xmlta transparency no. 1 query xml documents with xquery cheng-chia chen

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Query XML Documentswith XQuery

Cheng-Chia Chen

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Objectives

How XML generalizes relational databasesThe XQuery languageHow XML may be supported in databases

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XQuery 1.0

XML documents naturally generalize database relationsXQuery is the corresponding generalization of SQL

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Queries on XML documents

XML documents generalize relational data:

A database is composed of many tables A table (relation) is composed of tuples A tuple (record; rows) is composed of attributes An attribute (field; column) contains a primitve data (,

which may serves as a key/key reference) .

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From Relations to Trees

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Only Some Trees are Relations

A relation is a tree of height two with an unbounded number of children (rows) all of which have the same fixed number of child nodes (columns)

The database community has been looking for a richer data model than relations. Hierarchical, object-oriented, or multi-dimensional databases have emerged, but neither has reached consensus.

A DTD for RDB : <!ELEMENT PeopleDataBase (PeopleTable, …) > <!ELEMENT PeopleTable (PeopleRow*) > <!ELEMENT PeopleRow (%Column;) >

<!ENTITY % Column “FirstName, LastName, Age “ > <!ELEMENT FirstName (#PCDATA)> <!ELEMENT LastName (#PCDATA)> <!ELEMENT Age (#PCDATA)>

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Trees Are Not Relations

Not all XML Document trees satisfy the previous characterization Trees are ordered, while both rows and columns of tables

may be permuted without changing the meaning of the data

Trees may have height > 2. Trees is in general not homogeneous.

Elements of the same type may have different number and types of children.

This does not mean that we cannot store these documents in traditional data base but only means that a more complex encoding of xml documents into data base tables is required.

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An examlpe student grade records

<students> <student id="100026"> <name>Joe Average</name> <age>21</age> <major>Biology</major> <grades> <result course="Math 101" grade="C-"/> <result course="Biology 101" grade="C+"/> <result course="Statistics 101" grade="D"/> </grades> </student> <student id="100078"> <name>Jack Doe</name> <age>18</age> <major>Physics</major><major>XML Science</major> <grades> <result course="Math 101" grade="A"/> <result course="XML 101" grade="A-"/> <result course="Physics 101" grade="B+"/> <result course="XML 102" grade="A"/> </grades> </student></students>

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A Corresponding Student Database

key point of data base design: decompose general xml tree into flat data base tables

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For Relational Data base, we have well-established theory and practice, but can we have the same achievement for XML ? database XML DDL(DataDefintionLang) XML Schema DQL(DataQueryLang;SQL) XQuery

How should query languages like SQL be similarly generalized?

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XQuery Design Requirements

Must have at least one XML syntax and at least one human-readable syntax

Must be declarativeMust be namespace awareMust coordinate with XML SchemaMust support simple and complex datatypesMust be able to combine information from multiple

documentsMust be able to transform and create XML trees

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The XQuery language

Developed by W3C Recommendation.

Derived from several previous proposals: XML-QL ,YATL ,Lorel,Quilt

which all agree on the fundamental principles. XQuery relies on XPath and XML Schema

datatypes. There have been many

commercial/non-commerical implementations released. Exist : http://exist.sf.net AltovaXML (free) …

Two formats XQuery : plaintext syntax XQueryX : XML syntax.

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Relationship to XPath

XQuery 1.0 is a strict superset of XPath 2.0 Every XPath 2.0 expression is directly an XQuery 1.0

expression (a query)The extra expressive power is the ability to

join information from different sources and generate new XML fragments

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Relationship to XSLT

XQuery and XSLT are both domain-specific languages (DSL) for combining and

transforming XML data from multiple sourcesThey are vastly different in design, partly for historical

reasons XQuery is designed from scratch, XSLT is an intellectual descendant of CSS (cascaded

StyleSheet) – about how to render a document.Technically, they may emulate each other.XQuery Grammar

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XQuery conceptsA query in XQuery is an expression that:

reads a sequence of XML nodes or atomic values returns a sequence of XML nodes or atomic values

The principal forms of XQuery expressions are: path expressions element constructors

