Difference between / and /root-node

Question

My document looks like the following:

<a>
    whatever
</a>

If I run / or /a on the entire document is returned(at least effectively). If I run /a/.. the entire document is returned. But /.. returns an empty sequence Considering / and /a are returning the same node how come /a/.. and /.. are different?

Answer 1

The Document Node

The XML code you provided as document is actually wrapped in another node, the "document node". The document is another node kind, others are elements, attributes, text nodes, comments and processing instructions. Using XQuery/XPath 2.0 notation, it would look something like this:

document{
  <a>
      whatever
  </a>
}

Effects on Queries

• / selects the document node
• /a selects the root element, which is the only child of the document node
• /.. returns the empty sequence, as the document node has no parent node
• /a/.. again selects the parent node of the root element, which again is the document node
• /../a has no results, as we "stepped out of the tree" (compare with /..)

Why we Need a Document Node

The document node is important, as the XML specification allows other nodes to follow the root node, namely processing instructions and comments (and whitespace). From the XML grammar:

document       ::=           prolog element Misc*
Misc           ::=           Comment | PI | S

Without a document node, these elements wouldn't be reachable for XPath, as they are no elements of the "root element subtree".

So, this would also be a valid XML document (*):

document {
  <a>
      whatever
  </a>
  <!-- Just do nothing -->
  <?php foo(); ?>
}

(*) This isn't valid XPath 2.0 any more, as we would have to give a node sequence. I omitted the commas , after each node necessary for XPath 2.0, as this is only for demonstration purpose.

Answer 2

The expressions / and /a are not the same and don't return the entire document. / selects a node set containing the document root. The root node in XPath (or document node in XPath 2.0) is kind of a virtual node which sits above the document element./a selects a node set containing the document element.

The expression /a/.. selects the parent of the document element which is the root node. The expression /.. selects the parent of the root node. Since the root node has no parent, it returns the empty node set. This expression is also a common idiom to select the empty node set.