Introduction to DTD

Introduction to DTD

Kristian Torp

Department of Computer ScienceAalborg University

people.cs.aau.dk/˜[email protected]

November 3, 2015

daisy.aau.dk

Kristian Torp (Aalborg University) Introduction to DTD November 3, 2015 1 / 37

http://people.cs.aau.dk/~torp

mailto:[email protected]

http://daisy.aau.dk

Outline

1 Introduction

2 Elements

3 Attributes

4 DTD Find Errors

5 Putting it All Together

6 Summary


Learning Outcomes

Learning OutcomesBe able to read and understand a DTD

Be able to construct a DTD for a set of existing XML documents

Be able to validate an XML document against a DTD

Know the limitations of a DTD

Database FocusAll XML technologies are presented from a database perspective!


Outline

1 Introduction

2 Elements

3 Attributes

4 DTD Find Errors


6 Summary


Example: Course Catalog XML Document

User RequirementsMake a DTD for the course catalog

Use the DTD to validate our course catalog XML document

Example (Current Courses)<?xml vers ion= ” 1.0 ” ?><coursecata log>

<course c id= ’P4 ’><name>OOP< / name><semester>3< / semester><desc>Object−or iented programming< / desc>

< / course><course c id= ’P2 ’>

<name>DB< / name><semester>7< / semester><desc>Databases i n c l u d i n g SQL< / desc>

< / course>< / coursecata log>


Example: Course Catalog DTD

Example (DTD for Course Catalog)<?xml vers ion= ” 1.0 ” encoding= ”UTF−8” ?>< !ELEMENT coursecata log ( course )∗>< !ELEMENT course (name, semester , desc ) >< !ELEMENT name (#PCDATA)>< !ELEMENT semester (#PCDATA)>< !ELEMENT desc (#PCDATA)>< ! ATTLIST course c id ID #REQUIRED>

Informal DescriptionA course catalog consists of zero or more of coursesA course consists of a name, a semester, and a description

It is identified by an ID that is required

A (course) name is a string (leaf in XML document)

A semester is a string (leaf in XML document)

A description is a string (leaf in XML document)


Overview

PurposeDefine the document structure

Legal elements and attributes

Serves the same purpose as a create table statement in SQLStructure and type of dataIntegrity constraints!

Left over from SGMLIs not written in XML

If this is a requirement then use XML Schema

Still very widely usedBecause much simpler than XML Schema

NoteMany simple errors can be found using a DTD

A necessity if receiving XML documents from external sources


Outline

1 Introduction

2 Elements

3 Attributes

4 DTD Find Errors


6 Summary


Simplest EntityExample (Element Declaration)<!ELEMENT name (#PCDATA)>

Example (Allowed Values)<name>Hello Element</name>

<name/>

<name><![CDATA[ select ∗ from emp where sal > 10]]></name>

Example (Illegal Values)<name>> </name>

<name>></name>

<name><it>Hello</it></name>

Unknown element < it>, must be defined in DTD

NoteRoot, internal-node, and leafs in XML tree representation

Terminal and non-terminal in grammar terminology


Sequences of Child Elements

Example (Element Declaration)<!ELEMENT course (name, semester, desc)>

Example (Allowed XML Fragment, Why?)<course>

<name>OOP< / name><semester>7< / semester><desc> I n t r o d u c t i o n to OOP< / desc>