FLWOR ("flower") expressions list expressions (for,let) conditional expressions (where ) order by (o) return expr(r)

Xpath 2.0 + constructor: quantified expressions (some/every) datatype expressions (xs:int(“123”)); <e attr={…}>{…}</e>

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XQuery Modules and Prologs

An XQuery is a main module: Module ::= VersionDecl? ( MainModule | LibraryModule) VersionDecl:

xquery version "1.0" encoding "UTF-8" ; (: this is versiondecl; encoding is optional :)

a module may import other library modules. MainModule ::= Prolog QueryBoby LibraryModule ::= ModuleDecl Prolog

Ex: module namespace math = "http://example.org/math-functions";

Like XPath expressions, XQuery expressions are evaluated relatively to a context This is explicitly provided by a prolog Settings define various parameters for the XQuery

processor language, such as: declare bounndary-space (preserve | strip );

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More From the Prolog

Prolog ::= (: Prolog consists of two parts. :)( ( DefaultNamespaceDecl // declare default function or element/type NSs | Setter // settings for XQuery processing | NamespaceDecl // declare prefix-URI bindings | Import // import schema or other modules ) Separator // each ends with ‘;’ )*

( ( (: Second part contains variable decl, function decl and options :)

VarDecl // global variable declarations | FunctionDecl // function declarations | OptionDecl // implementation-defined options declaration ) Separator // each ends with ';')* optionDecl ::= declare option QName “aLiteral”

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Namespace Declarations

Format: // declare prefix-namexpace bindingdeclare namespace prefix = ”aURI” ;

Ex: declare namespace bk = ”http://www.book.org/” ; Predeclared namespaces prefixes:

(: for xml:lang, xml:space, etc :) declare namespace xml = "http://www.w3.org/XML/1998/namespace"; (: for xml schema and schema instance :) xs = "http://www.w3.org/2001/XMLSchema"; xsi = "http://www.w3.org/2001/XMLSchema-instance"; (: for xpath fucntion and data types ; this is the default function

namespace:) fn = "http://www.w3.org/2005/xpath-functions"; (: for xquery fucntions :) local="http://www.w3.org/2005/xquery-local-functions";

only xml prefix cannot be redefined.

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Setter [7] Setter ::= BoundarySpaceDecl | // preserve space b/t elements

declare boundary-space (preserve | strip); DefaultCollationDecl | // default collation for ordering declare default collation "aURILiteral" BaseURIDecl | declare base-uri "http://example.org"; ConstructionDecl | // preserve type of copied constructs? declare construction (strip | preserve); OrderingModeDecl | // ordered or not for return values declare ordering (ordered | unordered) ; EmptyOrderDecl | //empty or NaN as least/greatest key declare default order empty (greatest | least); CopyNamespacesDecl // copy prefix-NS bindings

see (here. and (3.7.1.3.1.e.2.D) for details) cf: constructed node ;copied nodes; original elements ex: <cn>{.//book}</cn>

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Default namespace declarations

Default namespace Defautl element/type namespace Default function namespace

Ex: (: can be disabled by ”” for element (and type as well) :) declare default element namespace ”http://a.b.c/d” ; declare default function namespace “aURI”; (: default is "http://www.w3.org/2005/xpath-functions" if not specified

or disabled by "" So f1(.) has the same effect as fn:f1(.) :)

Unprefixed attribute names and variable names are in no namespace.

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Import declarations

Import declarations // import element/type/attribute declared in a schema // may also declare prefix/default namespace for the target namespace // default namespace uri can be "" for importing noNamespaceSchema import schema import (library) module

Ex: import schema "targetNamespaceURI" ( at "schema_loc_uri1",

"schema_loc_uri2" )? ; import schema namespace prefix = "targetNamespaceURI" (at "loc1" )?; import schema default element namespace "targetNamespaceURI" ( at

"loc_uri" )? ; Ex:

// import a schema and use it as the default element/type namespace import schema default element namespace "http://example.org/abc"; // import a schema which has no target namespace and use it as default

namespace. import schema default element namespace "" at

"http://example.org/xyz.xsd";

