XSD - defining an element with a mandatory attribute of a certain value

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Is it possible to define a mandatory attribute with a certain value? In this case, I required at least one of the child element with \[code\]id\[/code\] attribute set to be "parent".\[code\]<root> <child id="parent" /> <!-- value "parent" must always exists in one of the child --> <child id="1" /> <child id="2" /></root>\[/code\]By specifying a restriction, it cannot have other values such as 1 and 2.. etc\[code\]<xs:attribute name="id" use="required" > <xs:simpleType> <xs:restriction base="xs:string"> <xs:enumeration value="http://stackoverflow.com/questions/12670499/parent"/> </xs:restriction> </xs:simpleType></xs:attribute>\[/code\]