Difference between group and sequence in XML Schema? - xsd

Question

What is the difference between an xs:group and an xs:sequence in XML Schema? When would you use one or the other?

Answer 1

xs:sequence - together with xs:choice and xs:all - is used to define the valid sequences of XML element in the target XML. E.g. the schema for this XML:
<mainElement>
<firstSubElement/>
<subElementA/>
<subElementB/>
</mainElement>
is something like:
<xs:element name='mainElement'>
<xs:complexType>
<xs:sequence>
<xs:element name="firstSubElement"/>
<xs:element name="subElementA"/>
<xs:element name="subElementB"/>
</xs:sequence>
</xs:complexType>
</xs:element>
xs:group is used to define a named group of XML element following certain rules that can then be referenced in different parts of the schema. For example if the XML is:
<root>
<mainElementA>
<firstSubElement/>
<subElementA/>
<subElementB/>
</mainElementA>
<mainElementB>
<otherSubElement/>
<subElementA/>
<subElementB/>
</mainElementB>
</root>
you can define a group for the common sub-elements:
<xs:group name="subElements">
<xs:sequence>
<xs:element name="subElementA"/>
<xs:element name="subElementB"/>
</xs:sequence>
</xs:group>
and then use it:
<xs:element name="mainElementA">
<xs:complexType>
<xs:sequence>
<xs:element name="firstSubElement"/>
<xs:group ref="subElements"/>
</xs:sequence>
</xs:complexType>
</xs:element>
<xs:element name="mainElementB">
<xs:complexType>
<xs:sequence>
<xs:element name="otherSubElement"/>
<xs:group ref="subElements"/>
</xs:sequence>
</xs:complexType>
</xs:element>