1

XML & related technologies

2

Outline

Markup Language XML DTD

API for XML DOM SAX

Related Technologies Name Space Xpath Xlink XSL

Query Languages for XML Quilt

3

A critique of HTML

Extraordinarily flexible, but low on structure

Fixed tag set (vocabulary)No automatic validationUnreliable use of the syntaxNobody uses the data model

4

How XML solves this

Define your own tags (vocabulary)Validate against the definitionError handling and strict definition of the

syntaxSmaller and simpler than SGMLStandardized APIs for working with itA data model specification is coming

5

XML background

A subset of SGMLSimplifies SGML by:

leaving out many syntactical options and variants

leaving out some DTD features leaving out some troublesome features

Recommendation approved by the W3C

6

A simple and complete element:

<address>

<street> 33, Terry Dr.</street>

<city> Morristown </city>

</address>

Elements

markup

Content

Start tag

End tag

7

Elements

Attach a meaning to a piece of a document

Have an element type (`example’, `name’) represented by a markup (tag).

Can be nested at any depth

8

Elements

Can contain: other elements (sub-elements)

<address>

<street> 33, Terry Dr.</street><city> Morristown </city>

</address>

text (data content)<street> 33, Terry Dr.</street>

a combination of them (mixed content)<par>Today, <date>05-06-2000</date> Mr. <name>Bill Gates<name> is in California to talk to ... </par>

9

Document Element

It is the outermost element containing all the elements in a documentexample:

<employee> … </employee>

It must always exist

10

Empty Elements

elements without content They do not have end tags Particular representation of start tags

example:

<medical-dossier …/>

11

Attributes

Used to annotate the element with extra information

Always attached to start tags:<el-name attr-name1=“v1” .. attr-name1=“v1” >……<el-name/>

Elements can have any number of

attributes, but all distinct

<Orders>

<SalesOrder SONumber="12345">

<Customer CustNumber="543">

<CustName>ABC Industries</CustName>

<Street>123 Main St.</Street>

<City>Chicago</City>

<State>IL</State>

<PostCode>60609</PostCode>

</Customer>

<OrderDate>981215</OrderDate>

<Line LineNumber="1">

<Part PartNumber="123">

<Description> Turkey wrench: Stainless steel,

one piece construction, lifetime guarantee.

</Description>

<Price>9.95</Price>

</Part>

<Quantity>10</Quantity>

</Line>

</SalesOrder>

</Orders>

13

An XML document

<?XML version=“1.0”><books>

<book><entry isbn=“1-55860-622-X”>

<title>Data on the Web:...</title><publisher>Morgan Kaufmann</publisher>

</entry><author> Serge Abiteboul</author> <author> Peter Buneman</author><author> Dan Suciu</author><bookRef to=“0-201-53771-O 1-55860-463-4”/><articleLink href=“http://…/articles.xml#id(Abi97)”>

</book> <book>

<entry isbn=“0-201-53771-O”> <title>Foundation of Databases</title>

<publisher>Addison Wesley</publisher></entry><author> Serge Abiteboul</author>...

</book>...

</books>

14

Elements Vs Attributes

An element, when: I need fast searching process

it is visible to everyone

it is relevant for the meaning of the document

An attribute, when: it is a choice it is visible only to the system

it is not relevant for the meaning of the document

Do I use an element or an attribute to store semantic info?

15

Other Stuff

Processing instructions, used mainly for extension purposes (<?target data?>)

Comments (<!-- … -->)Character references (&#163;)Entities:

named files or pieces of markup can be referred to recursively, inserted at

point of reference

16

Document Types

Basic idea: we need a type associated with a document, just like objects and values

A document type is a class of documents with similar structure and semanticsExamples: slide presentations, journal articles, meeting agendas, method calls, etc.

