We will use "unique" element to define, that the elements "a" under "root" element must have unique value of "id" attribute.

There is no such mechanism in Relax NG, but we can combine Relax NG and Schematron schemas.

We will use "key" element to define, that the elements "a" under "root" element must have unique value of "id" attribute and it must be present.

There is no such mechanism in Relax NG, but we can combine Relax NG and Schematron schemas.

In all checkings of "key", "unique", and "keyref" the values are compared, not string-values.

Schematron uses XPath functions, so we can force the conversion to a number.

We will use "key" element to define, that the elements "a" under "root" element must have unique value of "id" element.

We will use "unique" element to define, that the elements "a" under "root" element must have unique value of "id" element. "Unique" does not require the value to exist, so the element can be "nil".

The "keyref" lets you specify, that attribute/element refers to some node (which must be defined as "key" or "unique"). Now, the "key" element says: the elements "a" under "root" element must have unique value of "id" attribute and this attribute must be present. The "keyref" element specifies that element "b" refers to element "a" (via the attributes: b/@idref -> a/@id).

There is no such mechanism in Relax NG, but we can combine Relax NG and Schematron schemas.