XML data definition|V4.3.3| docs|Distributed Database

XML data definition

Last Updated：2024-12-02 03:48:26 Updated

XML syntax

The current version of OceanBase Database supports the XML 1.0 syntax, but does not support XML schemas or Document Type Definitions (DTDs). An XML document is mainly composed of two parts: the XML declaration and document content. The document content usually contains elements, attributes, comments, escape characters, and processing instructions.

An XML declaration has a fixed structure, such as <?xml version="1.0"?>. The XML document content has only one root element, which can contain any number of child elements. An element can contain attributes. For example, <book isbn="123-456-789">...<title>mybook</title><author>Alex</author></book> contains the isbn="123-456-789" attribute and two child elements: <title> and <author>. Child elements must be nested correctly. An empty element, such as <tag></tag>, can be represented by an empty-element tag, such as <tag/>.

An XML document uses identifiers, such as brackets (<>) and quotation marks (' or "). If the document content contains such special characters, you need to use &???; to escape them. For example, Java<tm> as the content of the child element name is escaped to <name>Java<tm></name>. The following table lists the built-in escape characters in XML 1.0.

Before escaping	After escaping
<	`<`
>	`>`
&	`&`
"	`"`
'	`'`

In an XML document, you can use the following syntax to define namespaces to avoid naming conflicts:

xmlns:namespace-prefix="namespaceURI"

The following sample XML document contains two table elements. Since the elements have different definitions, they must be distinguished by using namespaces. "http://www.example1.org/TR/html4/" and "http://www.example2.com.cn/furniture" are URLs that are used only as the unique names of the namespaces.

<doc>
  <h:table xmlns:h="http://www.example1.org/TR/html4/">
    <h:tr>
      <h:td>Apples</h:td>
      <h:td>Bananas</h:td>
    </h:tr>
  </h:table>
  <f:table xmlns:f="http://www.example2.com.cn/furniture">
    <f:name>African Coffee Table</f:name>
    <f:width>80</f:width>
    <f:length>120</f:length>
  </f:table>
</doc>

Note that when a namespace is defined in the start tag of an element, all child elements with the same prefix are associated with this namespace.

XPath

XML Path Language (XPath) is a language used to locate specific elements in an XML document. You can use XPath in EXTRACT, UPDATEXML, and other XML functions.

An XML document itself can be viewed as a node tree that contains element nodes, attribute nodes, and text nodes. XPath allows you to locate nodes in the data structure tree. XPath query results can be node sets (unordered collections without duplicate nodes), Boolean values (True or False), numbers, or strings.

The following sample code shows the complete XPath syntax. Different location nodes in an XPath is separated by /. You can omit / for the first node.

/axis::nodetest()

Here is an example:

/descendant::a, /child::text()

Parameters in the preceding example are described as follows:

descendant-or-self::para selects the para elements of the current node and all its descendants.
self::para selects the para element of the current node. If the current node does not have the para element, NULL is returned.
child::*/child::para selects all para grandchildren of the current node.
/ selects the root node.

Location steps

XPath uses location steps to select nodes in an XML document.

The following table describes the location steps supported by OceanBase Database.

Abbreviated syntax for location step	Description
nodename	Selects all children of only this node.
/	Selects from the root node.
//	Selects nodes in the document from the current node that match the selection no matter where they are.
.	Selects the current node.
..	Selects the parent of the current node.
@	Selects attributes.

Axes

An axis tells the XPath processor which direction to head from the current node.

The following table describes the axes supported by OceanBase Database. Among them, child is the default axis.

Axis	Description
ancestor	Selects all ancestors (parent, grandparent, and so on) of the current node.
ancestor-or-self	Selects all ancestors (parent, grandparent, and so on) of the current node and the current node itself.
attribute	Selects all attributes of the current node.
child	Selects all element children of the current node.
descendant	Selects all descendants (children, grandchildren, and so on) of the current node.
descendant-or-self	Selects all descendants (children, grandchildren, and so on) of the current node and the current node itself.
namespace	Selects all namespace nodes of the current node.
parent	Selects the parent of the current node.
self	Selects the current node.

