Mastering XML Schemas

The W3C XML Schema Language

Elliotte Rusty Harold

Wednesday, March 26, 2003

elharo@metalab.unc.edu

http://www.cafeconleche.org/

What are Schemas?

Generically, a document that describes what a correct document may contain
Specifically, a W3C Recommendation for an XML-document syntax that describes the permissible contents of XML documents

About Schemas

Created by W3C XML Schema Working Group based on many different submissions
No known patent, trademark, or other IP restrictions
XML Schema Part 1: Structures: http://www.w3.org/TR/xmlschema-1/
XML Schema Part 2: Datatypes: http://www.w3.org/TR/xmlschema-2/

What's Wrong with DTDs?

Unusual, non-XML like syntax
No data typing, especially for element content
Limited extensibility
Only marginally compatible with namespaces
Cannot use mixed content and enforce order and number of child elements
Cannot enforce number of child elements without also enforcing order. (i.e. no & operator from SGML)

greeting.xml

<?xml version="1.0"?>
<GREETING>
Hello XML!
</GREETING>

greeting.xsd

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="GREETING" type="xsd:string"/>

</xsd:schema>

Attaching the schema to the document without namespaces

xsi:noNamespaceSchemaLocation attribute on root element
xsi prefix is mapped to http://www.w3.org/2001/XMLSchema-instance URI

For example,

<?xml version="1.0"?>
<GREETING xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="greeting.xsd">
Hello XML!
</GREETING>

Other means of connecting schemas to documents are allowed

Validating the document with Xerces-J 2.1.0

D:\schemas\examples>java sax.Counter -v greeting2.xml
greeting2.xml: 701 ms (1 elems, 1 attrs, 0 spaces, 12 chars)

An Invalid Document

<?xml version="1.0"?>
<GREETING xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:noNamespaceSchemaLocation="greeting.xsd">
  <P>Hello XML!</P>
</GREETING>

Checking the Invalid Document

D:\speaking\Software Development 2003 West\schemas\examples>java sax.Counter -v greeting3.xml
[Error] greeting3.xml:4:6: Element type "P" must be declared.
[Error] greeting3.xml:5:13: Datatype error: In element 'GREETING' : Can not have
 element children within a simple type content.
greeting3.xml: 781 ms (2 elems, 1 attrs, 0 spaces, 14 chars)

Schema Aware Parsers

Xerces-J 2.x: http://xml.apache.org/xerces2-j
Xerces-J 1.4.4: http://xml.apache.org/xerces-j
Xerces-C++ 1.7.0: http://xml.apache.org/xerces-c
Oracle XML Parser for Java: http://technet.oracle.com/tech/xml/xdk_java/
Oracle XML Parser for C: http://technet.oracle.com/tech/xml/xdk_c/
Oracle XML Parser for C++: http://technet.oracle.com/tech/xml/xdk_cpp/

Complex vs. Simple Types

Complex types can have child elements and attributes
Simple types cannot have children or attributes

Four main schema elements:

xsd:element declares an element and assigns it a type
xsd:attribute declares an attribute and assigns it a type
xsd:complexType defines a new complex type
xsd:simpleType defines a new simple type

A More Complex Document

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="simple_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

A More Complex Schema

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="SongType"/>
 
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"
        minOccurs="1" maxOccurs="1"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        minOccurs="1" maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0" maxOccurs="1"/>
      <xsd:element name="LENGTH"    type="xsd:duration"
        minOccurs="1" maxOccurs="1"/>
      <xsd:element name="YEAR"      type="xsd:gYear"
        minOccurs="1" maxOccurs="1"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        minOccurs="1" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Validating the Song Document

D:\speaking\Software Development 2003 West\schemas\examples>java sax.Counter -v original_hotcop.xml [Error] original_hotcop.xml:10:25: Datatype error: In element 'LENGTH' : Value '6:20' is not legal value for current datatype. null. original_hotcop.xml: 1583 ms (10 elems, 2 attrs, 30 spaces, 98 chars)

Here's the problem:

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="simple_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>6:20</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

The LENGTH element not in the schema time duration format! which is ISO 8601 "PnYn MnDTnH nMnS, where nY represents the number of years, nM the number of months, nD the number of days, 'T' is the date/time separator, nH the number of hours, nM the number of minutes and nS the number of seconds. The number of seconds can include decimal digits to arbitrary precision. An optional preceding minus sign ('-') is allowed, to indicate a negative duration."