< / course>

Example (Disallowed XML Fragment, Why?)<course>

<semester>7< / semester><name>OOP< / name>

< / course>

Example (Is this allowed?)<course>< / course>






< / course>



< / course>







< / course>



< / course>



Choice Among Child Elements

Example (Element Declaration)<!ELEMENT circle (x, y, (radius | diameter))>

Example (Allowed XML Fragment)< c i r c l e>

<x>5< / x><y>9< / y><diameter>7< / d iameter>

< / c i r c l e>

Example (Illegal XML Fragment)< c i r c l e>

<x>4< / x><y>8< / y>< rad ius>3.5< / rad ius><diameter>7< / d iameter>

< / c i r c l e>


Symbols in a DTD

Symbols

Symbol Example

∗ <!ELEMENT coursecatalog (course)∗>

+ <!ELEMENT coursecatalog (course)+>

? <!ELEMENT coursecatalog (course)?>

, <!ELEMENT course (name, semester, desc) >

| <!ELEMENT course (name | semester | desc) >

NoteSymbols are mostly taken from regular expressions


Mixed Content

Example (Data Centric)< !ELEMENT coor ( x , y )>

Example (Allowed Fragment)<coor>

<x>5< / x><y>9< / y>

< / coor>

Example (Mixed Content)< !ELEMENT coor ( x , y , #PCDATA)∗>

Example (Allowed Fragment)<coor>

This i s the coord ina te(<x>5< / x> , <y>9< / y> ) wherethe t reasure i s hidden !

< / coor>

NoteData centric very table like

Mixed content also called narrative document


Element Declarations using ANY

Example (Any)< !ELEMENT coor (ANY)>< !ELEMENT x (#PCDATA)<!ELEMENT y (#PCDATA)

Example (Allowed Fragments)<coor/>

<coor>Hello World</coor>

<coor>Hello <x>1</x><x/>World<y>3</y><y>4</y></coor>

<coor>Hello <x>1</x><y>2</y>World<y>3</y><x>4</x></coor>

Example (Illegal Fragments)<coor><z>1</z></coor>

<coor><x>1</x><y>1<y/><z>1</z></coor>

NoteANY handy for narrative documents, e.g., HTML


Element Declarations using EMPTY

Example (Empty)< !ELEMENT coor EMPTY>

Example (Allowed?)<coor></coor>

<coor/>

<coor>Hello</coor>

<coor><x>Hello</x></coor>

<coor> </coor>


Summary: ElementsRepetition

Symbol Explanation Example

? zero-or-one <!ELEMENT person (address?)>

* zero-or-more <!ELEMENT person (address∗)>

+ one-or-more <!ELEMENT person (address+)>

once <!ELEMENT person (address)>

Sequence or ChoiceSymbol Explanation Example

, Sequence <!ELEMENT coor (x, y)>

| Choice <!ELEMENT coor (x | y)>

Data TypeSymbol Explanation Example

#PCDATA String <!ELEMENT name (#PCDATA)>

ANY What ever <!ELEMENT coor (ANY)>

EMPTY Empty <!ELEMENT room EMPTY>


Outline

1 Introduction

2 Elements

3 Attributes

4 DTD Find Errors


6 Summary


Attribute Declarations

Example (Circles)<?xml vers ion= ’ 1.0 ’ encoding= ’ u t f −8 ’ ?>< !ELEMENT drawing ( c i r c l e )∗>< !ELEMENT c i r c l e ( x , y , ( rad ius | diameter ) )>< ! ATTLIST c i r c l e c id ID #REQUIRED

name CDATA #IMPLIED >< !ELEMENT x (#PCDATA)>< !ELEMENT y (#PCDATA)>

< !ELEMENT rad ius (#PCDATA)>< ! ATTLIST rad ius u n i t (mm| cm |m) ”m”> < !−− Enum wi th d e f a u l t −−>< !ELEMENT diameter (#PCDATA)>< ! ATTLIST diameter u n i t (mm| cm |m) #REQUIRED> < !−− Enum no d e f a u l t −−>

NoteMandatory and optional attributes

One or more attributes

Enumeration with defaults


Example Document

Example (Circles)<?xml vers ion= ” 1.0 ” encoding= ’UTF−8 ’ ?>< !DOCTYPE drawing SYSTEM ” c i r c l e a t t . dtd ”><drawing>

< c i r c l e c id = ’C1 ’ name= ’ f o r e s t ’><x>8< / x> <y>8< / y>< rad ius>4< / rad ius> < !−− d e f a u l t u n i t−−>

< / c i r c l e>< c i r c l e c id= ’C2 ’> < !−− name not requ i red −−>

<x>5< / x> <y>5< / y>< rad ius u n i t = ”cm”>4< / rad ius> < !−− e x p l i c i t u n i t−−>

