url - Universität zu Lübeck

18 CHAPTER 2. FUNDAMENTALS
plies that the element has to appear exactly once.
The element is assigned to the user-defined type positiveNumber type
that is defined in line 15 as a restriction of the integer type. Another user-defined
type is assigned to the element . A description contains a
element that is of a mixed content type consisting of and elements.
Due to the keyword mixed="true" in line 26 an atomic text value can
appear between the elements and .
A great strength of XML Schemas is that they are written in XML syntax, so that
existing XML editors and parsers can be reused. XML Schema can be created and
processed by any software that works with XML documents. Especially an XML
Schema can be validated against the XML Schema that defines valid XML Schema
documents.
Because XML Schemas tend to be large XML documents (especially larger than
DTDs) it is difficult to perceive them. Therefore, a graphical representation is often
more comfortable. Figure 2.7 presents XMLSpys [4] visualization of the sample
schema.
Figure 2.7: A graphical representation of an XML Schema
Although it is semantically possible to create an XML Schema c that defines the
union of two XML Schemas a and b this is not processed as expected by the validating
parsers like XMLSpy . Syntactically, c can be defined using the inheritance
feature of XML Schema (see appendix 10.3). Therefore, as DTDs, XML Schema
are not closed under union; details can be found in [86, 87].