Fixed Hot Cop

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="simple_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <LENGTH>P0YT6M20S</LENGTH>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>
</SONG>

A Smaller Schema

Default value of minOccurs is 1
Default value of maxOccurs is 1

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="SongType"/>
 
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:duration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Anonymous Types

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG">
    <xsd:complexType>
      <xsd:sequence>
        <xsd:element name="TITLE"     type="xsd:string"/>
        <xsd:element name="COMPOSER"  type="xsd:string" 
          minOccurs="1" maxOccurs="unbounded"/>
        <xsd:element name="PRODUCER"  type="xsd:string" 
          minOccurs="0" maxOccurs="unbounded"/>
        <xsd:element name="PUBLISHER" type="xsd:string" 
          minOccurs="0" maxOccurs="1"/>
        <xsd:element name="LENGTH"    type="xsd:duration"/>
        <xsd:element name="YEAR"      type="xsd:string"/>
        <xsd:element name="ARTIST"    type="xsd:string" 
          minOccurs="1" maxOccurs="unbounded"/>
      </xsd:sequence>
    </xsd:complexType>
  </xsd:element>
 
</xsd:schema>

Data Typing

Consider this document:

<foo>
    <value>45.67</value>
</foo>

What is the type of value?

Possible types

A longitude or latitude
A decimal monetary type, as in COBOL
A fixed point number
An infinitely precise floating point number such as represented by the java.math.BigDecimal class
An IEEE754 double
A Java double
An IEEE 754 float
A VAX Fortran REAL
An imprecisely known decimal number with 4 significant digits that's plus or minus 1 in the last place.
An imprecisely known decimal number with 4 significant digits that's plus or minus 5 in the last place.
Build 67 of version 45 of Microsoft Word
A regular expression matching all strings that begin with the two characters '4' and '5', followed by a single character, followed by the two characters '6' and '7'.
A string of characters a monkey typed on a keyboard

Other interpretations are doubtless possible, and even make sense in particular contexts. There's no guarantee that the string 45.67 in fact represents any particular type.

The PSVI

A schema assigns an identifiable type to each element
Schema validation produces a Post Schema Validation Infoset, PSVI for short
Schema aware applications using schema aware parsers and APIs can make use of the types of elements

Primitive Data Types for Schemas

Boolean
String
URIs
Numeric types
Time types
XML types
No money types. However, these can be derived

Numeric Data Types for Schemas

XML Schema Built-In Numeric Simple Types
Name	Type	Examples
float	IEEE 754 32-bit floating point number	-INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN
double	IEEE 754 64-bit floating point number	-INF, 1.401E-90, -1E4, -0, 0, 12.78E-2, 12, INF, NaN, 3.4E42
decimal	arbitrary precision, decimal numbers	-2.7E400, 5.7E-444, -3.1415292, 0, 7.8, 90200.76, 3.4E1024
integer	an arbitrarily large or small integer	-500000000000000000000000, -9223372036854775809, -126789, -1, 0, 1, 5, 23, 42, 126789, 9223372036854775808, 456734987324983264987362495809587095720978
nonPositiveInteger	an integer less than or equal to zero	0, -1, -2, -3, -4, -5, ...
negativeInteger	an integer strictly less than zero	-1, -2, -3, -4, -5, ...
long	an eight-byte two's complement integer such as Java's `long` type	-9223372036854775808, -12678967543233, -1, 9223372036854775807
int	an integer that can be represented as a four-byte, two's complement number such as Java's `int` type	-2147483648, -1, 0, 1, 5, 23, 42, 2147483647
short	an integer that can be represented as a two-byte, two's complement number such as Java's `short` type	-32768, -1, 0, 1, 5, 23, 42, 32767
byte	an integer that can be represented as a one-byte, two's complement number such as Java's `byte` type	-128, -1, 0, 1, 5, 23, 42, 127
nonNegativeInteger	an integer greater than or equal to zero	0, 1, 2, 3, 4, 5, ...
unsignedLong	an eight-byte unsigned integer	0, 1, 2, 3, 4, 5, ...18446744073709551614, 18446744073709551615
unsignedInt	a four-byte unsigned integer	0, 1, 2, 3, 4, 5, ...4294967294, 4294967295
unsignedShort	a two-byte unsigned integer	0, 1, 2, 3, 4, 5, ...65534, 65535
unsignedByte	a one-byte unsigned integer	0, 1, 2, 3, 4, 5, ...254, 255
positiveInteger	an integer strictly greater than zero	1, 2, 3, 4, 5, 6, ...

Time Data Types for Schemas

XML Schema Built-In Time Simple Types
Name	Type	Examples
dateTime	a particular moment in Coordinated Universal Time; up to an arbitrarily small fraction of a second	1999-05-31T13:20:00.000-05:00
gMonth	A given month in a given year	2000-10
gYear	a given year	2000
gMonthDay	a date in no particular year, or rather in every year	--10-31
gDay	a day in no particular month, or rather in every mnonth	----31
duration	a length of time, without fixed endpoints, to an arbitrary fraction of a second	P2000Y10M31DT09H32M7.4312S
date	a specific day in history	2000-10-31
time	a specific time of day, that recurs every day	14:30:00.000, 09:30:00.000-05:00

