<bookinfo book_id="310494321">
  <category>programming</category>
  <title>Introduction to XML</title>
  <author>Sue Smith</author>
</bookinfo>

Nodes are given an ordering, called document order. In document order, the XML elements and attributes of each node are considered to be in the order in which they appear in the XML document. The document order rules are as follows:

1. Every node appears before any of its descendant nodes.
2. An attribute node appears immediately following its associated element node.
3. The order of siblings is the order in which they appear in the children property of the parent node.
4. Descendant nodes appear before sibling nodes.

The following figure shows the document order given to the nodes of the tree in Figure 1-12:

Figure 1-15 Example of document order

The following table lists the properties that nodes can have.

Table 1-44 Property types and their contents

No.	Property name	Explanation
1	node-name	Qualified name of an XML element or XML attribute
2	namespace	XML namespace to which the node belongs
3	parent	Parent node
4	children	XQuery sequence of child nodes
5	attribute	XQuery sequence of attribute nodes
6	type-name	Qualified name of the XQuery data type of an XML element or XML attribute
7	string-value	Value of the xs:string type character string representing the text that is specified as the node contents. See fn:string function.
8	typed-value	Value of the XQuery data type obtained from the string-value property. See fn:data function.
9	nilled	Property that indicates whether the contents of an XML element are empty. If the contents are empty, the children property does not contain any element nodes or text nodes.
10	processing-instruction-target	The application that is target of the XML processing instructions
11	content	Holds the contents of an XML comment, XML element, or XML processing instruction

The property types that a node has depends on what kind of node it is. The table below lists the properties that each node has. For details about the properties of each node, see 1.15.1(3) Node details.

Table 1-45 Properties of each node

No.	Property name	Document node	Element node	Attribute node	Processing instruction node	Comment node	Text node
1	node-name	N	Y	Y	N	N	N
2	namespace	N	Y	N	N	N	N
3	parent	N	Y	Y	Y	Y	Y
4	children	Y	Y	N	N	N	N
5	attribute	N	Y	N	N	N	N
6	type-name	N	Y	Y	N	N	N
7	string-value	Y	Y	Y	N	N	N
8	typed-value	Y	Y	Y	N	N	N
9	nilled	N	Y	N	N	N	N
10	processing-instruction-target	N	N	N	Y	N	N
11	content	N	N	N	Y	Y	Y

Legend:

Y: Has the property.

N: Does not have the property.

This subsection describes the six kinds of nodes that are in the XQuery data model.

(a) Document node

A document node holds the information for an XML document.

• Properties
  A document node has the following properties:
  

  Property name	Explanation
  children	XQuery sequence of child nodes of the document node
  string-value	Value of the result of concatenating, in document order, the values of the content properties of the descendant text nodes, or a character string of length zero if there are no text nodes descendants
  typed-value	Value of the string-value property as an xs:untypedAtomic type

• Constraints
  Document nodes must obey the following constraints:
  
  1. The only possible child nodes are element nodes, processing instruction nodes, comment nodes, and text nodes. The set of child nodes may be empty. Attribute nodes and document nodes cannot be child nodes.
  2. If node N is included in the children property of document node D, the parent property of node N is node D.
  3. If node N has document node D as its parent property, node N is set to the children property of node D.

(b) Element node

An element node holds the information for an XML element.

• Properties
  An element node has the following properties:
  

  Property name	Explanation
  node-name	Indicates the qualified name of the XML element.
  parent	Indicates the parent node of the element node. The parent node can be either a document node or an element node.
  type-name	Indicates the XQuery data type of the element node. If XML schema validation is not performed, or if XML schema validation results in an element type name that does not correspond to an XQuery data type provided by HiRDB, the value of the type-name property is undefined.
  children	Indicates the XQuery sequence of child nodes of the element node.
  attribute	Indicates the XQuery sequence of attribute nodes for the element node.
  namespace	Indicates the XML namespace to which the element node belongs.
  nilled	Indicates whether the contents of the element node are empty. If the contents are empty, the children property does not contain any element nodes or text nodes.
  string-value	Indicates the result of concatenating, in document order, the values of the content properties of the descendant text nodes. If the contents of the element are empty, the value of this property is a character string of length zero.
  typed-value	If the value of the type-name property is undefined, this is the actual value of the string-value property as an xs:untypedAtomic type. If the XML element is empty, the value of this property is an empty XQuery sequence. Otherwise, it is an XQuery sequence of atomic values obtained from the value of the string-value property based on the XQuery data type of the type-name property.