17

DTDs

DTDs provide a standardized means for declaratively describing the structure of a document type

This means: which (sub-)elements an element can contain whether it can contain text or not which attributes it can have some typing and defaulting of attributes

18

DTD

A DTD can be Internal: the DTD is in the document External: the DTD is in an external file and

included in the document mixed: part in the document and part outside

A DTD is logically composed of 2 parts: Element Type Definition Attribute List Declaration

19

Element Type Definition

The element type definition specifies: structure of the document allowed contents (content model) allowed attributes (by the meaning of attribute

list declarations)

20

Element Type Definition

<!ELEMENT A (B*, C, D?)> <!ELEMENT A (B | C+)> <!ELEMENT A (#PCDATA)> <!ELEMENT A EMPTY> <!ELEMENT A ANY> <!ELEMENT A (#PCDATA| B | C)*>

• The following are examples of possible declarations:

21

Attribute-List Declarations

It is the list of allowed attributes for each element.For each attribute: name, type, and other information.

Attribute types. Three groups: string types (CDATA) tokenized types (ID,IDREF,IDREFS,...) enumerated types (as the ones in Pascal)

22

Attribute-List Declarations

<!ATTLIST A a CDATA #IMPLIED> <!ATTLIST A a CDATA #IMPLIED b CDATA

#REQUIRED> <!ATTLIST A a CDATA #IMPLIED “aaa”> <!ATTLIST A a CDATA #REQUIRED “aaa”> <!ATTLIST A a CDATA #FIXED “aaa”> <!ATTLIST A a (aaa|bbb) #IMPLIED “aaa”> <!ATTLIST A id ID #REQUIRED> <!ATTLIST A ref IDREF #IMPLIED>

• <!ELEMENT A (#PCDATA)>

<!DOCTYPE Orders[

<!ELEMENT Orders(SalesOrder)+>

<!ELEMENT SalesOrder(Customer,OrderDate,Line*)>

<!ELEMENT Customer(CustName,Street,City,State,PostCode,tel*)>

<!ELEMENT CustName (#PCDATA)>

<!ELEMENT Street (#PCDATA)> <!ELEMENT State (#PCDATA)>

<!ELEMENT PostCode (#PCDATA)> <!ELEMENT tel (#PCDATA)>

<!ELEMENT OrderDate (#PCDATA)> <!ELEMENT Line (Part,Quantity)>

<!ELEMENT Part(Description,Price)> <!ELEMENT Quantity (#PCDATA)>

<!ELEMENT Description (#PCDATA)> <!ELEMENT Price (#PCDATA)>

<!ATTLIST SalesOrder SONumber CDATA #REQUIRED>

<!ATTLIST Customer CustNumer CDATA #REQUIRED>

<!ATTLIST Line LineNumber CDATA #IMPLIED>

<!ATTLIST Part PartNumber CDATA #REQUIRED>

]

A DTD

24

A DTD

<!DOCTYPE Books[

<!ELEMENT Books(book)+>

<!ELEMENT book(entry, author+, bookRef, articleLink*)>

<!ELEMENT entry(title, publisher)>

<!ELEMENT bookRef EMPTY>

<!ELEMENT articleLink EMPTY>

<!ELEMENT title (#PCDATA)>

<!ELEMENT author (#PCDATA)>

<!ELEMENT pubblisher (#PCDATA)>

<!ATTLIST entry isdn ID #REQUIRED>

<!ATTLIST bookRef to IDREFS #IMPLIED>

<!ATTLIST articleLink

xmlns:xlink CDATA #FIXED “http://w3c.org/xlink”

xlink:type CDATA #FIXED “simple”

xlink:href CDATA #REQUIRED>

]>

25

Well-formed and Valid Docs

A document is well-formed if it follows the grammar rules provided by W3C.

A document is valid if it conforms to a DTD which specifies the allowed structure of the document

26

Uses of XML Entities

Physical partition size, reuse, "modularity", … (both XML docs

& DTDs)

Non-XML data unparsed entities binary data

Non-standard characters character entities

Shorthand for phrases & markup, => effectively are macros

27

Types of Entities

Internal (to a doc) vs. External ( use URI)

General (in XML doc) vs. Parameter (in DTD)

Parsed (XML) vs. Unparsed (non-XML)

28

Entities & Physical Structure

A logical elementcan be split into

multiplephysical entities

Mylife.xml

DTD...