< / c i r c l e>< / drawing>

NoteUnique value is not an integer

Used that attribute name is optional in element circle


Uniqueness, Examples

Example (Circle/Points with IDs)<?xml vers ion= ’ 1.0 ’ encoding= ’ u t f −8 ’ ?>< !ELEMENT drawing ( po in t | c i r c l e )∗>< !ELEMENT po in t ( x , y )>< !ELEMENT c i r c l e ( x , y , ( rad ius | diameter ) )>< ! ATTLIST c i r c l e d id ID #REQUIRED>< ! ATTLIST po in t d id ID #REQUIRED>

Example (Circles)<drawing>

< c i r c l e d id= ’C1 ’><x>8< / x> <y>8< / y>< rad ius>4< / rad ius>

< / c i r c l e><po in t d id= ’P2 ’>

<x>5< / x> <y>5< / y>< / po i n t>

< / drawing>


Uniqueness, Errors

Example (Find the error 1!)<drawing>

< c i r c l e c id= ’C1 ’ name= ’ f o r e s t ’><x>8< / x> <y>8< / y>< rad ius>5< / rad ius>

< / c i r c l e>< c i r c l e c id= ’C1 ’>

<x>5< / x> <y>5< / y>< rad ius u n i t = ”cm”>8< / rad ius>




< / c i r c l e><po in t d id= ’C11 ’>

<x>5< / x> <y>5< / y>< / po i n t>

< / drawing>


< c i r c l e c id= ’C1 ’ name= ’ f o r e s t ’><x>8< / x> <y>8< / y>< rad ius>5< / rad ius>

< / c i r c l e>< c i r c l e c id= ’ 2C ’>

<x>5< / x> <y>5< / y>< rad ius u n i t = ”cm”>8< / rad ius>




< / c i r c l e><po in t d id= ’ C1111111111111111111111111111 ’>

<x>5< / x> <y>5< / y>< / po i n t>

< / drawing>


Uniqueness

LimitationsOnly attribute values unique not element values

Cannot be a integer, e.g., <circle did=’1’> not allowedOnly unique within a single document

Uniqueness not guaranteed across multiple documents

Only a single attribute uniqueness (no composite keys)Combination of x and y coordinates cannot be declared unique

NoteUniqueness quite restrictive compared to DBMS technology

XML Schema lifts most limitations on uniqueness


Empty Elements with Attributes

Example (Empty)< !ELEMENT coor EMPTY>< ! ATTLIST coor c id ID #REQUIRED

x CDATA #REQUIREDy CDATA #REQUIREDz CDATA #IMPLIED>

Example (Allowed?)<coor/>

<coor cid=’c1’ x=’1’ y=’1’ z=’1’ />

<coor cid=’c2’ x=’2’ y=’2’></coor>

<coor cid=’c3’ x=’3’ y=’3’> </coor>

<coor cid=’c4’ z=’4’ y=’4’ x=’4’ />

<coor z=’5’ y=’5’ x=’5’ />


Is something Wrong?