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Module imports

Import declarations // import function/global variables declared in an XQ library module import (library) module // may also declare its namespace prefix

Notes: Only variables and functions are imported; Hence in scope schema

definition and statically known namespaces are not imported. Module imports are not transitive: A imports B and B imports C do not imply

A imports C. Ex:

import module "http://book.org/ ";

// can give hints about module locations import module "http://book.org/” (at "http://www.book.org/book.xq",

"book2.xq” )? ;

// can also bind a prefix for imported module namespace uri. import module namespace book="http://book.org/” ( at "book2.xq” )? ;

import module namespace math = "http://example.org/math-functions";

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Variable and function declarations

VarDecl ::= declare variable $QName TypeDeclaration? ((":="

ExprSingle) | "external")

declare variable $math:pi xs:decimalopt := 3.14159 ; declare variable $math:e xs:double := 2.71E0 declare variable $ext:var1 external ;

FunctionDecl ::= declare function QName ( ParamList? ) (as SequenceType

)? (EnclosedExpr | external)

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Ex :

declare function local:summary // name

( $emps as element(employee)* ) // parameters

as element(dept) * // return type

{ //body

for $d in fn:distinct-values($emps/deptno) //a FLWOR expression

let $e := $emps[deptno = $d]

return

<dept>

<deptno>{$d}</deptno>

<headcount> {fn:count($e)} </headcount>

<payroll> {fn:sum($e/salary)} </payroll>

</dept>

};

Direct element

construction!

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Example

Suppose we have the file at loc "employee.xml"<employees> <employee>

<depno>dept2</depno><name>Wang</name><salary>100000</salary></employee>

<employee>

<depno>dept1</depno><name>Lee</name><salary>50000</salary></employee>

<employee>

<depno>dept3</depno><name>Chen</name><salary>100000</salary></employee>

<employee>

<depno>dept1</depno><name>Cheng</name><salary>150000</salary></employee>

<employees>

The following query sho how to use the function summary

xquery version "1.0" encoding "UTF-8" ;

(: Definition of fun summary inserted here or import it here :)

let $emps := doc("employees.xml")/employees/employee

return <summary>{summary($emps)}</summary> Result: <summary>

<dept><deptno>dept2</deptno><headCount>1</headcount><payroll>100000</payroll></dept>



</summary>

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Xquery Conecpts

QueryBody = Expr[31] Expr ::= ExprSingle ("," ExprSingle)*[32] ExprSingle ::=

FLWORExpr | QuantifiedExprxpath20

| TypeswitchExpr | IfExprxpath20 | OrExprxpath20+construction

Expressions are evaluated relative to a expression context: namespaces variables functions date and time context item,context position, context size …

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XPath Expressions

XPath expressions are also legal XQuery expressions same data model : sequence of items items : nodes or atomic data

The XQuery prolog gives the required static context namespace, function library, variable bindings

The initial context node, position, and size are undefined Since XQuery is intended to operate on multple

docuemnts. initial context info can be obtained by starting from

fn:doc()

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Datatype Expressions

Same atomic values as XPath 2.0Also lots of primitive simple values: xs:string("XML is fun") xs:boolean("true") xs:decimal("3.1415") xs:float("6.02214199E23") xs:dateTime("1999-05-31T13:20:00-05:00") xs:time("13:20:00-05:00") xs:date("1999-05-31") xs:gYearMonth("1999-05") xs:gYear("1999") xs:hexBinary("48656c6c6f0a") xs:base64Binary("SGVsbG8K") xs:anyURI("http://www.brics.dk/ixwt/") xs:QName("rcp:recipe") Functions for theses types can be found in fn:XXX().

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XML Expressions

XQuery expressions may produce new XML nodes, which do not exist in input documents.

Expressions may denote element, character data, comment, and processing instruction nodes

Each node is created with a unique node identityConstructors may be either

direct (called literal result element in XSLT) or computed Both may be nested with each other.