<mylife>Chap1.xml

Chap2.xml

</mylife>

<teen>yada yada</teen>

<adult>blah blah..</adult>

29

External Text Entities

<!ENTITY chap1 SYSTEM "http://...chap1.xml">

<mylife> &chap1; &chap2;</mylife>

External Text Entity Declaration

Entity Reference

<mylife> <teen>yada yada</teen><adult> blah blah</adult>

</mylife>

Logically equivalent to inlining file contents

URL

DTD

XML

30

Internal Text Entities

<!ENTITY WWW "World Wide Web">

<p>We all use the &WWW;.</p>

Internal Text Entity Declaration

Entity Reference

<p>We all use the World Wide Web.</p>

Logically equivalent to actually appearing

DTD

XML

31

Unparsed (& "Binary") Entities

<!ENTITY fusion SYSTEM "http://... fusion.ps" NDATA ps>

... and unparsed entity

<fullPaper source="fusion"/>

<!attlist fullPaper source ENTITY #REQUIRED>

Element with ENTITY attribute

Declare attribute type to be entity

<!NOTATION ps SYSTEM "ghostview.exe">

NOTATION declaration (helper app.)

Declare external...

DTD

XML

32

Processing XML

Non-validating parser: checks that XML doc is syntactically well-formed

Validating parser: checks that XML doc is also valid w.r.t. a given DTD

Parsing yields tree/object representation: Document Object Model (DOM) API

Or a stream of events (open/close tag, data): Simple API for XML (SAX)

33

API for handling XML Documents

DOM, SAX

34

DOM Structure Model and API

hierarchy of Node objects: document, element, attribute, text, comment, ...

language independent programming DOM API: get... first/last child, prev/next sibling, childNodes insertBefore, replace getElementsByTagName ...

alternative event-based SAX API (Simple API for XML) does not build a parse tree (reports events when

encountering begin/end tags) for (partially) parsing very large documents

35

DOM Summary

Object-Oriented approach to traverse the XML node tree

Automatic processing of XML docs

Operations for manipulating XML tree

Manipulation & Updating of XML on client & server

Database interoperability mechanism

36

SAX Event-Based API

Pros: The whole file doesn’t need to be loaded into

memory XML stream processing Simple and fast Allows you to ignore less interesting data

Cons: limited expressive power (query/update) when

working on streams=> application needs to build (some) parse-tree when

necessary

37

Related XML technologies

Namespace, Xlink, Xpath, XSL

38

Namespace

39

Namespace

Through namespaces it is possible to declare a set of names which meaning is not ambiguous, i.e. everyone agree on their meaning.

In other worlds, namespaces allow to distinguish two elements with the same name, but different meaning

Element/attribute name is a combination of 2 parts: prefix:name

40

Example: Namespace

<person><name>Rosalie Panelli</name><address>33 Terry Dr.</address>

</person><webSite>

<name>XML Italia</name><address>http://www.xml.it</address>

</webSite>

<person><name>Rosalie Panelli</name><address>33 Terry Dr.</address>

</person><webSite>

<name>XML Italia</name><address>http://www.xml.it</address>

</webSite>

<person xmlns:person=“http://namespaces.xml.it/person”><person:name>Rosalie Panelli</person:name><person:address>33 Terry Dr.</person:address>

</person><webSite

xmlns:webSite=“http://namespaces.xml.it/webSite”><webSite:name>XML Italia</webSite:name><webSite:address>http://www.xml.it</webSite:address>

</webSite>

<person xmlns:person=“http://namespaces.xml.it/person”><person:name>Rosalie Panelli</person:name><person:address>33 Terry Dr.</person:address>

</person><webSite

xmlns:webSite=“http://namespaces.xml.it/webSite”><webSite:name>XML Italia</webSite:name><webSite:address>http://www.xml.it</webSite:address>

</webSite>

41

Namespace declaration

Up to now, in order to parse a document containing a namespace against a DTD, it is necessary to include the prefixes in the element declarations:

<!ELEMENT person (person:name, person:address)><!ATTLIST person xmlns:person CDATA

#FIXED “http://namespaces.xml.it/person”><!ELEMENT person:name (#PCDATA)><!ELEMENT person:address (#PCDATA)>

Note: the address http://namespaces.xml.it/person might be dangling

42

Xlink

43

Xlink

Only internal links can be represented by the means of ID/IDREF(S) attributes

Xlink is a language that allows the definition of links among documents (external links) through Xlink elements

