XML data definition

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&lt;tm&gt;</name>. The following table lists the built-in escape characters in XML 1.0.

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.

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.

The following table provides examples of axes.

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.

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.

Wildcards

An XPath wildcard allows you to select any XML element.

The following table describes the wildcards supported by OceanBase Database.

The following table provides examples of wildcards.

Operators

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

Function expressions

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