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Uses the standard XML syntaxEx:

The expression

<foo><bar/>baz</foo> evaluates to the given XML fragment

Note that the expression (containing two direct elements)

<foo/> is <foo/> evaluates to false why ? ( unique identity of each node )

Direct Constructors

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Namespaces in Constructors (1/3)

The following four constructions have the same effect.

declare default element namespace "http://businesscard.org";

<card>

<name>John Doe</name>

<title>CEO, Widget Inc.</title>

<email>[email protected]</email>

<phone>(202) 555-1414</phone>

<logo uri="widget.gif"/>

</card>

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declare namespace b = "http://businesscard.org";

<b:card>

<b:name>John Doe</b:name>

<b:title>CEO, Widget Inc.</b:title>

<b:email>[email protected]</b:email>

<b:phone>(202) 555-1414</b:phone>

<b:logo uri="widget.gif"/>

</b:card>

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<card xmlns="http://businesscard.org">




<phone>(202) 555-1414</phone>


</card>

<b:card xmlns:b="http://businesscard.org">

<b:name>John Doe</b:name>

<b:title>CEO, Widget Inc.</b:title>

<b:email>[email protected]</b:email>

<b:phone>(202) 555-1414</b:phone>

<b:logo uri="widget.gif"/>

</b:card>

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Enclosed Expressions

XQ also allow us to embed some XQ expressions into direct elements : Such expression is called attribuate value template in XSLT. but can be appled to atribute value as well as element contents.

<foo>1 2 3 4 5</foo>

<foo>{1, 2, 3, 4, 5}</foo>

<foo>{1, "2", 3, 4, 5}</foo>

<foo>{1 to 5}</foo>

<foo>1 {1+1} {" "} {"3"} {" "} {4 to 5}</foo>

<foo bar="1 2 3 4 5"/>

<foo bar="{1, 2, 3, 4, 5}"/>

<foo bar="1 {2 to 4} 5"/> ‘{‘ and '}' now have special meaning and must be escaped

using { ( or {{ ) and } (or }} ).

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Direct Constructors v.s Computed constructors

Direct construction in XML syntax :

<card xmlns="http://businesscard.org">




<phone>(202) 555-1414</phone>


</card>• Non-XML alternative syntax (Computed Constructors)• syntaxdeclare default element namespace http://businesscard.org" ;element card { <name>John Doe</name>, (: may use direct contructor :) element title { text { "CEO, Widget Inc." } } , element email { text { "[email protected]" } }, element phone { text { "(202) 555-1414" } }, element logo { attribute uri { "widget.gif" } }}

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Computed QNames

Names of elements/attributes may need to be computed! Syntax: nodeType (QName| {NameExpr})? { ContentExpr } nodeType : element | attribute | processing-instruction – need a name | document | comment | text -- no name

declare default element namespace "http://businesscard.org"

element { lower-case("CARD") } {

element { "name" } { text { "John Doe" } },

element { "title" } { text { "CEO, Widget Inc." } },

element { "email" } { text { "[email protected]" } },

element { "phone" } { text { "(202) 555-1414" } },

element { "logo" } {

attribute { "uri" } { "widget.gif" },

"aLogo" } }

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Biliingual Business Cards

xquery version "1.0" encoding "big5" ;

declare default element namespace "uri:businesscard.org";

declare variable $lang := "zh_TW" ;

element { if ($lang="zh_TW") then "名片 " else "card" } {

element { if ($lang="zh_TW") then "姓名 " else "name" }{ "John Doe" },

element { if ($lang="zh_TW") then "頭銜 " else "title" }{ "CEO, Widget Inc." },

element { "email" }{ "[email protected]" },

element { if ($lang="zh_TW") then "電話 " else "phone" }

{"(202) 456-1414" },

element logo { attribute { "uri" } { "widget.gif" }}

}

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FLWOR Expressions

Used for general queries:Find the names of all students which have more than

one major and order them by student id. <doubles>

{ for $s in fn:doc("students.xml")//student

let $m := $s/major

where fn:count($m) ge 2

order by $s/@id

return <double>

{ $s/name/text() }

</double>

}

</doubles>

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The Difference Between For and Let (1/4)

for $x in (1, 2, 3, 4)

let $y := ("a", "b", "c")

return ($x, $y)