Unidirectional links can be defined as in HTML, but also other kinds

Based on a namespace specifically tailored by W3C

44

The Xlink Namespace

It consists of the following attributes: Type Href Role Title Show Actuate From to

By the means of these attributes it is possible to describe the different kind of links

45

Optional and Required Attributes

OTO

OFROM

OOACTUATE

OOSHOW

OOOOOTITLE

OOOOOROLE

ROHREF

RRRRRRTYPE

TITLEARCLOCATORRESOURCEEXTENDEDSIMPLEAttribute Link

46

Description of attributes

Type it is the type of Xlink element

Href URI address of the used resource

Role link description used from the application

Title link description used from the human user

Show It specifies the behavior of an application when cross the link

Actuate It specifies when the behavior selected by the show attribute must be executed

From/To they specify the role attributes of the sources and of the targets in an extended link

47

A simple link

<dsi xmlns:xlink=“http://www.w3c.org/”xlink:type=“simple”xlink:href=“http://www.dsi.unimi.it”xlink:show=“new”xlink:actuate=“onRequest”xlink:role=“DSI”xlink:title=“Dipartimento di Scienze dell’Info..”>Dipartimento di Scienze dell’Informazione

</dsi>

48

Actuate e Show

By the means of these attributes it is possible to specify when a particular link should be crossed and the behavior that an application should show when the link is effectively crossed

49

The values of show

New when the link is crossed, the resource is loaded in a new page

Replace when the link is crossed, the target resource is substituted in the current page

Embed the resource is included in the current document

Undefined the application is free to apply the behavior it likes

50

The values of actuate

onLoad the link is crossed when the document is loaded into the application

OnRequest the link is crossed when the user explicitly request to cross the link

Undefined the application crosses the link as it likes

51

The type of links

Simple: a link is simple when it is between two resources (one local, the other remote)

Extended: a link is extended when it is among different sources (both local and remote)

52

The simple links

They are similar to those of HTML:<html>

...<a href=“http://www.dsi.unimi.it/Index.html”> Computer Science Dept </a>...</html>

53

Example: Xlink declaration

<!Element a (#PCDATA)><!ATTLIST a

xmlns:xlink CDATA #FIXED “http://www.w3c.org”xlink:type (simple) #FIXED “simple”xlink:href CDATA #REQUIREDxlink:show (new|replace) #FIXED “replace”xlink:actuate (onRequest) #FIXED “onRequest”xlink:role CDATA #IMPLIEDxlink:title CDATA #IMPLIED

>

54

Example: Xlink Instance

<xml>

...<a href=“http://www.disi.unige.it/Index.html”> Computer Science Dept </a>...</xml>

55

Xpath

56

Xpath

It is a language for expressing path expression on the hierarchical structure of XML documents

Based on the concept of context node.A context node is the node from which an

expression is evaluatedExamples:

/books/book/author /child::books/child::book/child::author

57

Xpath: Location Path

The central construct is the location path, which is a sequence of location steps separated by /.

A location step is evaluated wrt some context resulting in a set of nodes.

A location path is evaluated compositionally, left-to-right, starting with some initial context.

Each node resulting from the evaluation of one step is used as context for evaluation of the next, and the results are combined together.

58

Location steps

A location step has the form

axis :: node-test [ predicate ]

Example: child::section[position()<6]/descendant::cite/attribute::href

selects all href attributes in cite elements in the first 5

sections of an article.

59

Axes

60

Node tests

Testing by node type: text() chardata nodes

comment() comment nodes

processing-instruction() processing instruction nodes

node() all nodes (not including attributes and namespace decl.s)

Testing by node name: name nodes with that name * any node

61

Predicates

expressions coerced to type boolean A predicate filters a node-set by

evaluating the predicate expression on each node in the set with that node as the context node

Different predefined predicates are available

Predicates can be conjunction, disjunction, negation of other predicates

62

Examples of Predicates

position() returns the sequential location of the element being tested

ex: author[position()=1] abbr. author[1]

last() returns the last siblingex: author[last()]

id(value) returns the element with id equal to “value” ex: entry[id(1-55860-622-X)]

text() returns the text content of a nodeex: author[text()=“Serge Abiteboul”]

63

Abbreviations

Normal syntax Abbreviation child:: nothing

attribute:: @

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

self::node() .

parent::node() ..

