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Defining data structures

In XML Schema, data units that are a collection of data fields are defined using complexType elements. Specifying a complex type requires three pieces of information:

  1. The name of the defined type is specified in the name attribute of the complexType element.

  2. The first child element of the complexType describes the behavior of the structure’s fields when it is put on the wire. See Complex type varieties.

  3. Each of the fields of the defined structure are defined in element elements that are grandchildren of the complexType element. See Defining the parts of a structure.

For example, the structure shown in Example 2.3 is be defined in XML Schema as a complex type with two elements.

Example 2.3. Simple Structure

struct personalInfo
{
  string name;
  int age;
};

Example 2.4 shows one possible XML Schema mapping for the structure shown in Example 2.3.

Example 2.4. A complex type

<complexType name="personalInfo">
  <sequence>
    <element name="name" type="xsd:string" />
    <element name="age" type="xsd:int" />
  </sequence>
</complexType>

XML Schema has three ways of describing how the fields of a complex type are organized when represented as an XML document and passed on the wire. The first child element of the complexType element determines which variety of complex type is being used. Table 2.1 shows the elements used to define complex type behavior.

Table 2.1. Complex type descriptor elements

ElementComplex Type Behavior
sequence All the complex type’s fields must be present and they must be in the exact order they are specified in the type definition.
all All of the complex type’s fields must be present but they can be in any order.
choice Only one of the elements in the structure can be placed in the message.

If a sequence element, an all element, or a choice is not specified, then a sequence is assumed. For example, the structure defined in Example 2.4 generates a message containing two elements: name and age.

If the structure is defined using a choice element, as shown in Example 2.5, it generates a message with either a name element or an age element.

Example 2.5. Simple complex choice type

<complexType name="personalInfo">
  <choice>
    <element name="name" type="xsd:string"/>
    <element name="age" type="xsd:int"/>
  </choice>
</complexType>

You define the data fields that make up a structure using element elements. Every complexType element should contain at least one element element. Each element element in the complexType element represents a field in the defined data structure.

To fully describe a field in a data structure, element elements have two required attributes:

In addition to name and type, element elements have two other commonly used optional attributes: minOcurrs and maxOccurs. These attributes place bounds on the number of times the field occurs in the structure. By default, each field occurs only once in a complex type. Using these attributes, you can change how many times a field must or can appear in a structure. For example, you can define a field, previousJobs, that must occur at least three times, and no more than seven times, as shown in Example 2.6.

Example 2.6. Simple complex type with occurrence constraints

<complexType name="personalInfo>
  <all>
    <element name="name" type="xsd:string"/>
    <element name="age" type="xsd:int"/>
    <element name="previousJobs" type="xsd:string:
             minOccurs="3" maxOccurs="7"/>
  </all>
</complexType>

You can also use the minOccurs to make the age field optional by setting the minOccurs to zero as shown in Example 2.7. In this case age can be omitted and the data will still be valid.

Example 2.7. Simple complex type with minOccurs set to zero

<complexType name="personalInfo>
  <choice>
    <element name="name" type="xsd:string"/>
    <element name="age" type="xsd:int" minOccurs="0"/>
  </choice>
</complexType>

In XML documents attributes are contained in the element’s tag. For example, in the complexType element name is an attribute. They are specified using the attribute element. It comes after the all, sequence, or choice element and are a direct child of the complexType element. Example 2.8 shows a complex type with an attribute.

Example 2.8. Complex type with an attribute

<complexType name="personalInfo>
  <all>
    <element name="name" type="xsd:string"/>
    <element name="previousJobs" type="xsd:string"
             minOccurs="3" maxOccurs="7"/>
  </all>
  <attribute name="age" type="xsd:int" use="optional" />
</complexType>

The attribute element has three attributes:

If you specify that the attribute is optional you can add the optional attribute default. The default attribute allows you to specify a default value for the attribute.

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