1, a, b, c, 2, a, b, c, 3, a, b, c, 4, a, b, c

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let $x in (1, 2, 3, 4)

for $y := ("a", "b", "c")

return ($x, $y)

1, 2, 3, 4, a, 1, 2, 3, 4, b, 1, 2, 3, 4, c

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for $x in (1, 2, 3, 4)

for $y in ("a", "b", "c")

return ($x, $y)

1, a, 1, b, 1, c, 2, a, 2, b, 2, c,

3, a, 3, b, 3, c, 4, a, 4, b, 4, c

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let $x := (1, 2, 3, 4)

let $y := ("a", "b", "c")

return ($x, $y)

1, 2, 3, 4, a, b, c

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Computing Joins

Find distinct names of recipses that make use of at least one stuff in a refrigerator?

declare namespace rcp = "http://www.brics.dk/ixwt/recipes";

fn:distinct-values(

for $r in fn:doc("recipes.xml")//rcp:recipe,

$i in $r//rcp:ingredient/@name,

$s in fn:doc("frige.xml")//stuff[text()=$i]

return $r/rcp:title/text() )<fridge> <stuff>eggs</stuff> <stuff>olive oil</stuff> <stuff>ketchup</stuff> <stuff>unrecognizable moldy thing</stuff></fridge>

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Computing Joins

Find distinct names of recipses that make use of at least one stuff in a refrigerator?

more efficient version:declare namespace rcp = "http://www.brics.dk/ixwt/recipes";

for $r in fn:doc("recipes.xml")//rcp:recipe

let $names := $r//rcp:ingredient/@name

let $stuffs := fn:doc("fridge.xml")//stuff/text() (: may be put at 2nd line:)

where $names = $s (: general comparison :)

return $r/rcp:title/text()

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Inverting a Relation

From Recipe ingredients to ingredients recipes


<ingredients>

{ for $i in distinct-values( (: for each ingredient i :)

fn:doc("recipes.xml")//rcp:ingredient/@name (: find all ingredients :)

)

return <ingredient name="{$i}">

{ for $r in fn:doc("recipes.xml")//rcp:recipe (: for each receipe :)

where $r//rcp:ingredient[@name=$i] (: which uses ingrediwent i :)

return <title>$r/rcp:title/text()</title>

}

</ingredient>

}

</ingredients>

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Sorting the Results


<ingredients>

{ for $i in distinct-values(

fn:doc("recipes.xml")//rcp:ingredient/@name

)

order by $i

return <ingredient name="{$i}">

{ for $r in fn:doc("recipes.xml")//rcp:recipe

where $r//rcp:ingredient[@name=$i]

order by $r/rcp:title/text()

return <title>$r/rcp:title/text()</title>

}

</ingredient>

}

</ingredients>

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A More Complicated Sorting

for $s in document("students.xml")//student

order by

fn:count($s/results/result[fn:contains(@grade,"A")]) descending,

fn:count($s/major) descending,

xs:integer($s/age/text()) ascending

return $s/name/text()

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Using Functions

declare function local:grade($g as xs:string ) {

if ($g="A") then 4.0 else if ($g="A-") then 3.7

else if ($g="B+") then 3.3 else if ($g="B") then 3.0

else if ($g="B-") then 2.7 else if ($g="C+") then 2.3

else if ($g="C") then 2.0 else if ($g="C-") then 1.7

else if ($g="D+") then 1.3 else if ($g="D") then 1.0

else if ($g="D-") then 0.7 else 0

};

declare function local:gpa($s as element(student) ) {

fn:avg( for $g in $s/results/result/@grade return local:grade($g) )

};

<gpas>

{ for $s in fn:doc("students.xml")//student

return <gpa id="{ $s/@id }" gpa="{local:gpa($s) }"/> }

</gpas>

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A Height Function

Find the height of an node tree. can use recursion !

declare function local:height($x as node() ) {

if (fn:empty($x/*))

then 0 (: an element without child element has height 0:)

else

fn:max(for $y in $x/* return local:height($y) )+1

} ;

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A Textual Outline

intended textual outline of a recipe unusual in that it generate plain text instead of xml output.