XML Data Types for Schemas

XML Schema Built-In XML Simple Types
Name	Type	Examples
ID	XML 1.0 ID attribute type	any XML name that's unique among ID type attributes
IDREF	XML 1.0 IDREF attribute type	any XML name that's used as an ID type attribute elsewhere in the document
ENTITY	XML 1.0 ENTITY attribute type	any XML name that's declared as an unparsed entity in the DTD
NOTATION	XML 1.0 NOTATION attribute type	any XML name that's declared as a notation name in the DTD
language	Permissible values for `xml:lang` as defined in XML 1.0	en-GB, en-US, fr
IDREFS	XML 1.0 IDREFS attribute type	a white space separated list of IDREF names
ENTITIES	XML 1.0 ENTITIES attribute type	a white space separated list of ENTITY names
NMTOKEN	XML 1.0 NMTOKEN attribute type	12 are you ready
NMTOKENS	XML 1.0 NMTOKENS attribute type	a white space separated list of name tokens
Name	An XML 1.0 Name	set, title, rdf, math, math123, href
QName	an optionally prefixed, namespace qualified name	song:title
NCName	a local name without any colons	title

Assorted Data Types for Schemas

XML Schema Built-In Simple Types
Name	Type	Examples
string	Parsed Character Data; #PCDATA	Hot Cop
normalizedString	A string whose normalized value does not contain any tabs, carriage returns, or linefeeds	PIC1, PIC2, PIC3, cow_movie, MonaLisa, Hello World , Warhol, red green
token	A string whose normalized value has no leading or trailing white space, no tabs, no linefeeds, and not more than one consecutive space	p1 p2, ss123 45 6789, _92, red, green, NT Decl, seventeenp1, p2, 123 45 6789, ^&^&_92, red green blue, NT-Decl, seventeen; Mary had a little lamb, The love of money is the root of all Evil.
boolean	C++'s `bool` type	true, false, 1, 0
anyURI	relative or absolute URI	http://www.w3.org/TR/2000/WD-xmlschema-2-20000407/#duration, /javafaq/reports/JCE1.2.1.html
hexBinary	Arbitrary binary data encoded in hexadecimal form	A4E345EC54CC8D52198000FFEA6C
base64Binary	Arbitrary binary data encoded in Base64	6jKpNnmkkWeArsn5Oeeg2njcz+nXdk0f9kZI892ddlR8Lg1aMhPeFTYuoq3I6neFlb BjWzuktNZKiXYBfKsSTB8U09dTiJo2ir3HJuY7eW/p89osKMfixPQsp9vQMgzph6Qa lY7j4MB7y5ROJYsTr1/fFwmj/yhkHwpbpzed1LE=

Derived Types

You can derive new simple types from existing types.
An xsd:simpleType element defines the subtype
The name attribute of xsd:simpleType assigns a name to the new type, by which it can be referred to in xsd:element type attributes.
An xsd:restriction child element derives by restricting the legal values of the base type
An xsd:list child element derives a type as a white space separated list of base type instances
An xsd:union child element derives by combining legal values from multiple base types

Deriving by restriction

You can derive new simple types types from existing types by restricting the type to a subset of its normal values
An xsd:simpleType element defines the restricted type
The name attribute of xsd:simpleType assigns a name to the new type.
An xsd:restriction child element specifies what type is being restricted via its base attribute.
Facet children of xsd:restriction specify the constraints on the type.

For example, this xsd:simpleType element defines a phonoYear as any year from 1877 (the year Edison invented the phonograph) on:

<xsd:simpleType name="phonoYear">
  <xsd:restriction base="xsd:gYear">
    <xsd:minInclusive value="1877"/>
  </xsd:restriction>
</xsd:simpleType>

Then you declare the year element like this:
<xsd:element type="phonoYear" />

Facets

Facets include:
- length
- minLength
- maxLength
- pattern
- enumeration
- whiteSpace
- maxInclusive
- maxExclusive
- minInclusive
- minExclusive
- totalDigits
- fractionDigits
Not all facets apply to all types.

Length Facets: length, minLength, maxLength

The number of units allowed in a value
For strings (string, normalizedString, token, QName, NCname, ID, IDREF, language, anyURI, ENTITY, NOTATION, and NMTOKEN) the units are characters
For lists (IDREFS, ENTITIES, and NMTOKENS) the units are tokens
For binary types (hexBinary, base64Binary) the units are bytes after decoding
Must be a non-negative integer

For example, to say that all names and titles must contain between 1 and 255 characters:

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="SongType"/>
 
  <xsd:simpleType name="Str255">
    <xsd:restriction base="xsd:string">
       <xsd:minLength value="1"/>
       <xsd:maxLength value="255"/>
    </xsd:restriction>
  </xsd:simpleType>
 
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="Str255"/>
      <xsd:element name="COMPOSER"  type="Str255"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="Str255" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="Str255" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:duration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="Str255" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Facets for ordered items: minExclusive, maxExclusive, minInclusive, maxInclusive

Determines the minimum and maximum allowed values
Applies to ordered simple types including byte, unsignedByte, integer, positiveInteger, negativeInteger, nonNegativeInteger, nonPositiveInteger, int, unsignedInt, long, number, unsignedLong, short, unsignedShort, number, float, double, time, dateTime, duration, date, gMonth, gYear, gDay, and gMonthDay.

