Chapter 9. Defining Uniqueness, Keys, and Key References
Like any storage system, a XML document needs to provide ways to identify and reference pieces of the information it contains. In this chapter, we will present and compare the two features that allow XML to do so with W3C XML Schema. One directly emulates the ID, IDREF, and IDREFs attribute types from the XML DTDs, while the other was introduced to provide more flexibility through the use of XPath expressions.
The first way to describe identifiers and references
with W3C XML Schema is inherited from XML’s DTDs. We
already discussed this in Chapter 5: the
xs:ID
,
xs:IDREF , and
xs:IDREFS datatypes introduced in W3C XML
Schema emulate the behavior of the XML DTD’s ID,
IDREF, and IDREFS attribute types.
Unlike their DTD counterparts, these simple types can be used to
describe both elements and attributes, but inherit the other
restrictions from the DTDs: their lexical space is the same as the
unqualified XML name (known as the
xs:NCName
datatype), and they are global to a
document, meaning that you won’t be allowed to use
the same ID value to identify, for instance, both an author and a
character within the same document.
The restriction on the lexical space can
often prevent you from using an existing node as an identifier. For
instance, in our library, we will not be able to use an ISBN number
as an ID since
xs:NCName cannot start with a number and whitespace is prohibited. We will therefore need to create ...
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