Example (Case 1)< !ELEMENT coor EMPTY>< ! ATTLIST coor

c id IDx CDATA #REQUIRED>

Example (Case 2!)< !ELEMENT coor EMPTY>< ! ATTLIST coor

c id ID #IMPLIEDx CDATA #REQUIRED>

Example (Case 3!)< !ELEMENT coor EMPTY>< ! ATTLIST coor

x CDATA #REQUIREDc id ID #REQUIRED>


c id ID ’ 42 ’x CDATA #REQUIRED>

Example (Case 5)< !ELEMENT coor (EMPTY)>< ! ATTLIST coor

c id ID #REQUIREDx CDATA #REQUIRED>


c id ID #REQUIREDx ID #REQUIRED>


Summary: AttributesGeneral Syntax

<!ATTLIST element−name attribute−name type [DefaultValue]>

Often used types

Type Example

CDATA <!ATTLIST course id CDATA>

ID <!ATTLIST course id ID #REQUIRED>

Enumeration <!ATTLIST course id (OOP | DB)>

Defaults

Type Example

#REQUIRED <!ATTLIST course id ID #REQUIRED>

#IMPLIED <!ATTLIST course id CDATA #IMPLIED>

#FIXED <!ATTLIST course id CDATA #FIXED ”1”>

A value <!ATTLIST course id (OOP | DB) ”DB”>


Outline

1 Introduction

2 Elements

3 Attributes

4 DTD Find Errors


6 Summary


A Buggy DTD

Example (DTD With Five Errors)<?xml vers ion= ’ 1.0 ’>< !ELEMENT users user+>< !ELEMENT user ( f i rs tname , lastname>< !ELEMENT f i r s tname (#PCDATA)>< !ELEMENT lastname>

Two-Minutes ExerciseWith your neighbor identify the errors in the DTD

Example (The Corrected DTD)<?xml vers ion= ’ 1.0 ’ encoding= ’ u t f −8 ’ ?>< !ELEMENT users ( user )+>< !ELEMENT user ( f i rs tname , lastname )>< !ELEMENT f i r s tname (#PCDATA)>< !ELEMENT lastname (#PCDATA)>


A Buggy DTD

Example (DTD With Five Errors)<?xml vers ion= ’ 1.0 ’>< !ELEMENT users user+>< !ELEMENT user ( f i rs tname , lastname>< !ELEMENT f i r s tname (#PCDATA)>< !ELEMENT lastname>

Two-Minutes ExerciseWith your neighbor identify the errors in the DTD

Example (The Corrected DTD)<?xml vers ion= ’ 1.0 ’ encoding= ’ u t f −8 ’ ?>< !ELEMENT users ( user )+>< !ELEMENT user ( f i rs tname , lastname )>< !ELEMENT f i r s tname (#PCDATA)>< !ELEMENT lastname (#PCDATA)>


Outline

1 Introduction

2 Elements

3 Attributes

4 DTD Find Errors


6 Summary


Uncertain About Content

Example (DTD for Courses with Flexible Description)<?xml vers ion= ” 1.0 ” encoding= ”UTF−8” ?>< !ELEMENT courses ( course )∗>< !ELEMENT course (name, desc )>< !ELEMENT name (#PCDATA)>< !ELEMENT desc ANY>

Example (DTD for Courses with Flexible Description)<?xml vers ion= ” 1.0 ” encoding= ”UTF−8” ?>< !DOCTYPE courses SYSTEM ” course . dtd ”><courses>

<course><name>OOP< / name><desc><name>ob jec t−or ien ted< / name><desc>programming< / desc> .

< / desc>< / course>

< / courses>


A University Example, Setup

Example (DTD)<?xml vers ion= ” 1.0 ” encoding= ”UTF−8” ?>< !ELEMENT u n i v e r s i t y ( courses ,

students ,f o l l o w s )>

< !ELEMENT courses ( course )+>< !ELEMENT course (name)>< ! ATTLIST course c id ID #REQUIRED>< !ELEMENT name (#PCDATA)>

< !ELEMENT students ( s tudent )+>< !ELEMENT student ( fname )>< ! ATTLIST student s id ID #REQUIRED>< !ELEMENT fname (#PCDATA)>

< !ELEMENT f o l l o w s ( takes )+>< !ELEMENT takes EMPTY>< ! ATTLIST takes s id IDREF #REQUIRED>< ! ATTLIST takes c ids IDREFS #REQUIRED>

Example (XML Fragment)<u n i v e r s i t y><courses><course c id= ’C111 ’><name>DB< / name>