For example, to say that the year must be between 1877 and 2100:

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="SongType"/>
 
  <xsd:simpleType name="phonoYear">
    <xsd:restriction base="xsd:gYear">
      <xsd:minInclusive value="1877"/>
      <xsd:maxInclusive value="2100"/>
    </xsd:restriction>
  </xsd:simpleType> 
 
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:duration"/>
      <xsd:element name="YEAR"      type="phonoYear"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

whiteSpace

Determines what the validator should do with white space before validating the value
Three possible values:
- preserve: The white space in the input document is left unchanged
- replace: Each tab, carriage return and linefeed is replaced with a single space.
- collapse: Each tab, carriage return and linefeed is replaced with a single space. Furthermore, after this replacement is performed, all runs of multiple spaces are condensed to a single space. Leading and trailing white space is deleted.
Applies to string, normalizedString and token type items
Per XML 1.0, white space in attributes is normalized irregardless of the schema

For example, to say that white space should be collapsed in all names and titles:

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="SongType"/>

  <xsd:simpleType name="CollapsedString">
    <xsd:restriction base="xsd:string">
       <xsd:whiteSpace value="collapse"/>
    </xsd:restriction>
  </xsd:simpleType>
 
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="CollapsedString"/>
      <xsd:element name="COMPOSER"  type="CollapsedString"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="CollapsedString" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="CollapsedString" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:duration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="CollapsedString" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Facets for decimal numbers: totalDigits and fractionDigits

The totalDigits facet specifies the maximum number of decimal digits in a number as a positive integer
The fractionDigits facet specifies the maximum number of decimal digits to the right of the decimal point as a non-negative integer
Applies to all types derived from decimal including byte, unsignedByte, integer, positiveInteger, negativeInteger, nonNegativeInteger, nonPositiveInteger, int, unsignedInt, long, unsignedLong, short, and unsignedShort.
Does not apply to float and double
You can specify at most two fractional digits or at most seven decimal digits, but not at least two fractional digits or exactly seven decimal digits

Enumeration

The enumeration facet lists all allowed values
Applies to all simple types except boolean

For example, to say that the publisher must be one of the oligopoly that controls 90% of U.S. music (Warner-Elektra-Atlantic, Universal Music Group, Sony Music Entertainment, Inc., Capitol Records, Inc., BMG Music)

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:element name="SONG" type="SongType"/>
 
  <xsd:simpleType name="oligopolyMember">
    <xsd:restriction base="xsd:string">
      <xsd:enumeration value="Warner-Elektra-Atlantic"/>
      <xsd:enumeration value="Universal Music Group"/>
      <xsd:enumeration value="Sony Music Entertainment, Inc."/>
      <xsd:enumeration value="Capitol Records, Inc."/>
      <xsd:enumeration value="BMG Music"/>
    </xsd:restriction>
  </xsd:simpleType> 
 
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string"
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="oligopolyMember" 
        minOccurs="0"/>
      <xsd:element name="LENGTH"    type="xsd:duration"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>
 
</xsd:schema>

Adding a Price

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="priced_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
  <PRICE>$1.35</PRICE>  
</SONG>

The pattern facet

Suppose you want a money type to specify that the PRICE element content must look like $1.35 or ¥11000
Derive this from the xsd:string type by restriction
Use the pattern facet to specify a regular expression instances must match

Regular Expressions

More or less Perl-like including the Unicode extensions introduced in Perl 5.6
The money regular expression:
\p{Sc}\p{Nd}+(\.\p{Nd}\p{Nd})?

\p{Sc}

Any Unicode currency indicator; e.g. $, ¥, £, ¤, etc.

\p{Nd}
A Unicode decimal digit character

\p{Nd}+
One or more Unicode decimal digit characters

\.
The period character

(\.\p{Nd}\p{Nd})
A string with a period followed by two decimal digits like .35

(\.\p{Nd}\p{Nd})?
Zero or one strings of the form .35

The Price Schema

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:simpleType name="money">
    <xsd:restriction base="xsd:string">             
      <xsd:pattern value="\p{Sc}\p{Nd}+(\.\p{Nd}\p{Nd})?"/>
    <!-- 
       Regular Expression:
       \p{Sc}             Any Unicode currency indicator; e.g. $, &#xA5, &#xA3, &#A4, etc.
       \p{Nd}             A Unicode decimal digit character
       \p{Nd}+            One or more Unicode decimal digit characters
       \.                 The period character
       (\.\p{Nd}\p{Nd})
       (\.\p{Nd}\p{Nd})?  Zero or one strings of the form .35
       
       This works for any decimalized currency. 
       
