An XSD Example
This chapter will demonstrate how to write an XML Schema. You
will also learn that a schema can be written in different ways.
An XML Document
Let's have a look at this XML document called "shiporder.xml":
<?xml version="1.0" encoding="ISO-8859-1"?>
<shiporder orderid="889923"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="shiporder.xsd">
<orderperson>John Smith</orderperson>
<shipto>
<name>Ola Nordmann</name>
<address>Langgt 23</address>
<city>4000 Stavanger</city>
<country>Norway</country>
</shipto>
<item>
<title>Empire Burlesque</title>
<note>Special Edition</note>
<quantity>1</quantity>
<price>10.90</price>
</item>
<item>
<title>Hide your heart</title>
<quantity>1</quantity>
<price>9.90</price>
</item>
</shiporder>
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The XML document above consists of a root element, "shiporder",
that contains a required attribute called "orderid". The "shiporder" element contains three
different child
elements: "orderperson", "shipto" and "item". The "item" element
appears twice, and it contains a "title", an optional "note" element, a "quantity", and a "price"
element.
The line above: xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
tells the XML parser that this document should be validated against a schema.
The line: xsi:noNamespaceSchemaLocation="shiporder.xsd" specifies WHERE
the schema resides (here it is in the same folder as "shiporder.xml").
Create an XML Schema
Now we want to create a schema for the XML document above.
We start by opening
a new file that we will call "shiporder.xsd". To create the schema we could simply follow
the structure in the XML document and define each element as we find it. We
will start with the standard XML declaration followed by the xs:schema element that defines a schema:
<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
...
...
</xs:schema>
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In the schema above we use the standard namespace (xs), and the URI
associated with this namespace is the Schema language definition, which has the
standard value of http://www.w3.org/2001/XMLSchema.
Next, we have to define the "shiporder" element. This element has an
attribute and it contains other elements, therefore we consider it as a complex
type. The child elements of the "shiporder" element is surrounded by a
xs:sequence element that defines an ordered sequence of sub elements:
<xs:element name="shiporder">
<xs:complexType>
<xs:sequence>
...
...
</xs:sequence>
...
</xs:complexType>
</xs:element>
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Then we have to define the "orderperson" element as a simple type (because it does
not contain any attributes or other elements). The type (xs:string) is prefixed
with the namespace prefix associated with XML Schema that indicates a predefined
schema data type:
<xs:element name="orderperson" type="xs:string"/>
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Next, we have to define two elements that are of the complex type: "shipto" and
"item". We start by defining the "shipto" element:
<xs:element name="shipto">
<xs:complexType>
<xs:sequence>
<xs:element name="name" type="xs:string"/>
<xs:element name="address" type="xs:string"/>
<xs:element name="city" type="xs:string"/>
<xs:element name="country" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
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With schemas we can define the number of possible occurrences for an element
with the maxOccurs and minOccurs attributes. maxOccurs specifies the maximum
number of occurrences for an element and minOccurs specifies the minimum number
of occurrences for an element. The default value for both maxOccurs and
minOccurs is 1!
Now we can define the "item" element. This element can appear multiple times
inside a "shiporder" element. This is specified by setting the maxOccurs
attribute of the "item" element to "unbounded" which means that there can be as
many occurrences of the "item" element as the author wishes. Notice that the
"note" element is optional. We have specified this by setting the minOccurs
attribute to zero:
<xs:element name="item" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="title" type="xs:string"/>
<xs:element name="note" type="xs:string" minOccurs="0"/>
<xs:element name="quantity" type="xs:positiveInteger"/>
<xs:element name="price" type="xs:decimal"/>
</xs:sequence>
</xs:complexType>
</xs:element>
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We can now declare the attribute of the "shiporder" element. Since
this is a required attribute we specify use="required".
Note: The attribute declarations must
always come last:
<xs:attribute name="orderid" type="xs:string" use="required"/>
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Here is the complete listing of the schema file called "shiporder.xsd":
<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:element name="shiporder">
<xs:complexType>
<xs:sequence>
<xs:element name="orderperson" type="xs:string"/>
<xs:element name="shipto">
<xs:complexType>
<xs:sequence>
<xs:element name="name" type="xs:string"/>
<xs:element name="address" type="xs:string"/>
<xs:element name="city" type="xs:string"/>
<xs:element name="country" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="item" maxOccurs="unbounded">
<xs:complexType>
<xs:sequence>
<xs:element name="title" type="xs:string"/>
<xs:element name="note" type="xs:string" minOccurs="0"/>
<xs:element name="quantity" type="xs:positiveInteger"/>
<xs:element name="price" type="xs:decimal"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
<xs:attribute name="orderid" type="xs:string" use="required"/>
</xs:complexType>
</xs:element>
</xs:schema>
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Divide the Schema
The previous design method is very simple, but can be difficult to read and
maintain when documents are complex.