Note

The current version of OceanBase Database does not support the following-sibling, following, preceding, or preceding-sibling axis.

The following table provides examples of axes.

Example	Description
child::book	Selects all `book` nodes that are children of the current node.
attribute::lang	Selects the `lang` attribute of the current node.
child::*	Selects all element children of the current node.
attribute::*	Selects all attributes of the current node.
child::text()	Selects all text node children of the current node.
child::node()	Selects all children of the current node.
descendant::book	Selects all `book` descendants of the current node.
ancestor::book	Selects all `book` ancestors of the current node.
ancestor-or-self::book	Selects all `book` ancestors of the current node and itself if it is a `book` node.
child::*/child::price	Selects all `price` grandchildren of the current node.

Node tests

A node test is used together with an axis to tell the XPath processor which nodes need to be selected. By default, element nodes are selected.

The following table describes the node tests supported by OceanBase Database.

Node test	Description
para_name	Selects the node with the specified name. If the axis is `attribute`, this node test specifies an attribute name. If the axis is `namespace`, this node test specifies a namespace name. For other axes, this node test specifies an element name.
text()	Selects `text` nodes (without specifying the name).
node()	Selects all types of nodes (without specifying the name).
comment()	Selects `comment` nodes (without specifying the name).
processing-instruction(literal)	Selects `pi` nodes. When `literal` is not specified, the node test matches all `pi` nodes. Otherwise, it matches only the `pi` node of the specified name.

Predicates

A predicate tells the XPath processor to select a specific node or a node that contains a specific value.

The following table provides some examples of predicates supported by OceanBase Database.

Path expression	Description
//title[@lang='eng']	Selects all `title` elements whose `lang` attribute is `eng`.
/bookstore/book[price>35.00]	Selects all `book` elements whose `price` is greater than 35.00 in the `bookstore` element.
/bookstore/book[price>35.00]/title	Select all `title` elements of the `book` element whose `price` is greater than 35.00 in the `bookstore` element.

Wildcards

An XPath wildcard allows you to select any XML element.

The following table describes the wildcards supported by OceanBase Database.

Wildcard	Description
*	Matches any element node.
@*	Matches any attribute node.
node()	Matches any type of node.
text()	Matches any text node.

The following table provides examples of wildcards.

Path expression	Description
/bookstore/*	Selects all element children of the `bookstore` element.
//*	Selects all elements in the document.
//title[@* = "attribute"]	Selects all `title` elements with attributes.

Operators

The following table describes the operators supported for XPaths in OceanBase Database.

Operator	Description	Example	Return value
=	Equal	price=9.80	If `price` is `9.80`, `true` is returned. Otherwise, `false` is returned.
!=	Not equal	price!=9.80	If `price` is not `9.80`, `true` is returned. Otherwise, `false` is returned.
<	Less than	price<9.80	If `price` is less than `9.80`, `true` is returned. Otherwise, `false` is returned.
<=	Less than or equal to	price<=9.80	If `price` is less than or equal to `9.80`, `true` is returned. Otherwise, `false` is returned.
>	Greater than	price>9.80	If `price` is greater than `9.80`, `true` is returned. Otherwise, `false` is returned.
>=	Greater than or equal to	price>=9.80	If `price` is greater than or equal to `9.80`, `true` is returned. Otherwise, `false` is returned.
or	Or	price=9.80 or price=9.70	If `price` is `9.80` or `9.70`, `true` is returned. Otherwise, `false` is returned.
and	And	price>9.00 and price<9.90	If `price` is greater than `9.00` and less than `9.90`, `true` is returned. Otherwise, `false` is returned.

Function expressions

The following table describes the function expressions supported for XPath definitions in the current version of OceanBase Database.

Category	Function expression	Description
Boolean	boolean true()	Returns `TRUE`.
	boolean false()	Returns `FALSE`.