    -->
    </xsd:restriction>
  </xsd:simpleType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRICE" type="money"/>      
     </xsd:sequence> 
  </xsd:complexType>
  
</xsd:schema>

Complex Types

Elements that contain child elements or have attributes or both
Defined by an xsd:complexType element

A Document with Attributes

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="attribute_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

Declaring Attributes

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <!-- An empty element -->
  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

Element Content

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="nested_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>
    <NAME>
      <GIVEN>Jacques</GIVEN>
      <FAMILY>Morali</FAMILY>
    </NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>
      <GIVEN>Henri</GIVEN>
      <FAMILY>Belolo</FAMILY>
    </NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>
      <GIVEN>Victor</GIVEN>
      <FAMILY>Willis</FAMILY>
    </NAME>
  </COMPOSER>
  <PRODUCER>
    <NAME>
      <GIVEN>Jacques</GIVEN>
      <FAMILY>Morali</FAMILY>
    </NAME>
  </PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

Declaring Complex Types

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="ComposerType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:sequence>
             <xsd:element name="GIVEN"  type="xsd:string"/>
             <xsd:element name="FAMILY" type="xsd:string"/>  
          </xsd:sequence>    
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="ProducerType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:sequence>
            <xsd:element name="GIVEN"  type="xsd:string"/>
            <xsd:element name="FAMILY" type="xsd:string"/>
          </xsd:sequence>      
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="ComposerType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="ProducerType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

Sharing Content Models

PRODUCER and COMPOSER are really the same type.

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PersonType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:sequence>
             <xsd:element name="GIVEN"  type="xsd:string"/>
             <xsd:element name="FAMILY" type="xsd:string"/>  
          </xsd:sequence>    
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="PersonType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="PersonType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

Mixed Content

Schemas let you enforce order and appearance of elements in mixed content.

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="mixed_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>
    <NAME>Mr. <GIVEN>Jacques</GIVEN> <FAMILY>Morali</FAMILY> Esq.</NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>Mr. <GIVEN>Henri</GIVEN> L. <FAMILY>Belolo</FAMILY>, M.D.</NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME>Mr. <GIVEN>Victor</GIVEN> C. <FAMILY>Willis</FAMILY></NAME>
  </COMPOSER>
  <PRODUCER>
    <NAME>Mr. <GIVEN>Jacques</GIVEN> S. <FAMILY>Morali</FAMILY></NAME>
  </PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

Declaring Mixed Content

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PersonType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType mixed="true">
          <xsd:sequence>
            <xsd:element name="GIVEN"  type="xsd:string"/>
            <xsd:element name="FAMILY" type="xsd:string"/>
           </xsd:sequence>      
         </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="PersonType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="PersonType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

When Order Doesn't Matter

<?xml version="1.0"?>
<SONG xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="unordered_song.xsd">
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>
    <NAME><FAMILY>Morali</FAMILY> <GIVEN>Jacques</GIVEN></NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME><GIVEN>Henri</GIVEN> <FAMILY>Belolo</FAMILY></NAME>
  </COMPOSER>
  <COMPOSER>
    <NAME><FAMILY>Willis</FAMILY> <GIVEN>Victor</GIVEN></NAME>
  </COMPOSER>
  <PRODUCER>
    <NAME><GIVEN>Jacques</GIVEN> <FAMILY>Morali</FAMILY></NAME>
  </PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

The xsd:all Group

Each element in the xsd:all group must occur zero or once; that is minOccurs and maxOccurs must each be 0 or 1
The xsd:all group must be the top level element of its type
The xsd:all group may contain only individual element declarations; no choices or sequences

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PersonType">
    <xsd:sequence>
      <xsd:element name="NAME">
        <xsd:complexType>
          <xsd:all>
            <xsd:element name="GIVEN"  type="xsd:string"/>
            <xsd:element name="FAMILY" type="xsd:string"/> 
          </xsd:all>     
        </xsd:complexType>
      </xsd:element>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="PersonType" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="PersonType" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <!-- An empty element -->
  <xsd:complexType name="PhotoType" content="empty">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

Choices

xsd:choice requires exactly one of a group of specified elements to appear
The choice can have minOccurs and maxOccurs attributes that adjust this from zero to any given number.

<xsd:complexType name="SongType">
  <xsd:sequence>
    <xsd:element name="TITLE" type="xsd:string"/>
    <xsd:element name="COMPOSER" type="PersonType"/>
    <xsd:choice minOccurs="0" maxOccurs="unbounded">
      <xsd:element name="PRODUCER" type="PersonType"/>
      <xsd:element name="COMPOSER" type="PersonType"/>
      <xsd:element name="ARTIST"   type="xsd:string"/>
    </xsd:choice>
    <xsd:element name="ARTIST" type="xsd:string"/>
    <xsd:element name="PUBLISHER" type="xsd:string"
                 minOccurs="0"/>
    <xsd:element name="LENGTH" type="xsd:string"/>
    <xsd:element name="YEAR"   type="xsd:string"/>
    <xsd:element name="PRICE" type="xsd:string" minOccurs="0"/>
  </xsd:sequence>
</xsd:complexType>

Sequences

xsd:sequence requires each child element it specifies to appear in the specified order
The sequence can have minOccurs and maxOccurs attributes that repeat each sequence zero to any given number of times.

