5.3 XML Schema Constraints vs. XML 1.0 ID AttributesXML Schema Constraints vs. XML 1.0 ID Attributes
[XML 1.0]
provides a mechanism for ensuring uniqueness using
the ID attribute and its associated attributes IDREF and
IDREFS. This mechanism is also provided in XML Schema
through the ID, IDREF, and IDREFS simple types which can be used
for declaring XML 1.0-style attributes. XML Schema also
introduces new mechanisms that are more flexible and
powerful. For example, XML Schema's mechanisms can be
applied to any element and attribute content, regardless of
its type. In contrast, ID is a type of attribute
and so it cannot be applied to attributes, elements or
their content. Furthermore, Schema enables you to specify
the scope within which uniqueness applies whereas the scope
of an ID is fixed to be the whole document. Finally, Schema
enables you to create
keys or a
keyref from combinations of element and
attribute content whereas ID has no such facility.
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