A complex element contains other elements and/or attributes.
What is a Complex Element?
A complex element is an XML element that contains other elements and/or attributes.
There are four kinds of complex elements:
- empty elements
- elements that contain only other elements
- elements that contain only text
- elements that contain both other elements and text
Note: Each of these elements may contain attributes as well!
Examples of Complex Elements
A complex XML element, "product", which is empty:
<product pid="1345"/>
A complex XML element, "employee", which contains only other elements:
<employee>
<firstname>John</firstname>
<lastname>Smith</lastname>
</employee>
A complex XML element, "food", which contains only text:
<food type="dessert">Ice cream</food>
A complex XML element, "description", which contains both elements and text:
<description>
It happened on <date lang="norwegian">03.03.99</date> ....
</description>
How to Define a Complex Element
Look at this complex XML element, "employee", which contains only other elements:
<employee>
<firstname>John</firstname>
<lastname>Smith</lastname>
</employee>
We can define a complex element in an XML Schema two different ways:
- The "employee" element can be declared directly by naming the element, like this:
<xs:element name="employee">
<xs:complexType>
<xs:sequence>
<xs:element name="firstname" type="xs:string"/>
<xs:element name="lastname" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:element>
If you use the method described above, only the "employee" element can use the specified complex type. Note that the child elements, "firstname" and "lastname", are surrounded by the <sequence> indicator. This means that the child elements must appear in the same order as they are declared. You will learn more about indicators in the XSD Indicators chapter.
- The "employee" element can have a type attribute that refers to the name of the complex type to use:
<xs:element name="employee" type="personinfo"/>
<xs:complexType name="personinfo">
<xs:sequence>
<xs:element name="firstname" type="xs:string"/>
<xs:element name="lastname" type="xs:string"/>
</xs:sequence>
</xs:complexType>
If you use the method described above, several elements can refer to the same complex type, like this:
<xs:element name="employee" type="personinfo"/>
<xs:element name="student" type="personinfo"/>
<xs:element name="member" type="personinfo"/>
<xs:complexType name="personinfo">
<xs:sequence>
<xs:element name="firstname" type="xs:string"/>
<xs:element name="lastname" type="xs:string"/>
</xs:sequence>
</xs:complexType>
You can also base a complex type on an existing complex type and add some elements, like this:
<xs:element name="employee" type="fullpersoninfo"/>
<xs:complexType name="personinfo">
<xs:sequence>
<xs:element name="firstname" type="xs:string"/>
<xs:element name="lastname" type="xs:string"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="fullpersoninfo">
<xs:complexContent>
<xs:extension base="personinfo">
<xs:sequence>
<xs:element name="address" type="xs:string"/>
<xs:element name="city" type="xs:string"/>
<xs:element name="country" type="xs:string"/>
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>