Schemas and Namespaces

Namespaces in XML rules and definitions:
- Elements and attributes that are in namespaces are called qualified
- All unprefixed attributes are unqualified
- All prefixed elements are qualified
- Unprefixed elements may or may not be qualified. They are qualified if they are in a default namespace.
Each schema has a target namespace
Each schema can define elements in attributes in its target namespace
A schema can also define unqualified attributes of elements in its target namespace.
A schema can also define unqualified child elements of elements in its target namespace. Unqualified child elements are called local elements. This is a very bad idea!
A schema may not define elements and attributes in namespaces other than the target namespace; i.e., for each namespace there must be at least one schema
Schemas can reference global elements and attributes defined in other schemas by importing the schema with xsd:import and referencing the global elements and attributes defined therein.

Default Namespace

<?xml version="1.0"?>
<GREETING 
  xmlns="http://www.cafeconleche.org/schemas/greeting/"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://www.cafeconleche.org/schemas/greeting/
                      greeting_defaultNS.xsd">
  Hello XML!
</GREETING>

The targetNamespace attribute

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.cafeconleche.org/schemas/greeting/"
>
 
  <xsd:element name="GREETING" type="xsd:string"/>

</xsd:schema>

A Song with a Namespace

All elements are in the default namespace and are therefore qualified.
All attributes are unprefixed and are therefore unqualified.

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<SONG xmlns="http://www.cafeconleche.org/namespace/song"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation = 
       "http://www.cafeconleche.org/namespace/song namespace_song.xsd"
>
  <TITLE>Hot Cop</TITLE>
  <PHOTO ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

A Schema for a Document that Uses the Default Namespace

The elementFormDefault attribute must have the value qualified to indicate that the locally defined elements—i.e., TITLE, PHOTO, COMPOSER, PRODUCER, PUBLISHER, YEAR, and ARTIST but not SONG—are qualified.
Globally defined elements such as SONG are always qualified by the target namespace
The default value of elementFormDefault is unqualified. This is exactly backwards from what it should be.
The default value of attributeFormDefault is unqualified. This is correct.
This mess was caused by an misguided attempt to make local elements and attributes equivalent.
Simplest approach: either don't use namespaces at all or make sure all elements are namespace qualified and set elementFormDefault="qualified"

<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.cafeconleche.org/namespace/song"
  elementFormDefault="qualified"
  attributeFormDefault="unqualified"
>
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE" type="xsd:string"/>
      <xsd:element name="PHOTO" type="PhotoType"  
        minOccurs="0"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>    
      <xsd:element name="YEAR" type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

  <xsd:complexType name="PhotoType">
    <xsd:complexContent>
       <xsd:restriction base="xsd:anyType">
         <xsd:attribute name="ALT"    type="xsd:string"/>
         <xsd:attribute name="WIDTH"  type="xsd:positiveInteger"/>
         <xsd:attribute name="HEIGHT" type="xsd:positiveInteger"/> 
      </xsd:restriction>
    </xsd:complexContent>
  </xsd:complexType>
  
</xsd:schema>

Multiple Namespaces, Multiple Schemas

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<SONG xmlns="http://www.cafeconleche.org/namespace/song"
      xmlns:xlink="http://www.w3.org/1999/xlink"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation = 
       "http://www.cafeconleche.org/namespace/song xlink_song.xsd
        http://www.w3.org/1999/xlink xlink.xsd"
>
  <TITLE>Hot Cop</TITLE>
  <PHOTO xlink:type="simple" xlink:href="hotcop.jpg"
         xlink:actuate="onLoad" xlink:show="embed"
    ALT="Victor Willis in Cop Outfit" WIDTH="100" HEIGHT="200"/>
  <COMPOSER>Jacques Morali</COMPOSER>
  <COMPOSER>Henri Belolo</COMPOSER>
  <COMPOSER>Victor Willis</COMPOSER>
  <PRODUCER>Jacques Morali</PRODUCER>
  <PUBLISHER>PolyGram Records</PUBLISHER>
  <YEAR>1978</YEAR>
  <ARTIST>Village People</ARTIST>  
</SONG>

XLink Schema

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.w3.org/1999/xlink"
>

  <xsd:attribute name="type" type="xsd:string" 
                 fixed="simple" />
  <xsd:attribute name="href" type="xsd:anyURI" />
  <xsd:attribute name="actuate" type="xsd:string"
                 fixed="onLoad" />
  <xsd:attribute name="show" type="xsd:string"
                 fixed="embed" />

