XML Schema

Chapter 5. Creating Simple Datatypes

So far, we have used only predefined datatypes. In this chapter, we will see how to create new simple types, taking advantage of the different derivation mechanisms and facets of derivation by restriction.

W3C XML Schema has defined three independent and complementary mechanisms for defining our own custom datatypes, using existing datatypes as starting points. These new user datatypes that are built upon existing predefined datatypes or on other user datatypes are called “derivation.”

The three derivation methods are derivation by restriction (where constraints are added on a datatype without changing its original semantic or meaning), derivation by list (where new datatypes are defined as being lists of values belonging to a datatype and take the semantic of list datatypes), and derivation by union (where new datatypes are defined as allowing values from a set of other datatypes and lose most of their semantic).

As with the xs:complexType, definitions (which we saw in our Russian doll design) and xs:simpleType(global definition) can be either named or anonymous. Despite this similarity, simple and complex types are very different. A simple type is a restriction on the value of an element or an attribute (i.e., a constraint on the content of a set of documents) while a complex type is a definition of a content model (i.e., a constraint on the markup). This is why the derivation methods for simple and complex types are very different, even ...