Complex Elements

A complex element is an XML element that contains other elements or attributes. There are four types of complex elements: empty elements, elements that contain other elements, elements that contain only text and attributes, and elements that contain both text and other elements.

In the following XML code block <Tree> is consider a complex element because it contains other elements:

Trees.xml

<?xml version="1.0" ?>
<TreeInventory xsi:noNamespaceSchemaLocation="Trees.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    
    <Tree tid="1">
        <ItemNumber>1047</ItemNumber>
        <TreeName>
            <CommonName>Colorado Blue Spruce</CommonName>
            <ScientificName>Picea pungens</ScientificName>
        </TreeName>
        <Description>A magnificent sight of silver blue-green spruce.</Description>
        <Price>1.99</Price>
        <Quantity>50</Quantity>
        <Type>Evergreen</Type>
        <Picture>1047.jpg</Picture>
    </Tree>

</TreeInventory>

Listing 3-17. Complex elements.

A complex element is declared using the <xsd:complexType> designation. It is coded immediately below the element declaration. If a complex element contains other elements, the <xsd:sequence> is used to indicate that the parent element contains a sequence of child elements.

Trees.xsd

<xsd:element name="Tree">
    <xsd:complexType>
        <xsd:sequence>
            <xsd:element name="ItemNumber" type="xsd:integer"/>
            <xsd:element name="TreeName" type="xsd:string"/>
            ..... other elements .....
        </xsd:sequence>
    </xsd:complexType>
</xsd:element>

Listing 3-18. Complex element declaration.

The partial Schema above declares Tree a complex element that contains other elements. Its child elements are listed between the open and closing <xsd:sequence> block. Here ItemNumber and TreeName are defined as child elements of the Tree element. In the XML file, the sequence of child elements must appear in the order defined by the Schema.

Recall that an empty element that contains attributes is considered a complex element. Suppose an XML empty element named Tree is defined and assigned an attribute tid:

<Tree tid="1111"/>

Listing 3-19. Defining complex elements.

In the Schema below, we define the Tree element followed by the complexType element. Since this element does not include content, the Tree element is included. The complexContent element signals that we intend to restrict or extend the content model of a complex type, and the restriction of integer declares one attribute but does not introduce any element content.

<xsd:element name="Tree">
    <xsd:complexType>
        <xsd:complexContent>
            <xsd:restriction base="xsd:integer">
                <xsd:attribute name="tid" type="xsd:integer"/>
            </xsd:restriction>
        </xsd:complexContent>
    </xsd:complexType>
</xsd:element>

Listing 3-20. Restricting or extending complex type models.

An "elements-only" complex type contains an element that contains only other elements. Below the <TreeName> element contains two other elements.

Trees.xml

<?xml version="1.0" ?>

<TreeName>
    <CommonName>Colorado Blue Spruce</CommonName>
    <ScientificName>Picea pungens</ScientificName>
</TreeName>

Listing 3-21. "Elements-only" complex type (XML).

The Schema is defined as follows:

Trees.xsd

<xsd:element name="TreeName">
    <xsd:complexType>
        <xsd:sequence>
            <xsd:element name="CommonName" type="xsd:string"/>
            <xsd:element name="ScientificName" type="xds:string"/>
        </xsdsequence>
    </xsd:complexType>
</xsd:element>

Listing 3-22. "Elements-only" complex type (XSD).

First the <TreeName> element is defined followed by the complexType designation. Since the <TreeName> element contains two other elements in sequence, they are defined within the sequence designation. In the XML file, all instances of the <TreeName> element must contain a common name and a scientific name. Also, common name must always preceed scientific name.

In addition to complex elements that are empty elements, and elements that contain other elements, some complex elements contain only text and an attribute. Recall that elements that contain only text and no attributes are considered Simple Elements.

<Tree tid="1111"/>White Oak</Tree>

Listing 3-23. Simple content.

This type contains only simple content (text and attributes), therefore we add a simpleContent element around the content. When using simple content, you must define an extension or a restriction within the simpleContent element:

<xsd:element name="Tree">
    <xsd:complexType>
        <xsd:simpleContent>
            <xsd:extension base="xsd:string">
                <xsd:attribute name="tid" type="xsd:integer"/>
            </xsd:extension>
        </xsd:simpleContent>
    </xsd:complexType>
</xsd:element>

Listing 3-24. Defining extensions and restrictions within simple elements.

The last type of complex element is an element that contains both text and other elements. This is also known as a mixed type. The example below uses the fields of a tree order form to demonstrate a mixed type.

<TreeOrder>
You ordered a<Tree>Red Maple</Tree>.
The tree id is <tid>1222</tid>
The cost is <price>4.00</price>.
</TreeOrder>

Listing 3-25. Mixed complex element.

The Schema below is used to define the elements of the XML document:

<xsd:element name="TreeOrder">
    <xsd:complexType mixed="true">
        <xsd:sequence>
            <xsd:element name="Tree" type="xsd:string"/>
            <xsd:element name="tid" type="xsd:integer"/>
            <xsd:element name="price" type="xsd:decimal"/>
        </xsd:sequence>
    </xds:complexType>
</xsd:element>

Listing 3-26. Defining XML document elements.

To enable character data to appear between the child-elements of the TreeOrder element, the mixed attribute must be set to "true". The <xsd:sequence> defines the elements that appear within the TreeOrder (Tree, tid, price). Again, when coded in the XML document, these child-elements must appear in the correct order to ensure that the XML content is valid.

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