CH 20
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XML SCHEMA
Objective
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 What’s wrong with DTDs?
 What is a schema?
 The W3C XML Schema Language
 Hello schemas
 Complex types
 Simple types
 Deriving simple types
What’s wrong with DTDs?
3
 XML is being used for object serialization, stock
trading, remote procedure calls, vector graphics, and
many more things
1. First issue - DTD lack data type, especially for
element content
a) - DTD can’t say that a PRICE element must contain
a number
b) There’s no way to say that a MONTH element must
be an integer between 1 and 12
c) There’s no way to indicate that a TITLE must
contain between 1 and 255 characters
What’s wrong with DTDs? Cont…
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 Data type is not needed for SGML was aimed
 For computer-to-computer exchange of information,
data type is needed
2. The second problem is that DTDs have an unusual
non-XML syntax
 Parsers and APIs that read an XML document can’t
read a DTD
What’s wrong with DTDs? Cont…
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<!ELEMENT TITLE
(#PCDATA)>
 This above example is not a legal XML. Reason
 You can’t begin an element name with an
exclamation point
 TITLE is not an attribute. Neither is (#PCDATA)
3. The third problem is that DTDs are only
marginally extensible and don’t scale very well
What’s wrong with DTDs? Cont…
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 It’s difficult to combine independent DTDs
together in a sensible way
 It’s difficult to combine independent DTDs
together in a sensible way
 XML applications can be defined before the entire
DTD becomes completely unmanageable and
incomprehensible
4. DTDs don’t allow you to do things that it
really feels like you ought to be able to do
What’s wrong with DTDs? Cont…
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 DTDs cannot enforce the order or number of child
elements in mixed content
 You can’t enforce constraints such as each
PARAGRAPH element must begin with exactly one
SUMMARY element that is followed by plain text
What’s wrong with DTDs? Cont…
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 Schemas are an attempt to solve all these problems
by defining a new XML-based syntax for describing
the permissible contents of XML documents that
includes the following
1. Powerful data typing including range checking
2. Namespace-aware validation based on namespace
URIs rather than on prefixes
3. Extensibility and scalability
What Is a Schema?
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 Schema simply mean form or shape
 Schema was a description of all the tables in a
database(Oracle, MySQL, etc..) and the fields in the table
 Schema also described what type of data each field could
contain. E.g. CHAR,INT, DATE and so on.
 An XML Schema describes the structure of an XML
document
 Different kinds of schemas from different technologies,
including vocabulary schemas, RDF schemas,
organizational schemas, X.500 schemas, and of course,
XML schemas
The W3C XML Schema Language
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 Was created by the W3C XML Schema Working
Group based on many different submissions from a
variety of companies and individuals
 There are no known patent, trademark, or other
intellectual property restrictions that would prevent
you from doing anything you might reasonably want
to do with schemas
Hello Schemas cont…
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 Schema can be written and saved in any text editor
that knows how to save Unicode files
 Schema documents are XML documents and have all
the privileges and responsibilities of other XML
documents
Hello Schemas
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<?xml
version=”1.0”?>
<GREETING>
Hello XML!
</GREETING>
 The greeting schema
<?xml version=”1.0”?>
<xsd:schema
xmlns:xsd=”http://www.w3.org/2001/XMLSchema”>
<xsd:element name=”GREETING” type=”xsd:string”/>
</xsd:schema>
Save as
.xsd
Hello Schemas
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 The root element of this and all other schemas is
schema
 This must be in the
http://www.w3.org/2001/XMLSchema namespace
 Namespace is bound to the prefix xsd or xs
 Elements are declared using xsd:element elements
Hello Schemas
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 The name attribute specifies which element is being
declared
 This xsd:element element also has a type attribute
whose value is the data type of the element
A first Look
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Complex Types
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 The W3C XML Schema Language divides elements




into complex and simple types
A simple type element is one such as GREETING
that can only contain text and does not have any
attributes
It cannot contain any child elements
Complex type elements can have attributes and can
have child elements
Most documents need a mix of both complex and
simple elements
Complex Types cont…
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<?xml version=”1.0”?>
<SONG xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance”
xsi:noNamespaceSchemaLocation=”song.xsd”>
