advanced databases - xml

Advanced Databases

Mourad Gridach

Filière : Génie Informatique. EST d’Agadir

Année Universitaire : 2019 - 2020

XML Databases

Outlineü Well-Formed XML

u DOMu SAX

ü Print XMLu CSSu XSLT

ü Validating XMLu DTDu XSD

ü Querying XMLu XPathu XQuery

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Introduction

ü XML : eXtensible Markup Language

ü Semi-structured (hierarchical) model

ü More popular these days

ü Based on trees

ü Standard for data representation and exchange

ü Document format similar to HTML

Ø Tags describe content instead of formatting

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Document XML Example

Basic Concepts

ü Tagged elements (nested)

ü Attributes

ü Text

Relationnel XML

Structure

Schema

Queries

Implementation

Relationnel Model versus XML

Tables Hierarchy, trees, graphs

Fixed beforeFlexible ( see example before)

Simple (RA, SQL) J Complex L

RDBMS New, integrated in the most RDBMS

“Well-Formed” XML (1)§ An XML document respect some basic structural requirements

ü Single root element (“Bookstore” in the example below)ü Structured tags, proper nestingü Unique attributes within elements

“Well-Formed” XML (2)

XMLParser

XMLDocument Parsed XML

“Not well-formed”

§ Example of XML parsers

ü DOM (Document Object Model)

ü SAX (Simple API for XML)

Displaying XML Document (1)§ Use rule-based language to translate to HTML

ü Cascading StyleSheets (CSS)

ü Extensible Stylesheet Language (XSL)

Displaying XML Document (2)

CSS/XSLinterpreter

XMLDocument

(data)

Rules

HTMLDocument(look at)

Parser

XML Data

Validating XML

DTDs and XSD

“Validating” an XML Document§ Adheres to basic structural requirements (one single root, structured tags, proper

nesting, unique attributes)§ Also adheres to content-specific specification

ü Document Type Descriptor (DTD)ü XML Schema (XSD) (XML Schema Description)

“Validating” an XML Document

Validating XMLParser

XMLDocument Parsed XML

“Not valid”

DTD or XSD

Document Type Descriptor (DTD)§ Grammar-like language for specifying elements, attributes, nesting, ordering,

#occurrences

Document XML

DTD

Examples –Demos

ü Validating an XML document :

u xmllint --valid --noout Bookstore-DTD.xml

ü Examples

1. Change : “Edition CDATA #IMPLIED>” to “Edition CDATA #REQUIRED>”.

2. Add “Edition="2nd"” to the document.

3. Switch the order of “first_name” and “last_name”

ü <First_Name>Jeffrey</First_Name>

ü <Last_Name>Ullman</Last_Name>15

Examples (2)

1. Add “remark” to the first book :

<remark> </remark>

2. Add a magazine :

<Magazine Month="January" Year="2015">

<Title> Artificial Intelligence Magazine </Title>

</Magazine>

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XML Schema (XMD)

Validating XML Documents

XML Schema (Reminder)

Validating XML

Parser

XMLDocument Parsed XML

“Not valid”

DTD or XSD

XML Schema Descriptor (XSD)

§ Extensive language

§ Like DTDs, can specify elements, attributes, nesting, ordering, #occurrences

§ Also data types, keys, (typed) pointers, and more

ü XSD is written in XML

ü The XSD file is written separately from the XML file (not like DTDs)

Basic Concepts

§Data type

§Keys

§References

§Occurrences

Data Type§ In an XML Schema, we can specify the data type (integer, string, etc.)

§ In this case, « Price » is an « Integer ».

§ If we modify the value of « Price » into « string », we will have an error.

Keys

§ The equivalent primary keys in relational model

§ We used the keyword “key”

§ It must be unique

§ « @ISBN » is a key for « Book ».

§ « @Ident » is a key for « Author ».

§ Remark: if 2 authors have the same « @Ident », we get an error.

References § We used « keyRef » to specify a reference

§ It must be unique

§ We use XPath (see later)

§ The attribute « @authIdent » must a reference for « AuthorKey ».

§ The attribute « @book » must a reference for « BookKey ».

Occurrences (1) § Indicates the number of times an element can appear in an XML document

§ By default, it’s “1”.

§ « minOccurs » specifies the minimum number for an element to appear in an XML documents.

§ « maxOccurs » specifies the maximum number for an element to appear in an XML documents.

Occurrences (2) - Example

§ For an author, « maxOccurs = unbouded » : many authors.

§ Absence of « minOccurs » : by default is « 1 ».

§ « minOccurs = 0 » : the minimum number of remarks in an XML document.

§ Absence of « maxOccurs » : by default is « 1 ».