</xsd:schema>

Song Schema with XLink Support

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  xmlns:xlink="http://www.w3.org/1999/xlink"
  targetNamespace="http://www.cafeconleche.org/namespace/song"
  elementFormDefault="qualified"
>

  <xsd:import namespace="http://www.w3.org/1999/xlink"
              schemaLocation="xlink.xsd"/>

  <xsd:element name="SONG" type="SongType"/>

  <xsd:complexType name="PhotoType">
    <xsd:attribute name="WIDTH"  type="xsd:nonNegativeInteger"
                   use="required" />
    <xsd:attribute name="HEIGHT" type="xsd:nonNegativeInteger"
                   use="required" />
    <xsd:attribute name="ALT"    type="xsd:string"
                   use="required" />
    <xsd:attribute ref="xlink:type"/>
    <xsd:attribute ref="xlink:href" use="required"/>
    <xsd:attribute ref="xlink:actuate"/>
    <xsd:attribute ref="xlink:show"/>                 
  </xsd:complexType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="PHOTO"     type="PhotoType"/>
      <xsd:element name="COMPOSER"  type="xsd:string"
                   maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string"
                   minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string"
                   minOccurs="0"/>
      <xsd:element name="YEAR"      type="xsd:gYear"/>
      <xsd:element name="ARTIST"    type="xsd:string"
                   maxOccurs="unbounded"/>
    </xsd:sequence>
  </xsd:complexType>

</xsd:schema>

Annotations

An xsd:annotation element describes its parent element.
A top-level xsd:annotation element describes the entire schema.
Its xsd:documentation child element describes the schema for human readers.
Its xsd:appInfo child element describes the schema for computer programs; e.g. stylesheet instructions.

<?xml version="1.0"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema">

  <xsd:annotation>
    <xsd:documentation>
      Song schema example for XML Schemas tutorial
      Copyright 2001 Elliotte Rusty Harold. 
    </xsd:documentation>
  </xsd:annotation>
 
  <xsd:element name="SONG" type="SongType"/>

  <xsd:simpleType name="money">
    <xsd:restriction base="xsd:string">             
      <xsd:pattern value="\p{Sc}\p{Nd}+(\.\p{Nd}\p{Nd})?">
        <xsd:annotation>
          <xsd:documentation>
       \p{Sc}             Any Unicode currency indicator; e.g. $, &#xA5, &#xA3, &#A4, etc.
       \p{Nd}             A Unicode decimal digit character
       \p{Nd}+            One or more Unicode decimal digit characters
       \.                 The period character
       (\.\p{Nd}\p{Nd})
       (\.\p{Nd}\p{Nd})?  Zero or one strings of the form .35
       
       This works for any decimalized currency.           
          </xsd:documentation>
        </xsd:annotation>
      </xsd:pattern>
    </xsd:restriction>
  </xsd:simpleType>

  <xsd:complexType name="SongType">
    <xsd:sequence>
      <xsd:element name="TITLE"     type="xsd:string"/>
      <xsd:element name="COMPOSER"  type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRODUCER"  type="xsd:string" 
        minOccurs="0" maxOccurs="unbounded"/>
      <xsd:element name="PUBLISHER" type="xsd:string" 
        minOccurs="0"/>
      <xsd:element name="YEAR"   type="xsd:gYear"/>
      <xsd:element name="ARTIST" type="xsd:string" 
        maxOccurs="unbounded"/>
      <xsd:element name="PRICE" type="money"/>      
     </xsd:sequence> 
  </xsd:complexType>
  
</xsd:schema>

What Schemas don't do

Cannot declare entities
Parent models
Extra-document validation

W3C XML Schema Language Alternatives

Rick Jelliffe's Schematron
Murato Makoto's RELAX
James Clark's TREX
RELAX + TREX -> RELAX NG
Rick Jelliffe's Hook
xlinkit
Eric van der Vlist's Examplotron
DTDs

Schematron

According to Schematron inventor Rick Jelliffe:
The Schematron differs in basic concept from other schema languages in that it not based on grammars but on finding tree patterns in the parsed document. This approach allows many kinds of structures to be represented which are inconvenient and difficult in grammar-based schema languages.
Language is based on XPath
Validator is implemented in XSLT
W3C Schemas are conservative: everything not permitted is forbidden.
Schematron is liberal: everything not forbidden is permitted.
No data typing; validation only
Handles unordered structures very well
Handles descendant constraints very well
Almost self-documenting
http://www.ascc.net/xml/resource/schematron/schematron.html

A Schematron schema for songs

A schema contains a title and a pattern
Each pattern contains rule child elements
Each rule contains assert and report elements and has a context attribute
Each assert and report element has a test attribute containing an XPath expression whihc returns a boolean.
The contents of each assert element is printed if the assertion test fails
The contents of each report element is printed if the report test succeeds

