 2002)


• Framework– Syntax: uri#pointer1 … pointer_n

pointer ::= ( idValue | linkSchema '(' schemaData ')' )*• IdValue references an element

• The values "xpointer", "xmlns" and "element" are allowed as

4.3 XPointer

• The values "xpointer", "xmlns" and "element" are allowed asvalues for LinkSchema

– Provides extensibility

– Resolving of multiple references from left to right• BUT: Break on success

– Context of XPointer expressions: root node of theXML document referenced by URI


• xpointer() scheme: ranges and points– Every location expression in XPath returns a node set– XPointer can identify parts of documents which can not

be represented as XPath node set– Additional concept: location and location setsLocation: Point, range or standard XPath node

4.3 XPointer

Location: Point, range or standard XPath node• Point location: standard node and index (points either to a child

node or to a character)• Range location: 2 points, seperated by the keyword "to"

– 2 new node tests: range() and point()– New functions working on ranges and points:range-to, string-range, range, range-inside, start-point, end-point, here andorigin


• xmlns() scheme

– Namespaces are used only with the xmlns() scheme

– xmlns() scheme expressions

• always fail

4.3 XPointer

• assign a namespace to a prefix

– Following schema expressions can use the prefix

– Syntax: NamespacePrefix = namespace URI


Examplexmlns(xlink=http://www.w3.org/1999/xlink)

xpointer(//*[@xlink:role="rollen.xml#author"])

• element() scheme

– Identify ranges in documents by id values and / orelement numbers (position of child nodes)

– Examples:

4.3 XPointer


Element node with the id value xmldat06032005

element(xmldat06032005)

2nd child of that node

element(xmldat06032005/2)

2nd child of the 2nd child of the root node

Element(1/2/2)

4.1 Introduction

4.2 XPath

4.3 XPointer


4.4 XLink

4.5 Overview

4.6 References


• XML Linking Language


• be as easy as HTML links

• have multiple end points

• be stored independently of the referenced documents

4.4 XLink

• be stored independently of the referenced documents

– Allows to add metadata to links

– Uses XPointer and XPath

– [XLink] W3C Recommendation 27 June 2001http://www.w3.org/TR/xlink


• XLink namespace

– The XLink namespace has to be declared, so thatapplications and tools can recognize XLink markup:

4.4 XLink

XLink namespace

• Simple links– … link exactly 2 resources with a reference from the local

to the remote– … are indicated by the attributexlink:type='simple'

– … contain special attributes with information about:

4.4 XLink

– … contain special attributes with information about:• The remote resource: xlink:href• Properties of the link: xlink:role rsp. Xlink:arcrole

(the value must be an URI)• The meaning of the link: xlink:title• The type of presentation of the remote resource: xlink:show

(possible values: new, replace, embed, other, none)• When the link shall be followed: xlink:actuate (values can

be: onLoad, onRequest, other, none)


•

4.4 XLink

Simple links – examplesHuckleberry Finn

• Extended XLinks– Components:

• Element with attribute xlink:type containing all otherlink components

• Elements representing local resources( )

4.4 XLink

• Elements representing local resources(xlink:type='resource')

• Elements representing remote resources(xlink:type='locator')

• Elements representing the actual link between 2 resources(xlink:type='arc')

• Elements containing descriptions of the links (xlink:type='title')

• Some other attributes


4.4 XLink


•

4.4 XLink

simple extended locator arc resource title

type R R R R R R

href O R

role O O O O

arcrole O O

title O O O O O

R: required, O: optional

54[XLink] XML Databases – Silke Eckstein – Institut für Informationssysteme – TU Braunschweig

title O O O O O

show O O

actuate O O

label O O

from O

to O

4.4 XLink

55[XLink] XML Databases – Silke Eckstein – Institut für Informationssysteme – TU Braunschweig

• Extended links – example

xlink:type (extended) #FIXED "extended"

xlink:role CDATA #IMPLIED

xlink:title CDATA #IMPLIED>

xml:lang CDATA #IMPLIED>

4.4 XLink

xml:lang CDATA #IMPLIED>

xlink:href CDATA #REQUIRED

xlink:role CDATA #IMPLIED

xlink:title CDATA #IMPLIED

xlink:label NMTOKEN #IMPLIED>

xlink:href CDATA #REQUIRED

xlink:role CDATA #FIXED "http://www.example.com/linkprops/course"





Course Load for Pat Jones

3.5


• Simple vs. Extended links

– Simple links can not

• link an arbitrary number of localand remote resources

• specify a link from a remote

4.4 XLink

• specify a link from a remote resource to a local resource

• set a title to a fixed link

• set a role or title to the localresource

• set a role or totle to the link itself


1. Introduction

2. XML Basics

3. Schema definition

4. XML query languages I

5. Mapping relational

8. XML query languages II

9. XML storage I

10. XML storage - index

11. XML storage - native

3.6 Overview

üüüü

üüüü

5. Mapping relational data to XML

6. SQL/XML

7. XML processing

11. XML storage - native

12. Updates / Transactions

13. Systems

14. XML Benchmarks


• http://www.w3.org/ [W3C]

– XPath, XPointer, XLink, XQuery, XSLT

• M. Scholl, "XML and Databases", Lecture, Uni Konstanz, WS07/08 [Scholl07]

• XML und Datenmodellierung [EE04]

3.7 References

• XML und Datenmodellierung [EE04]– R. und S. Eckstein

– Dpunkt-Verlag, 2004, ISBN 3898642224

• XML in a Nutshell [HM04]– Harold & Means

– O'Reilly, 2004, ISBN 0596007647


• Now, or ...

• Room: IZ 232

• Office our: Tuesday, 12:30 – 13:30 Uhr

Questions, Ideas, Comments

• Office our: Tuesday, 12:30 – 13:30 Uhr

or on appointment

• Email: [email protected]
