XML Reference Guide

Mar 14, 2003

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In XML development, there are actually several "XML Schema" proposals available on the market today. We'll cover some of the others in future sections, but here we're going to discuss the XML Schema Recommendation put out by the W3C.

XML Schemas solve three problems presented by Document Type Definitions:

DTDs use an unfamiliar, non–XML-based syntax.
There's no way to specify two different elements with the same name using a DTD, even if they appear in different places within the document hierarchy.
DTDs have limited datatyping capabilities; it's difficult, if not impossible, to specify that the contents of an element must be a number or a date.

The first two problems are simple; an XML Schema document consists of a valid XML document that describes not only individual elements, as a DTD does, but also their place within the document structure.

The third is arguably where XML Schemas show their power. Not only do XML Schemas provide an opportunity to specify that the content of an element or attribute must conform to one of 47 basic datatypes such as integer or timestamp, but they enable you to create entire element hierarchies as types, and then assign those types to a particular element.

Take our order structure from the earlier "Validation" section:

<order orderid="THX1138" customerNumber="3263827">
  <lineitem itemid="C33">
    <quantity>36</quantity>
    <unitprice currency="dollars">.35</unitprice>
  </lineitem>
  <lineitem itemid="M48">
    <quantity>1</quantity>
    <unitprice currency="dollars">2200</unitprice>
  </lineitem>
</order>

As with a DTD, we can build up an XML schema piece by piece. To start with, let's look at the lineitem element:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

 <xs:element name="order">
  <xs:complexType>
   <xs:sequence>
     <xs:element ref="lineitem" minOccurs="0" maxOccurs="unbounded"/>
   </xs:sequence>
   <xs:attribute name="orderid" type="xs:string" use="required"/>
   <xs:attribute name="customerNumber" type="xs:string" use="required"/>
  </xs:complexType>
 </xs:element>

</xs:schema>

Here we've created a schema document and defined a single element named order. An element that contains only text, and has no attributes, is said to be a simpleType. Because the order element contains other elements, and because it has attributes, it's a complexType, so we need to specify that.

Within the complexType, we first specify that order contains a sequence of other elements. In this case there's just one other element in that sequence, but we'll see more in a moment. The element is a lineitem, and it's optional (as indicated by a minOccurs value of 0). It may also appear any number of times (as indicated by the maxOccurs value of unbounded).

Next we specify the attributes, orderid and customerNumber. Both are designated as type string, one of the predefined types within the XML Schema specification. We've also indicated that both of them are required.

In this case, we've simply referenced an element, lineitem, which we can then define:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

 <xs:element name="order">
  <xs:complexType>
   <xs:sequence>
     <xs:element ref="lineitem" minOccurs="0" maxOccurs="unbounded"/>
   </xs:sequence>
   <xs:attribute name="orderid" type="xs:string" use="required"/>
   <xs:attribute name="customerNumber" type="xs:string" use="required"/>
  </xs:complexType>
 </xs:element>

 <xs:element name="lineitem">
  <xs:complexType>
   <xs:sequence>
     <xs:element name="quantity" type="xs:string"/>
     <xs:element name="unitprice" type="priceType"/>
   </xs:sequence>
   <xs:attribute name="itemid" type="xs:string" use="required"/>
  </xs:complexType>
 </xs:element>

</xs:schema>

The lineitem element is also a complexType, but in this case we're defining most of it inline (within the definition of the lineitem element itself). Notice that we've specified the elements directly by name, rather than by reference. Also notice that this time we've referred to a type (priceType) that's not within the XML Schema namespace (prefixed by xs:). This is a custom type that we can then define explicitly:

<?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

 <xs:element name="order">
  <xs:complexType>
   <xs:sequence>
     <xs:element ref="lineitem" minOccurs="0" maxOccurs="unbounded"/>
   </xs:sequence>
   <xs:attribute name="orderid" type="xs:string" use="required"/>
   <xs:attribute name="customerNumber" type="xs:string" use="required"/>
  </xs:complexType>
 </xs:element>

 <xs:element name="lineitem">
  <xs:complexType>
   <xs:sequence>
     <xs:element name="quantity" type="xs:string"/>
     <xs:element name="unitprice" type="priceType"/>
   </xs:sequence>
   <xs:attribute name="itemid" type="xs:string" use="required"/>
  </xs:complexType>
 </xs:element>

 <xs:complexType type="priceType">
  <xs:simpleContent>
   <xs:extension base="xs:string">
    <xs:attribute name="currency">
     <xs:simpleType>
      <xs:restriction base = "xs:string">
       <xs:enumeration value="dollars"/>
       <xs:enumeration value="pounds"/>
      </xs:restriction>
     </xs:simpleType>
    </xs:attribute>
   </xs:extension>
  </xs:simpleContent>
 </xs:complexType>

</xs:schema>

The actual definition is a bit beyond the scope of this discussion, but notice that we've defined the complexType independently, so we can refer to it from any element we wish. Simply put, priceType describes an element that may have only text content (simpleContent) but has an attribute (currency) that must consist of either the string value dollars or the string value pounds.

In this way, you can create a schema document that's much more flexible than a DTD. To reference the schema from an instance document, for an XML document to be validated against the schema, it's customary to add the schema location via attributes on the root element:

<?xml version="1.0"?>
<order orderid="THX1138" customerNumber="3263827"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:noNamespaceSchemaLocation="myschema.xsd">
  <lineitem itemid="C33">
    <quantity>36</quantity>
    <unitprice currency="dollars">.35</unitprice>
  </lineitem>
  <lineitem itemid="M48">
    <quantity>1</quantity>
    <unitprice currency="dollars">2200</unitprice>
  </lineitem>
</order>

This is known as a hint. In most cases, you can also specify the schema directly from within the application.

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