The next design method is based on defining all elements and attributes
first, and then referring to them using the ref attribute.
Here is the new design of the schema file ("shiporder.xsd"):
<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<!-- definition of simple elements -->
<xs:element name="orderperson" type="xs:string"/>
<xs:element name="name" type="xs:string"/>
<xs:element name="address" type="xs:string"/>
<xs:element name="city" type="xs:string"/>
<xs:element name="country" type="xs:string"/>
<xs:element name="title" type="xs:string"/>
<xs:element name="note" type="xs:string"/>
<xs:element name="quantity" type="xs:positiveInteger"/>
<xs:element name="price" type="xs:decimal"/>
<!-- definition of attributes -->
<xs:attribute name="orderid" type="xs:string"/>
<!-- definition of complex elements -->
<xs:element name="shipto">
<xs:complexType>
<xs:sequence>
<xs:element ref="name"/>
<xs:element ref="address"/>
<xs:element ref="city"/>
<xs:element ref="country"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="item">
<xs:complexType>
<xs:sequence>
<xs:element ref="title"/>
<xs:element ref="note" minOccurs="0"/>
<xs:element ref="quantity"/>
<xs:element ref="price"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="shiporder">
<xs:complexType>
<xs:sequence>
<xs:element ref="orderperson"/>
<xs:element ref="shipto"/>
<xs:element ref="item" maxOccurs="unbounded"/>
</xs:sequence>
<xs:attribute ref="orderid" use="required"/>
</xs:complexType>
</xs:element>
</xs:schema>
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Using Named Types
The third design method defines classes or types, that enables us to reuse element
definitions. This is done by naming the simpleTypes and complexTypes elements,
and then point to them through the type attribute of the element.
Here is the third design of the schema file ("shiporder.xsd"):
<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:simpleType name="stringtype">
<xs:restriction base="xs:string"/>
</xs:simpleType>
<xs:simpleType name="inttype">
<xs:restriction base="xs:positiveInteger"/>
</xs:simpleType>
<xs:simpleType name="dectype">
<xs:restriction base="xs:decimal"/>
</xs:simpleType>
<xs:simpleType name="orderidtype">
<xs:restriction base="xs:string">
<xs:pattern value="[0-9]{6}"/>
</xs:restriction>
</xs:simpleType>
<xs:complexType name="shiptotype">
<xs:sequence>
<xs:element name="name" type="stringtype"/>
<xs:element name="address" type="stringtype"/>
<xs:element name="city" type="stringtype"/>
<xs:element name="country" type="stringtype"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="itemtype">
<xs:sequence>
<xs:element name="title" type="stringtype"/>
<xs:element name="note" type="stringtype" minOccurs="0"/>
<xs:element name="quantity" type="inttype"/>
<xs:element name="price" type="dectype"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="shipordertype">
<xs:sequence>
<xs:element name="orderperson" type="stringtype"/>
<xs:element name="shipto" type="shiptotype"/>
<xs:element name="item" maxOccurs="unbounded" type="itemtype"/>
</xs:sequence>
<xs:attribute name="orderid" type="orderidtype" use="required"/>
</xs:complexType>
<xs:element name="shiporder" type="shipordertype"/>
</xs:schema>
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The restriction element indicates that the datatype is derived from a W3C XML
Schema namespace datatype. So, the following fragment means that the value of
the element or attribute must be a string value:
<xs:restriction base="xs:string">
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The restriction element is more often used to apply restrictions to elements.
Look at the following lines from the schema above:
<xs:simpleType name="orderidtype">
<xs:restriction base="xs:string">
<xs:pattern value="[0-9]{6}"/>
</xs:restriction>
</xs:simpleType>
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This indicates that the value of the element or attribute must be a string, it must be exactly six characters in a row, and those characters must be a
number from 0 to 9.
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