Syntactic sugar: convenient notation for common situations

64

Examples

/books/book/author //author //author[1] //book[entry/@isbn =“1-5586..” or author =

“...”] //book[author][last()>2]

65

XSLeXtensible Stylesheet Language

66

Why Stylesheets?

XML is not a fixed tag set (like HTML) and has no (application) semantics

XML markup does not (usually) include formatting information

Reuse: the same content can look different in different contexts

Multiple output formats: different media (paper, online), different sizes (manuals, reports), different classes of output devices (palmtops, workstations, cellurars)

67

Why Style sheets? (Cont.)

Standardized styles: corporate style sheets can be applied to the content at any time

Freedom from style issues for content authors: technical writers need not be concerned with layout issues because the correct style can be applied later

Therefore there must be something in addition to the XML document that provides information on how to present or otherwise process the XML

68

XSL

XSL stands for eXtensible Style Sheet Language

XSL has currently on preparation by W3CXSL is a style sheet language allowing one

to address the previous issues

69

XSL - More than a Style Sheet

XSL consists of two parts: a method for transforming XML documents a method for formatting XML documents

70

Transforming XML documents

An input document can be transformed into another document by: generation of constant text suppressing of content moving text (e.g. exchanging the order of the first and

last name) duplicating text (e.g. copying titles to make a table of

contents) sorting more complex transformations that “compute” new

information in terms of the existing information

71

Formatting XML documents

A description of how to present the transformed information (i.e. specification of what properties to associate to each of the various parts of the transformed info)

This includes: Specification of the general screen or page layout Assignment of the transformed content into basic

“content container types” (e.g. lists, paragraphs, inline text)

Specification of formatting properties (spacing, margins, alignment, fonts, etc.) for each resulting “container”

72

Components of XSL

The full XSL language logically consists ot 3 component languages which described in 3 W3C recommandations: Xpath XSLT: XSL Transformation -- a language for describing

how to transform one XML document into another XSL: Extensible Stylesheet Language -- XSLT + a

description of a set of Formatting Objects and Formatting Properties

73

XSLT Processing Model

XML source tree XML,HTML, text… result tree

XSLT stylesheet

Transformation

74

XSLT Elements

<xsl:stylesheet version="1.0” xmlns:xsl="http://www.w3.org/1999/XSL/Transform">

root element of an XSLT stylesheet "program"

<xsl:template match=pattern name=qname priority=number mode=qname>

...template...</xsl:template>

declares a rule: (pattern => template)

<xsl:apply-templates select = node-set-expression mode = qname>

apply templates to selected children (default=all) optional mode attribute

   <xsl:call-template name=qname>

75

XSLT Processing Model

XSL stylesheet: collection of template rules template rule: (pattern template)main steps:

match pattern against source tree (expressed in Xpath)

instantiate template (replace current node “.” by the template in the result tree)

select further nodes for processing (expressed in Xpath)

control can be a mix of recursive processing (<xsl:apply-templates> ...) program-driven (<xsl:foreach> ...)

76

<xsl:template match="product"> <table> <xsl:apply-templates select="sales/domestic"/> </table> <table> <xsl:apply-templates select="sales/foreign"/> </table> </xsl:template>

Template Rule: Example

(i) match pattern: process <product> elements(ii) instantiate template: replace each product element with two HTML tables(iii) select the <product> grandchildren (“sales/domestic”, “sales/foreign”) for further processing

pattern

template

77

An XML document<?xml version="1.0"?><books>

<book category="reference"><author>Nigel Rees</author><title>Sayings of the Century</title><price>8.95</price>

</book><book category="fiction">

<author>Evelyn Waugh</author><title>Sword of Honour</title><price>12.99</price>

</book><book category="fiction">

<author>Herman Melville</author><title>Moby Dick</title><price>8.99</price>

</book><book category="fiction">

<author>J. R. R. Tolkien</author><title>The Lord of the Rings</title><price>22.99</price>