< / course><course c id= ’C222 ’><name>OOP< / name>

< / course>< / courses><students><student s id= ’S11 ’><fname>Ann< / fname>

< / s tudent><student s id= ’S22 ’><fname>Bar t< / fname>

< / s tudent><student s id= ’S33 ’><fname>Curt< / fname>

< / s tudent>< / s tudents>


A University Example, Referencing

Example (DTD)<?xml vers ion= ” 1.0 ” encoding= ”UTF−8” ?>< !ELEMENT u n i v e r s i t y ( courses ,

students ,f o l l o w s )>

< !ELEMENT courses ( course )+>< !ELEMENT course (name)>< ! ATTLIST course c id ID #REQUIRED>< !ELEMENT name (#PCDATA)>

< !ELEMENT students ( s tudent )+>< !ELEMENT student ( fname )>< ! ATTLIST student s id ID #REQUIRED>< !ELEMENT fname (#PCDATA)>

< !ELEMENT f o l l o w s ( takes )+>< !ELEMENT takes EMPTY>< ! ATTLIST takes s id IDREF #REQUIRED>< ! ATTLIST takes c ids IDREFS #REQUIRED>

Example (XML Fragment)< f o l l o w s><takes s id= ’S11 ’ c ids= ’C111 C222 ’ /><takes s id= ’S22 ’ c ids= ’C222 ’ /><takes s id= ’S33 ’ c ids= ’C111 ’ />

< / f o l l o w s>

NoteID cannot start with digit

sid is a single ID

cids is a set of IDs

No overlap between IDs

Separator is space (not ,)


Quiz: IDREFS

Example (University XML)<u n i v e r s i t y><courses><course c id= ’C111 ’><name>DB< / name>

< / course><course c id= ’C222 ’><name>OOP< / name>

< / course>< / courses><students><student s id= ’S11 ’><fname>Ann< / fname>

< / s tudent><student s id= ’S22 ’><fname>Bar t< / fname>

< / s tudent><student s id= ’S33 ’><fname>Curt< / fname>

< / s tudent>< / s tudents>

Example (Allowed One?)< f o l l o w s><takes s id= ’S11 ’ c ids= ’C111 C222 C111 ’ />

< / f o l l o w s>

Example (Allowed Two?)< f o l l o w s><takes s id= ’S11 ’ c ids= ’C333 C222 C111 ’ />

< / f o l l o w s>

Example (Allowed Three?)< f o l l o w s><takes s id= ’S11 ’ c ids= ’C111 ’ /><takes s id= ’S11 ’ c ids= ’C222 ’ />

< / f o l l o w s>

Example (Allowed Four?)< f o l l o w s><takes s id= ’S11 ’ c ids= ’ ’ /><takes s id= ’S22 ’ c ids= ’ c111 ’ />

< / f o l l o w s>


Using an Internal DTDExample (DTD for Courses with Flexible Description)<?xml vers ion= ” 1.0 ” standalone= ” yes ” ?>< !DOCTYPE courses [<!ELEMENT courses ( course )∗>< !ELEMENT course (name, desc )>< !ELEMENT name (#PCDATA)>< !ELEMENT desc ANY>]><courses>

<course><name>OOP< / name><desc><name>ob jec t−or ien ted< / name><desc>programming< / desc> .

< / desc>< / course>

< / courses>

NoteBenefit: All information in one file

Drawback: DTD is not reused (maintenance nightmare)


Outline

1 Introduction

2 Elements

3 Attributes

4 DTD Find Errors


6 Summary


Summary: DTD

LimitationsOnly very basic data types supported

Only single-column keys (for uniqueness)

Uniqueness only guaranteed within a single document

Very limited support for integrity constraints

NoteDTD is widely used

DTD is being replaced by XML Schema when documents are complex

There are problems using XML Namespace and DTD

AdviseNever build a new DTD if an existing (standard) can be used!


RDBMS vs. XML

RDBMS vs. XML

Query SchemaSQL DML DDLXML XQuery DTD/XML Schema


Summary: DTD versus XML Schema

DTDOwn format

Compact notation

Simple data types

From SGML

Support entities

No support namespaces

XML SchemaXML format

Very verbose

Advanced data types

Invented for XML

Does not support entities

Support namespaces

AdviceStart with a DTD

Move on to XML Schema for later iterations


Technology

Introduction to DTD