<?xml version="1.0"?>
<schema xmlns="http://www.ascc.net/xml/schematron">
  <title>A Schematron Schema for Songs</title>
  <pattern>
    <rule context="SONG">
      <assert test="TITLE">
        A SONG must contain an initial TITLE element.
      </assert>
      <assert test="TITLE[position()=1]">
        The TITLE element must be the initial element of the SONG element.
      </assert>
      <assert test="COMPOSER">
        A SONG must contain at least one COMPOSER element.
      </assert>
      <assert test="ARTIST">
        A SONG must contain at least one ARTIST element.
      </assert>
    </rule>
  </pattern>
</schema>

RELAX NG

Combination of RELAX and TREX
OASIS standard
Uses externally defined data types including W3C Schema simple data types + facets
Mostly DTD-like structures
Allows fully unordered content; e.g. a name must contain one FirstName, zero or one LastNames, and any number of MiddleNames in any order.
http://www.oasis-open.org/committees/relax-ng/

A RELAX-NG Song Schema

<?xml version="1.0"?>
<element name="SONG" xmlns="http://relaxng.org/ns/structure/1.0"
    datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
  <interleave>
  
    <element name="TITLE">
      <text/>
    </element>
    
    <zeroOrMore>
      <element name="COMPOSER">
        <text/>
      </element>
    </zeroOrMore>
    
    <zeroOrMore>
      <element name="PRODUCER">
        <text/>
      </element>
    </zeroOrMore>
    
    <optional>
      <element name="PUBLISHER">
        <text/>
      </element>
    </optional>

    <element name="LENGTH">
      <data type="duration"/>
    </element>

    <element name="YEAR">
      <data type="gYear"/>
    </element>

    <oneOrMore>
      <element name="ARTIST">
        <text/>
      </element>
    </oneOrMore>

    <optional>
      <element name="PRICE">
        <data type="string">
          <param name="pattern">\p{Sc}\p{Nd}+(\.\p{Nd}\p{Nd})?</param>
        </data>
      </element>
    </optional>
    
  </interleave>
</element>

Non-XML RELAX NG Syntax

element SONG {
  element TITLE { text }
  & element COMPOSER { text }*
  & element PRODUCER { text }*
  & element PUBLISHER { text }?
  & element LENGTH { xsd:duration }
  & element YEAR { xsd:gYear }
  & element ARTIST { text }+
  & element PRICE {
      xsd:string { pattern = "\p{Sc}\p{Nd}+(\.\p{Nd}\p{Nd})?" }
    }?
}

Hook

Rick Jelliffe's Hook: A One-Element Language for Validation of XML Documents based on Partial Order
XSLT/XPath based
No data typing; validation only

A Hook schema for XHTML (adapted from Rick Jelliffe):

<hook:order targetNamespace="http://www.w3.org/1999/xhtml" >
  html head  [ title; meta. link. base. ]   body
  [ a br. blockquote caption; div  dl; h1; h2; h3; h4; h5; h6;  
    img. ol; p; pre; table; ul; ]  
  [ tr;  dt; dd; li; ]  td 
  [ a br. blockquote div  form img. ol; ul; li; ]  
  [ input; label; select; textarea; ]  [ option. ]
  [ abbr acronym address cite code dfn em kbd q samp span strong var object; ] 
  param 
</hook:order>

http://www.ascc.net/xml/hook/

Examplotron

Invented by Eric van der Vlist
Schemas look like the documents they model
Validator is an XSLT stylesheet
http://www.examplotron.org

<?xml version="1.0"?>
<SONG xmlns:eg="http://examplotron.org/0/">
  <TITLE>Put title here</TITLE>
  <COMPOSER eg:occurs="+">Author of song</COMPOSER>
  <PRODUCER eg:occurs="*">Put producer here</PRODUCER>
  <PUBLISHER eg:occurs="?">Put publisher here</PUBLISHER>
  <LENGTH>Length of song</LENGTH>
  <YEAR>Year song was published</YEAR>
  <ARTIST eg:occurs="+">Singers and musicians</ARTIST>  
</SONG>

DTDs aren't Dead!

To Learn More

XML in a Nutshell, 2nd Edition
- Elliotte Rusty Harold and W. Scott Means
- O'Reilly & Associates, 2002
- ISBN 0-596-00292-0
XML Bible, Gold edition, Chapter 23
- Elliotte Rusty Harold
- Hungry Minds, 2001
- ISBN 0-7645-4819-0
- http://www.cafeconleche.org/books/biblegold/chapters/ch24.html
This presentation: http://www.cafeconleche.org/slides/sd2003west/schemas
W3C Schema Primer: http://www.w3.org/TR/xmlschema-0/
XML Schema Part 1: Structures: http://www.w3.org/TR/xmlschema-1/
XML Schema Part 2: Datatypes: http://www.w3.org/TR/xmlschema-2/

Index | Cafe con Leche