</book></books>

78

An XSL stylesheet<?xml version="1.0" ?><xsl:stylesheet

xmlns:xsl="http://www.w3.org/1999/XSL/Transform"version="1.0">

<xsl:template match="books"><html><body><h1>A list of books</h1><table width="640"><xsl:apply-templates/></table></body></html>

</xsl:template><xsl:template match="book">

<tr><td><xsl:number/></td><xsl:apply-templates/></tr>

</xsl:template><xsl:template match="author | title | price">

<td><xsl:value-of select="."/></td></xsl:template></xsl:stylesheet>

79

The result (textual version)

<html> <body> <h1>A list of books</h1> <table width="640"> <tr> <td>1</td> <td>Nigel Rees</td> <td>Sayings of the Century</td> <td>8.95</td> </tr> <tr> <td>2</td> <td>Evelyn Waugh</td> <td>Sword of Honour</td> <td>12.99</td> </tr> …... </table> </body></html>

80

The result (shown on a browser)

81

Query Languages for XML

82

Querying XML

Different XML QL paradigms depending on the community: (relational, oo, semistructured) database

perspectiveLorel, YaTL, XML-QL, XMAS, FLORA/FLORID, ...

document processing perspectiveXQL, XSL(T), XPath, ...

functional programming perspectiveQLs with structural recursion, …

Patching desirable features together: Quilt

83

Important QL Features (DB Perspective)

typical parts of a query: (match) pattern (selects parts of the source XML tree

without looking at data)filter condition (selects further, now looking at the data)answer construction (putting the results together,

possibly reordered, grouped, etc.)

reordering based on nested queries, grouping, sorting

tag variables, path expressions for defining the patterns without requiring knowledge of the DTD

84

Querying XML No "official" W3C XML QL yet (but bits and pieces) numerous quite different XML QLs are popping up some XML QL overviews, comparisons, and resources:

XML Query Languages: Experiences and Exemplarshttp://www-db.research.bell-labs.com/user/simeon/xquery.html (co-authored by several XML QL gurus)

XML and Query Languages (Oasis Cover Pages)http://www.oasis-open.org/cover/xmlQuery.html

Comparative Analysis of Five XML Query Languages (A. Bonifati, S. Ceri) http://www.acm.org/sigmod/record/issues/0003/bonifati.pdf.gz

A Data Model and Algebra for XML Query (Philip Wadler et.al. “functional (Haskell) perspective”)http://cm.bell-labs.com/cm/cs/who/wadler/topics/xml.html

XML-QL vs XSLT queries (Geert Jan Bex and Frank Neven)http://alpha.luc.ac.be/~gjb/xml-ql2xslt.html

children of the “(semistructured) database(s) crowd”: XML-QL, YaTL, Lorel, …

… from the “functional crowd”:

… from the “document processing folks”: XQL, XSL(T), XPath, ...

XPath: W3C Recommendation Powerful pattern language for selecting parts of XML docs Used by XSL(T), XPointer, and XQL

XQL based on XPath,

Browser:IE5 XML DBs: Excelon, Tamino, Perl, …

85

Quilt Source: Daniela Florescu, Jérôme Siméon, VLDB 2000

Goals: Put together the most effective features of several existing

and proposed Query languages Design a small, clean, implementable language

Antecedent: Xpath and XQL XML-QL by A. Deutsch, M. Fernandez, D. Florescu, A. Levy,

D. Suciu SQL and OQL

86

Antecedents: Xpath and XQL

Closely-related languages for navigating in a hierarchy

A path expression is a series of stepsEach step moves along an axis (children,

ancestors, attributes, etc.) and may apply a predicate

XQL has some additional operators: BEFORE, AFTER

87

Antecedents: XML-QL

WHERE-clause binds variables according to a pattern, CONSTRUCT-clause generates output document

WHERE<part pno = $pno> $pname </> in “parts.xml”,<supp sno = $sno> $sname </> in “supp.xml”,<sp pno = $pno sno = $sno> $numofpart </> in “sp.xml”

CONSTRUCT<purchase> <partname> $pname </> <suppname> $sname </> <numofparts> $numofpart </></purchase>

88

Antecedents SQL and OQL

SQL and OQL are database query languagesSQL derives a table from other tables by a

stylized series of clauses: SELECT-FROM-WHERE

OQL is a functional language a query is an expression Expressions can take several forms Expressions can be nested and combined SELECT-FROM-WHERE is one form of OQL-

expression

89

An Example of Document: bib.xml

<bib><book>

<title>…</title><author>…</author>...<publisher>…</publisher><year>…</year><price>…</price>

</book>….

