What Is "keyref" Identity-Constraint?

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∟Identity-Constraints: unique, key and keyref

∟What Is "keyref" Identity-Constraint?

This section describes the 'keyref' identity-constraint, where a combination of certain sub elements or attributes is defined as a key reference identity, which must have values that reference to an existing key identity.

What Is "keyref" Identity-Constraint? A "keyref" Identity-Constraint is a constraint on an XML element, where a combination of certain sub elements or attributes is defined as a key reference identity, which must have values that reference to an existing key identity.

"keyref" identity-constraint in XML document is very similar to the concept of FOREIGN KEY constraint on multiple columns in a database table.

Here is the syntax on how a "keyref" identity-constraint should be defined in XSD schema:

 <xs:element name="parent" ...>
  ...
  <xs:keyref name="constraint-name" refer="key-name>
   <xs:selector xpath="sub-element-path"/>
   <xs:field xpath="field-1-path"/>
   <xs:field xpath="field-2-path"/>
   ...
  </xs:keyref>
 </xs:element>

Note that:

The "keyref" declaration must be placed inside the "element" statement which declares the parent element where the constraint will be applied.
"keyref[@name]" attribute is required to provide a name for this key reference identity constraint.
"keyref[@refer]" attribute is required to provide the name of a key identity constraint.
"selector" is required to specify a relative XPath to match a sequence of sub elements from which identity values will be constructed and validated.
At least 1 "field" is required.
Each "field" specifies a relative XPath pointing to an attribute or a simple element content, which will be used as part of the identity construction.

For example, I have the following XML document, keyref_identity.xml:

<?xml version="1.0" encoding="utf-8"?>
<!-- keyref_identity.xml
 - Copyright (c) 2002-2013 HerongYang.com. All Rights Reserved.
-->
<survey>
<questions>
 <question code="Java">What is Java?</question>
 <question code="John">Who is John?</question>
 <question code="Jim1">Where is Jim?</question>
 <question code="Jim2">What is Jim doing?</question>
</questions>
<answers>
 <answer qcode="Java">Java is an island of Indonesia.</answer>
 <answer qcode="Java">Java is programming language.</answer>
 <answer qcode="John">John is a character in Atlas Shrugged.</answer>
 <answer qcode="Jojo">Jojo is a singer-songwriter.</answer>
 <answer qcode="Jim1">Jim is in Java island.</answer>
 <answer qcode="Jim2">Jim is having a coffee.</answer>
 <answer>These are good questions.</answer>
</answers>
</survey>

For this XML document, I want to have an XSD schema that has two identity-constraints to ensure values in "question[@code]" attributes can be used as a key identity. And values in "answer[@qcode]" attributes are referring to an existing value in "question[@code]" attribute.