<TITLE>Yes I Am</TITLE>
<COMPOSER>Melissa Etheridge</COMPOSER>
<PRODUCER>Hugh Padgham</PRODUCER>
<PUBLISHER>Island Records</PUBLISHER>
<LENGTH>4:24</LENGTH>
<YEAR>1993</YEAR>
<ARTIST>Melissa Etheridge</ARTIST>
<PRICE>$1.25</PRICE>
</SONG>
 yesiam.xml
Complex Types cont…
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 Schema that describes the xml
song.xsd
<?xml version=”1.0”?>
<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”/>
<xsd:element name=”PRODUCER” type=”xsd:string”/>
<xsd:element name=”PUBLISHER” type=”xsd:string”/>
<xsd:element name=”LENGTH” type=”xsd:string”/>
<xsd:element name=”YEAR” type=”xsd:string”/>
<xsd:element name=”ARTIST” type=”xsd:string”/>
<xsd:element name=”PRICE” type=”xsd:string”/>
</xsd:sequence>
</xsd:complexType>
</xsd:schema>
Complex Types cont…
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 The xsd:complexType element defines a new type
Complex Types cont…
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 minOccurs and maxOccurs
 Specify the minimum and maximum number of
instances of the element that may appear at that
point in the document
 The value of each attribute is an integer greater
than or equal to zero
 The maxOccurs attribute can also have the value
unbounded
 unbounded to indicate that an unlimited number
 of the particular element may appear
 See List 20-7
Complex Types cont…
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This schema says that every SongType element must have, in order:
1. Exactly one TITLE (minOccurs=”1” maxOccurs=”1”)
2. At least one, and possibly a great many, COMPOSERs (minOccurs=”1”
maxOccurs=”unbounded”)
3. Any number of PRODUCERs, although possibly no producer at all
(minOccurs=”0” maxOccurs=”unbounded”)
4. Either one PUBLISHER or no PUBLISHER at all (minOccurs=”0”
maxOccurs=”1”)
5. Exactly one LENGTH (minOccurs=”1” maxOccurs=”1”)
6. Exactly one YEAR (minOccurs=”1” maxOccurs=”1”)
7. At least one ARTIST, possibly more (minOccurs=”1”
maxOccurs=”unbounded”)
8. An optional PRICE, (minOccurs=”0” maxOccurs=”1”)
Complex Types cont…
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 This is much more flexible and easier to use than the
limited ?, *, and + that are available in DTDs
 If minOccurs and maxOccurs are not present, the
default value of each is 1
Complex Types cont…
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 Element content
 When elements contain other elements
 Listing 20-9
Complex Types cont…
 Element Content
<TITLE>Hot Cop</TITLE>
<COMPOSER>
<NAME>Jacques Morali</NAME>
</COMPOSER>
<COMPOSER>
<NAME>Henri Belolo</NAME>
</COMPOSER>
<COMPOSER>
<NAME>Victor Willis</NAME>
</COMPOSER>
<PRODUCER>
<NAME>Jacques Morali</NAME>
</PRODUCER>
<PUBLISHER>PolyGram
Records</PUBLISHER>
<LENGTH>6:20</LENGTH>
<YEAR>1978</YEAR>
<ARTIST>Village People</ARTIST>
</SONG>
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<xsd:complexType
name=”ComposerType”>
<xsd:sequence>
<xsd:element
name=”NAME”
type=”xsd:string”/>
</xsd:sequence>
</xsd:complexType>
<xsd:complexType
name=”ProducerType”>
<xsd:sequence>
<xsd:element
name=”NAME”
type=”xsd:string”/>
</xsd:sequence>
</xsd:complexType>
Complex Types cont…
 Sharing content models
<TITLE>Hot Cop</TITLE>
<COMPOSER>
<NAME>Jacques Morali</NAME>
</COMPOSER>
<COMPOSER>
<NAME>Henri Belolo</NAME>
</COMPOSER>
<COMPOSER>
<NAME>Victor Willis</NAME>
</COMPOSER>
<PRODUCER>
<NAME>Jacques Morali</NAME>
</PRODUCER>
<PUBLISHER>PolyGram
Records</PUBLISHER>
<LENGTH>6:20</LENGTH>
<YEAR>1978</YEAR>
<ARTIST>Village People</ARTIST>
</SONG>
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<xsd:complexType
name=“PersonType”>
<xsd:sequence>
<xsd:element
name=”NAME”
type=”xsd:string”/>
</xsd:sequence>
</xsd:complexType>
Complex Types cont…
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 Mixed Content
<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>
<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>
Complex Types cont…
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 Mixed Content de
<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>
Simple Types
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<xsd:element name=”LENGTH” type=”xsd:duration”/>
<xsd:element name=”YEAR” type=”xsd:gYear”/>
<xsd:element name=”ARTIST” type=”xsd:string”
maxOccurs=”unbounded”/>
<xsd:element name=”PRICE” type=”xsd:string”
minOccurs=”0”/>
</xsd:sequence>
</xsd:complexType>
 xsd:gYear and xsd:duration
 These declarations say that it’s no longer okay for the
YEAR and LENGTH elements to contain just any old
string of text
 Instead, they must contain strings in particular formats
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Simple Types
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 There are 44 built-in simple types in the W3C XML
Schema Language
 These can be unofficially divided into seven groups
✦ Numeric types
✦ Time types
✦ XML types
✦ String types
✦ The boolean type
✦ The URI reference type
✦ The binary types
Go to the Book for Examples
Attributes
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references
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Schema
 http://www.w3schools.com/schema/
Attribute
 http://www.w3schools.com/schema/schema_simple_attri
butes.asp