Complex data types

Complex type elements fall down into the following categories:

• empty elements containing only attributes
• elements containing text only and attributes, but no child elements
• elements containing child elements, but no attributes
• elements containing both child elements and attributes

A general structure of a complex type declaration is as follows:

Empty elements and attributes

The syntax is as follows:

Example:

The following XML element:
<person name="Joe Doe" age="50" />
has two attributes which can be declared as follows:
<xs:element name="person">
  <xs:complexType>
    <xs:attribute name="name" type="xs:string" />
    <xs:attribute name="age" type="xs:string" />
  </xs:complexType>
</xs:element>

Elements with attributes but no child elements

If the element is not empty and contains attributes and no child elements, the syntax is as follows:

Extension is a tool for creating complex custom data types.

Example:

The following XML element:
<person gender="male" age="50">Joe Doe</person>
has two attributes which can be declared as follows:
<xs:element name="person">
  <xs:complexType>
    <xs:simpleContent>
      <xs:extension base="xs:string">
        <xs:attribute name="gender" type="xs:string" />
        <xs:attribute name="age" type="xs:string" />
    </xs:simpleContent>
  </xs:complexType>
</xs:element>

Cross-referencing elements and attributes

Rather than nesting the attribute declarations within an element, one can create references to them by following the general syntax:

Thus, the above example can be rewritten as follows:

Example:

The following XML element:
<person gender="male" age="50">Joe Doe</person>
has two attributes which can be declared as follows:
<xs:attribute name="gender" type="xs:string" />
<xs:attribute name="age" type="xs:string" />
<xs:element name="person">
  <xs:complexType>
    <xs:simpleContent>
      <xs:extension base="xs:string">
        <xs:attribute ref="gender" use="required" />
        <xs:attribute ref="age" use="required" />
    </xs:simpleContent>
  </xs:complexType>
</xs:element>

Elements with child elements but no attributes

For such elements we use the compositor

XML schema supports the following compositors:

• sequence - defines an order for the child elements
• choice - allows only ONE element from the list to appear as a child element
• all - allow any of the child elements to appear with no restriction on the order and occurrence.

Example:

<element name="address">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="street" style="xs:string" />
      <xs:element name="city" style="xs:string" />
      <xs:element name="state" style="xs:string" />
      <xs:element name="country" style="xs:string" />
    </xs:sequence>
  </xs:complexType>
<element>

Example:

<element name="sponsor">
  <xs:complexType>
    <xs:choice>
      <xs:element name="parent" style="xs:string" />
      <xs:element name="guardian" style="xs:string" />
    </xs:choice>
  </xs:complexType>
<element>

Example:

<element name="parents">
  <xs:complexType>
    <xs:all>
      <xs:element name="father" style="xs:string" />
      <xs:element name="mother" style="xs:string" />
    </xs:all>
  </xs:complexType>
<element>

Compositors can be nested and combined with each other:

Example:

<element name="account">
  <xs:complexType>
    <xs:sequence>
      <xs:choice>
        <xs:element name="person" style="xs:string" />
        <xs:element name="company" style="xs:string" />
      </xs:choice>
      <xs:choice>
        <xs:element name="checking" style="xs:string" />
        <xs:element name="savings" style="xs:string" />
      </xs:choice>
    </xs:sequence>
  </xs:complexType>
<element>

Specifying the item occurrence

This can be done by using the attributed minOccurs and maxOccurs (case sensitive!).

The maxOccurs attribute, if present, can take on any positive value or the value "unbounded". If some of those attributes are missing, their default values are set up as follows:

Elements with child elements and attributes

The general syntax is as follows:

In the below example the element and attributed declarations can be used instead of the references. Note that comment as it is defined, is an optional child element.

Example:

<element name="patient">
  <xs:complexType>
    <xs:sequence>
      <xs:element ref="lastName" />
      <xs:element ref="firstName" />
      <xs:element ref="id" />
      <xs:element ref="comment" minOccurs="0" maxOccurs="unbounded" />
    </xs:sequence>
  <xs:attribute ref="age" />
  <xs:attribute ref="gender" />
  </xs:complexType>
<element>

Mixed content

The attribute mixed="true" of element complexType specifies the element containing both text and child elements. The structure of the child elements can be specified by the methods described above.

Example:

The XML content
<summary>
  Patient <name>Joe Doe</name> was taken by
  <personnel>Mary Brown</personnel> on 8/3/2005.
</summary>
can be declared as follows:
<element name="summary">
  <xs:complexType mixed="true">
    <xs:sequence>
      <xs:element ref="name" />
      <xs:element ref="personnel" />
    </xs:sequence>
  </xs:complexType>
<element>