Xml schema, how to make sure one element exists with a specific attribute value

Question

How do I enforce the existing of an element with a specific attribute value in the XML?

For example:

Answer 1

If you're using XML Schema 1.0, you cannot express the constraint directly. You could do it outside of XML Schema 1.0 via Schematron or XSLT directly.

If you're using XML Schema 1.1, you can specify co-occurrence constraints via xs:assert:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           version="1.0">
  <xs:element name="events">
    <xs:complexType>
      <xs:sequence>
        <xs:element name="event" minOccurs="1" maxOccurs="unbounded">
          <xs:complexType>
            <xs:attribute name="type" type="xs:string"/>
            <xs:attribute name="desc" type="xs:string"/>
          </xs:complexType>
        </xs:element>
      </xs:sequence>
      <xs:assert test="count(event[@type = 'system']) = 1"/>
    </xs:complexType>
  </xs:element>
</xs:schema>