Exemples - Demos

• For validation, we use the command :

xmllint --schema Bookstore.xsd --noout Bookstore-xsd.xml

• Examples

1. Suppose we want to change the type of « Price » into type « string » and let us try to validate the document.

<Book ISBN = "ISBN-0-13-713526-2" Price = ”hi">

2. Let us test the keys :

replace “<Author Ident="JU">” by “<Author Ident="HG">”.

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Querying XML

N.B. The Demos are done using “editiX-XML Editor”. You can download it for free

Introduction

ü Not nearly as mature as Querying Relational

u Newer

u No underlying algebra

ü We will explore :

u XPath : expressions for a path + conditions.

u XQuery : based on XPath (close to SQL)

u Other : XSLT, XLink, XPointer (out of the scope of this course).

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Querying XML

XPath

XPathü XPath : expressions for a path + conditions.

u The XML document is considered as a tree

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Bookstore

Book Magazine

@ISBN @Price Title Authors

Author

Last_Name First_Name

"Jeff""Ullman"

Book

Author"ISBN-22" "100" "AI"

XPath : Basic Concepts(1)

1. Expressionsü « / » (slash): root element or separator.

u Example : Bookstore/Book.ü « * » : any thing.

u Example : Bookstore/Book/*ü « @Att » : represents an attribute.

u Example : @ISBNü « // » : represents a descendant of an element (the element is included).

u Example : Bookstore/Book// : the elements « Book », « Title », « Authors », « last name », « first name ».

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XPath : Concepts de Bases (2)

2. Conditions

ü The conditions are written between brackets « [condition] ».

ü Example : [@Price < 50].

3. Built in Functions (many)

u « contains (s1, s2) » : if « s1 » contains « s2 ».

u « name() » : return the current tag of an element.32

XPath : Concepts de Bases (2)

4. Navigation « axes » (13 axes)

u « parent:: » : return the parent element.

u « following-sibling:: » : return the child element.

u « descendants:: » : return the descendants.

u « self:: » : return the current element.

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XPath Examples (1)ü Books’ titles :

u /Bookstore/Book/Title

ü We can use « * » :u /Bookstore/ * /Title

ü All the titles :u //Title

ü All the elements :u //*

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XPath Examples (2)

ü The « ISBN » of books :

u /Bookstore/Book/@ISBN

ü Books which cost less than 80 Dh :

u /Bookstore/Book[@Price < 80]

ü Books’ titles which cost less than 80 Dh :

u /Bookstore/Book[@Price < 80]/Title35

XPath Examples (3) (Stop here)

ü Books’ titles which have a remark :

u /Bookstore/Book[Remark]/Title

ü Books’ titles which cost less than 90 DH and “Ullman” is an author:

u /Bookstore/Book[@Price < 90 and Authors/Author/Last_Name= "Ullman"]/Title

ü Books’ titles where the remark contains the word “great”:

u //Book[contains(Remark, "great")]/Title

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Querying XML

XQuery

XQuery

ü Expression language (compositional)

ü Each expression operates on & returns sequence of elements

ü XPath is one type of expression used by XQuery

ü More complex than SQL

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XQuery : L’Expression FLWOR

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For $var in expr

Let $var := expr

Where condition

Order By expr

Return expr

ü All are optional except for « Return ».

ü « For » and « Let » can be repeated and interleaved (intercalés)

ü Close to SQL Produce a set of un « N » elements

Assign one element to variable « var »

Return « M » elements as results

Mixing Queries and XML

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ü <Result> { …query goes here… } </Result>

ü { …query… } will be evaluated by the XML

processor.

XQuery Example (1)

ü Books’ titles which cost less than 90 DH and « Ullman » is an author:

for $b in /Bookstore/Bookwhere $b/@Price < 90

and $b/Authors/Author/Last_Name = "Ullman"return $b/Title

ü In this case « expr » and « for » is an « XPath » expression

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XQuery Example (2)

ü Titles and price of books ordered by price

for $b in /Bookstore/Bookorder by xs:int($b/@Price) return <Book>

{ $b/Title }<Price> { $b/data(@Price) } </Price>

</Book>

ü « xs:int($b/@Price) » : to convert a String « @Price » to an Integer.42

XQuery Examples (3)

ü All authors’ names

for $n in //Last_Name

return $n

ü Remark : We get duplicates.

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XQuery Examples (3)

ü Use the function « distinct-values() ».

for $n in distinct-values(//Last_Name)

return $n

ü Change the display:

for $n in distinct-values(//Last_Name)

return <Last_Name> {$n} </Last_Name>

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Conclusion

ü XML queries are still in progress compared to the relational model

u Newer

u No relational algebra behind the language

ü We explored:

u XPath : expressions for a path + conditions.

u XQuery : based on XPath (close to SQL)

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