</bib>

90

Some examples of queries

“Find all the books published in 1998 by Jackson”

FOR $b IN document(“bib.xml”)//bookWHERE $b/year = “1998” and $b/pablisher = “Jackson”RETURN $b SORTEDBY(author, title)

“Find title of books that have no authors”

<orphan_books>FOR $b IN document(“bib.xml”)//bookWHERE empty($b/author)RETURN $b/title

</orphan_books>

91

Some examples of queries (cont.)

“Invert the hierarchy from publishers inside books to books inside publishers”

FOR $p IN distinct(//publisher) RETURN

<publisher><name> $p/text() </name>,

FOR $b IN //book[publisher = $p]RETURN <book> $b/title, $b/price </book> SORTEDBY(price DESCENDENT)

</publisher> SORTBY(name)

92

Some examples of queries (cont.)

“Find the description and average price of each red part that has at least 10 orders”

FOR $p IN document(“parts.xml”)//part[color=“Red”]LET $o := document(“orders.xml”)//order[partno = $p/partno]WHERE count ($o) >= 10RETURN

<important_red_part>$p/description,<avg_price> avg($o/price)</avg_price>

</important_red_part>

93

Quilt data model

A document in Quilt is a treeIn order to model parts of a document and

collection of documents Quilt supports the concept of forest of trees

E

T

A

EE

E

E

E

T

T T

TT A

A

denotes element

denotes text node

denotes attribute

a document a forest

94

Quilt Expressions

Like OQL, Quilt is a functional language (a query is an expression, and expressions can be composed)

Some types of Quilt expressions: A path expression (using Xpath syntax):

document(“bids.xml”)//bis[itemno =“47”]/bid_amount

An expression using operators and functions:($x + $y) * foo($z)An element constructor

<bid><userid> $u </userid><bid_amount> $a </bid_amount>

<bid>A “FLWR” expression

95

Expression results

The result of the evaluation of a Quilt expression can be: an XML document (i.e. a tree) a set of elements (i.e. a forest) a primitive value (i.e. a string)

96

A FLWR Expression

A FLWR expression binds some variables, applies a predicate, and constructs a new result

FOR … LET … WHERE … RETURN …

RETURNCLAUSE

LETCLAUSE

WHERECLAUSE

FORCLAUSE

97

FOR clause

FOR is used for iterating over one or more collections

Each expression evaluates to a collection of nodes

The FOR clause produces many binding-tuples from the Cartesian product of these collections

In each tuple, the value of each variable is one node and its descendents

The order of the tuples preserves document order unless some expression contains a non-order-preserving function such as distinct()

FOR var IN expr

,

98

LET clause

LET is used for binding variables (without iteration) A LET clause produce one binding for each variable

(therefore it does not effect the number of binding-tuples)

The variable is bound to the value of expression, which may contain many nodes (i.e. the set of nodes returned by the expression evaluation are linked into a forest of nodes)

document order is preserved among the nodes in each bound collection, unless some expression contains a non-order-preserving function such as distinct()

LET var := expr

,

99

WHERE clause

Applies a predicate to the tuples of bound variabes Retains only tuples that satisfy the predicate Preserves order of tuples, if any May contain AND, OR, NOT Applies scalar conditions to scalar variables:

$color = “Red” Applies set conditions to variables bound to sets:

avg($emp/salary) > 1000

WHERE boolean-expression

100

RETURN clause

Constructs the result of the FLWR expression Executed once for each tuple of bound variables Preserves order of tuples, if any, … OR, can impose a new order using a SORTBY

clause Often uses an element constructor:

<item>$item/itemno,<avg_bid> avg($b/bid_amount) </avg_bid>

</item> SORTBY itemno

RETURN expr