Cailles en Sarcophages //recipe title

pastry // ingredient

chilled unsalted butter // sub ingredient

flour

salt

ice water

filling

baked chicken

marinated chicken

small chickens, cut up

Herbes de Provence

dry white wine

orange juice

minced garlic

truffle oil

...

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Computing Textual Outlines


declare function local:ingredients($i,$p){ fn:string-join( (:$p is ident spaces :) for $j in $i/rcp:ingredient return fn:string-join( ($p, $j/@name,"
", local:ingredients($j, fn:concat($p,"

"))), ""), "") };declare function local:recipes($r) { fn:concat( $r/rcp:title/text(), "
", local:ingredients($r," ") )};(: Query body :) for $r in

fn:doc("http://www.brics.dk/ixwt/examples/recipes.xml")//rcp:recipe[5] return local:recipes($r)

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Sequence Types

2 instance of xs:integer2 instance of item()2 instance of xs:integer?() instance of empty()() instance of xs:integer*(1,2,3,4) instance of xs:integer*(1,2,3,4) instance of xs:integer+<foo/> instance of item()<foo/> instance of node()<foo/> instance of element()<foo/> instance of element(foo)<foo bar="baz"/> instance of element(foo)<foo bar="baz"/>/@bar instance of attribute()<foo bar="baz"/>/@bar instance of attribute(bar)fn:doc("recipes.xml")//rcp:ingredient instance of element()+fn:doc("recipes.xml")//rcp:ingredient instance of element(rcp:ingredient)+

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An Untyped Function

declare function local:grade($g) { if ($g="A") then 4.0 else if ($g="A-") then 3.7 else if ($g="B+") then 3.3 else if ($g="B") then 3.0 else if ($g="B-") then 2.7 else if ($g="C+") then 2.3 else if ($g="C") then 2.0 else if ($g="C-") then 1.7 else if ($g="D+") then 1.3 else if ($g="D") then 1.0 else if ($g="D-") then 0.7 else 0};

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A Default Typed Function

declare function local:grade($g as item()*) as item()* { if ($g="A") then 4.0 else if ($g="A-") then 3.7 else if ($g="B+") then 3.3 else if ($g="B") then 3.0 else if ($g="B-") then 2.7 else if ($g="C+") then 2.3 else if ($g="C") then 2.0 else if ($g="C-") then 1.7 else if ($g="D+") then 1.3 else if ($g="D") then 1.0 else if ($g="D-") then 0.7 else 0};

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A Precisely Typed Function

declare function local:grade($g as xs:string) as xs:decimal { if ($g="A") then 4.0 else if ($g="A-") then 3.7 else if ($g="B+") then 3.3 else if ($g="B") then 3.0 else if ($g="B-") then 2.7 else if ($g="C+") then 2.3 else if ($g="C") then 2.0 else if ($g="C-") then 1.7 else if ($g="D+") then 1.3 else if ($g="D") then 1.0 else if ($g="D-") then 0.7 else 0};

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Another Typed Function

declare function local:grades($s as element(students)) as attribute(grade)* { $s/student/results/result/@grade};

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Runtime Type Checks

Type annotations are checked during runtimeA runtime type error is provoked when

an actual argument value does not match the declared type a function result value does not match the declared type a valued assigned to a variable does not match the

declared type

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Built-In Functions Have Signatures

fn:contains($x as xs:string?, $y as xs:string?) as xs:boolean

op:union($x as node()*, $y as node()*) as node()*

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XQueryX

for $t in fn:doc("recipes.xml")/rcp:collection/rcp:recipe/rcp:titlereturn $t

<xqx:module xmlns:xqx="http://www.w3.org/2003/12/XQueryX" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.w3.org/2003/12/XQueryX xqueryx.xsd"> <xqx:mainModule> <xqx:queryBody> <xqx:expr xsi:type="xqx:flwrExpr"> <xqx:forClause> <xqx:forClauseItem> <xqx:typedVariableBinding> <xqx:varName>t</xqx:varName> </xqx:typedVariableBinding> <xqx:forExpr> <xqx:expr xsi:type="xqx:pathExpr"> <xqx:expr xsi:type="xqx:functionCallExpr"> <xqx:functionName>doc</xqx:functionName> <xqx:parameters> <xqx:expr xsi:type="xqx:stringConstantExpr"> <xqx:value>recipes.xml</xqx:value> </xqx:expr> </xqx:parameters>