• Constraints
  Element nodes must obey the following constraints:
  
  1. The only possible child nodes are element nodes, processing instruction nodes, comment nodes, and text nodes. The set of child nodes may be empty. Attribute nodes and document nodes cannot be child nodes.
  2. The qualified name of the XML attribute for each attribute node whose parent property contains the same element node must be distinct.
  3. If node N is included in the children property of element node E, the parent property of node N is set to node E.
  4. If a node N that is not an attribute node has an element node E as its parent property, node N is included in the children property of node E.
  5. If attribute node A is included in the attribute property of element node E, the parent property of node A is node E.
  6. If attribute node A has element node E as its parent property, node A is included in the attribute property of node E.
  7. If the type-name property of an element node is undefined, the type-name property of all its descendant element nodes is also undefined, and the type-name property of all of its attribute nodes is xs:untypedAtomic.
  8. If the type-name property of an element node is undefined, the nilled property is FALSE.
  9. If the nilled property is TRUE, the children property does not contain element nodes or text nodes.
  10. For any qualified name in the node-name property of element node E or in the node-name property of any attribute node A of node E, there must be an XML namespace prefix associated with the XML namespace URI for node E.

(c) Attribute node

An attribute node holds the information for an XML attribute.

• Properties
  An attribute node has the following properties:
  

  Property name	Explanation
  node-name	Indicates the qualified name of the XML attribute.
  parent	Indicates the parent node of the attribute node. The parent node is the node of the elements to which the XML attributes belong.
  type-name	Indicates the name of the XQuery data type of the attribute node.
  string-value	Indicates the values held by the attribute node. This property cannot be empty.
  typed-value	If type-name is xs:untypedAtomic, this is the actual value of the string-value property as an xs:untypedAtomic type. Otherwise, it is an XQuery sequence of atomic values obtained from the string-value property based on the XQuery data type resulting from validation against the XML schema.

• Constraints
  Attribute nodes must obey the following constraints:
  
  1. If attribute node A is included in the attribute property of element node E, the parent property of node A is node E.
  2. If attribute node A has element node E as its parent property, node A is included in the attribute property of node E.

(d) Processing instruction node

A processing instruction node holds information for XML processing instructions.

• Properties
  A processing instruction node has the following properties:
  

  Property name	Explanation
  processing-instruction-target	Indicates the target to which the processing instructions are applied.
  content	Indicates the contents of the processing instructions.
  parent	Indicates the parent node of the processing instruction node.

• Constraints
  A processing instruction node must obey the following constraint:
  
  1. The processing instruction target is a local name.

(e) Comment node

A comment node holds information for an XML comment.

• Properties
  A comment node has the following properties:
  

  Property name	Explanation
  content	Indicates the contents of the XML comment.
  parent	Indicates the parent node of the comment node.

(f) Text node

A text node holds the information for the contents of an XML element.

• Properties
  A text node has the following properties:
  

  Property name	Explanation
  content	Indicates the character string contents of the XML element.
  parent	Indicates the parent node of the text node.

• Constraints
  Text nodes must obey the following constraints:
  
  1. If a text node has a parent node, the value of the content property cannot be a character string of length zero. If the text node does not have a parent node, the value will be a character string of length zero.

The following tables indicate which kinds of nodes can act as the parent node or child nodes of which other kinds of nodes.

Table 1-46 Possible parent nodes for each kind of node

Node kind	Parent node kind
	Document node	Element node	Attribute node	Processing instruction node	Comment node	Text node
Document node	N	N	N	N	N	N
Element node	Y	Y	N	N	N	N
Attribute node	N	Y	N	N	N	N
Processing instruction node	Y	Y	N	N	N	N
Comment node	Y	Y	N	N	N	N
Text node	Y	Y	N	N	N	N

Legend:

Y: Can be parent node.

N: Cannot be parent node.

Table 1-47 Possible child nodes for each node kind

Node kind	Child node kind
	Document node	Element node	Attribute node	Processing instruction node	Comment node	Text node
Document node	N	Y	N	Y	Y	Y
Element node	N	Y	N	Y	Y	Y
Attribute node	N	N	N	N	N	N
Processing instruction node	N	N	N	N	N	N
Comment node	N	N	N	N	N	N
Text node	N	N	N	N	N	N

Legend:

Y: Can be a child node.

N: Cannot be a child node.