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XSD Indicators

How to use XSD indicators (all, choice, sequence, maxOccurs, minOccurs, group, and attributeGroup) to control element ordering, occurrence, and grouping in XML Schema definitions.

Indicators control the way how elements are to be organized in an XML document.

Indicators

There are seven types of indicators that fall into three broad categories.

Order Indicators

  • All: Child elements can occur in any order.
  • Choice: Only one of the child element can occur.
  • Sequence: Child element can occur only in specified order.

Occurrence Indicators

  • maxOccurs: Child element can occur only maxOccurs number of times.
  • minOccurs: Child element must occur minOccurs number of times.

Group Indicators

  • Group: Defines related set of elements.
  • attributeGroup: Defines related set of attributes.

Order Indicators

Order indicators are used to define the order of the elements.

All Indicator

The <xs:all> indicator specifies that the child elements can appear in any order, and that each child element must occur only once:

<xs:element name="person">
	<xs:complexType>
		<xs:all>
			<xs:element name="firstname"  type="xs:string"/>
			<xs:element name="lastname"  type="xs:string"/>
		</xs:all>
	</xs:complexType>
</xs:element>

Choice Indicator

The <xs:choice> indicator specifies that either one child element or another can occur:

<xs:element name="person">
	<xs:complexType>
		<xs:choice>
			<xs:element name="employee"  type="employee"/>
			<xs:element name="member"  type="member"/>
		</xs:choice>
	</xs:complexType>
</xs:element>

Sequence Indicator

The <xs:sequence> indicator specifies that the child elements must appear in a specific order:

<xs:element name="person">
	<xs:complexType>
		<xs:sequence>
			<xs:element name="firstname"  type="xs:string"/>
			<xs:element name="lastname"  type="xs:string"/>
		</xs:sequence>
	</xs:complexType>
</xs:element>

Occurrence Indicators

Occurrence indicators are used to define how often an element can occur.

maxOccurs Indicator

The maxOccurs indicator specifies the maximum number of times an element can occur:

<xs:element name="person">
	<xs:complexType>
		<xs:sequence>
			<xs:element name="full_name"  type="xs:string"/>
			<xs:element name="child_name"  type="xs:string" maxOccurs="10"/>
		</xs:sequence>
	</xs:complexType>
</xs:element>

minOccurs Indicator

The minOccurs indicator specifies the minimum number of times an element can occur:

<xs:element name="person">
	<xs:complexType>
		<xs:sequence>
			<xs:element name="full_name" type="xs:string"/>
			<xs:element name="child_name" type="xs:string" minOccurs="0" maxOccurs="10"/>  
		</xs:sequence>
	</xs:complexType>
</xs:element>

Group Indicators

Group indicators are used to define related sets of elements.

Element Groups

Element groups are defined with the group declaration in this way:

<xs:group name="groupname">  
	...  
</xs:group>

For example:

<xs:group name="persongroup">
	<xs:sequence>
		<xs:element name="firstname" type="xs:string"/>
		<xs:element name="lastname" type="xs:string"/>
		<xs:element name="birthday" type="xs:date"/>
	</xs:sequence>
</xs:group>

<xs:element name="person" type="personinfo"/>

<xs:complexType name="personinfo">
	<xs:sequence>
		<xs:group ref="persongroup"/>
		<xs:element name="country" type="xs:string"/>
	</xs:sequence>
</xs:complexType>

Attribute Groups

Attribute groups are defined with the attributeGroup declaration in this way:

<xs:attributeGroup name="groupname">  
	...  
</xs:attributeGroup>

For example:

<xs:attributeGroup name="personattrgroup">
	<xs:attribute name="firstname" type="xs:string"/>
	<xs:attribute name="lastname" type="xs:string"/>
	<xs:attribute name="birthday" type="xs:date"/>
</xs:attributeGroup>
  
<xs:element name="person">
	<xs:complexType>
		<xs:attributeGroup ref="personattrgroup"/>
	</xs:complexType>
</xs:element>
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