<xqx:stepExpr> <xqx:xpathAxis>child</xqx:xpathAxis> <xqx:elementTest> <xqx:nodeName> <xqx:QName>rcp:collection</xqx:QName> </xqx:nodeName> </xqx:elementTest> </xqx:stepExpr> <xqx:stepExpr> <xqx:xpathAxis>child</xqx:xpathAxis> <xqx:elementTest> <xqx:nodeName> <xqx:QName>rcp:recipe</xqx:QName> </xqx:nodeName> </xqx:elementTest> </xqx:stepExpr> <xqx:stepExpr> <xqx:xpathAxis>child</xqx:xpathAxis>

xqx:nodeName> <xqx:QName>rcp:title</xqx:QName> </xqx:nodeName> </xqx:elementTest> </xqx:stepExpr> </xqx:expr> </xqx:forExpr> </xqx:forClauseItem> </xqx:forClause> <xqx:returnClause> <xqx:expr xsi:type="xqx:variable"> <xqx:name>t</xqx:name> </xqx:expr> </xqx:returnClause> </xqx:expr> </xqx:elementContent> </xqx:expr> </xqx:queryBody> </xqx:mainModule></xqx:module>

XMLTA

Transparency No. 60

XML Databases

How can XML and databases be merged?

Several different approaches: extract XML views of relations use SQL to generate XML shred XML into relational databases

XMLTA

Transparency No. 61

The Student Database Again

XMLTA

Transparency No. 62

Automatic XML Views (1/2)

Columns as attributes

<Students>

<record id="100026" name="Joe Average" age="21"/>

<record id="100078" name="Jack Doe" age="18"/>

</Students>

XMLTA

Transparency No. 63

Automatic XML Views (2/2)

Column as elements

<Students>

<record>

<id>100026</id>

<name>Joe Average</name>

<age>21</age>

</record>

<record>

<id>100078</id>

<name>Jack Doe</name>

<age>18</age>

</record>

</Students>

XMLTA

Transparency No. 64

Programmable Views

wrap query results by xml elements SQL/XML (http://sqlx.org)

An extension of SQL allowing constructin of XML data as a resultof SQL queries.

xmlelement(name "Students",

select xmlelement(name "record",

xmlattributes(s.id, s.name, s.age) )

from Students s

)

xmlelement(name "Students",

select xmlelement(name "record",

xmlforest(s.id, s.name, s.age) )

from Students s

)

XMLTA

Transparency No. 65

XML Shredding

Shredding an XML documents into multiple data base Tables

Each element type is represented by a relationEach element node is assigned a unique key in

document order so siblings can be ordered.

Each element node contains the key of its parent so parent –child relations can be maintained

The possible attributes are represented as fields, where absent attributes have the null value

Contents consisting of a single character data node (#PCDATA only) is inlined as a field.

XMLTA

Transparency No. 66

From XQuery to SQL

Any XML document can be faithfully representedThis takes advantage of the existing database

implementationQueries must now be phrased in ordinary SQL rather

than XQueryBut an automatic translation is possible

//rcp:ingredient[@name="butter"]/@amount

select ingredient.amountfrom ingredientwhere ingredient.name="butter"

XMLTA

Transparency No. 67

Summary

XML trees generalize relational tablesXQuery similarly generalizes SQL

XQuery and XSLT have roughly the same expressive power

But they are suited for different application domains: data-centric vs. document-centric

XMLTA

Transparency No. 68

Essential Online Resources

http://www.w3.org/TR/xquery/http://www.w3.org/XML/Query/implementations:

http://www.galaxquery.org/ http://exists.sf.net http://saxon.sf.net AltovaXML