What is an XML schema?
·
An XML schema
consists of various definitions including
o elements and
attributes that appear in a document
o child elements
o sequence of
elements
o number of child
elements
o whether an element
can include text or has to be empty
o default values of
attributes
·
More or less, an XML schema,
just like a DTD, defines the legal building blocks of XML documents
XML Schema basic elements:
·
Complex Type
o Elements having
element content can be controlled by <complexType> definitions. Within
each <complexType> definition, we specify the allowable element content
for the declaration
·
Element
o When declaring an
element, we specify the element name and define the content that is allowed in
that element.
·
Sequence
o Sequences are used
to impose order on the child elements.
Global versus Local Types:
·
XML Schemas allow us to specify an
element’s type in two ways : Local or Global
·
Global declarations are declarations appearing as direct
children of the <schema> element. These global element declarations can
be reused throughout the XML Schema
·
Local declarations don’t have the <schema> element as
their direct parent and thus, can be used only in their specific context
XML Schema Advantages:
·
Learning XML schemas
is easy
·
Understanding XML Schemas
is easy as the schema itself is written in XML
·
Support provided for more data types
·
Support for namespaces is provided.
·
Objected-oriented in a sense
o We can use the old
data types by extending them to create new custom data types or we can restrict
them (deriving new types on the basis of the old definitions)
·
We can model sets, i.e., we can define the occurrences of the
child elements in any order (just as in a set)
·
We can maintain uniqueness while writing the content of the
element (keys on content) and uniqueness within a specific region
·
We can declare the elements that don’t take any content (empty
elements)
·
substitutable elements can be defined
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