Choose First match from an XPath - xpath

Question

The XPath (bookstore/book/title|bookstore/book/author) selects title, author if both of them exist
How can I select just the first match of these two and not both, and get this value for all the 'book' nodes in the document
(bookstore/book/title|bookstore/book/author)[1] limits the result to just the first 'title' in the first book.But I need to be able to get results from other book nodes in the document

Answer 1

I'm assuming that by 'first' you mean 'first in document order', not 'first referenced in my XPath expression.'
In XPath 2.0, you can say
bookstore/book/((title|author)[1])
If you only have XPath 1.0, let us know and we can proceed from there. Also let us know something of the broader environment (XSLT? XQuery? Javascript?) because some of this may have to be done outside of XPath.
Update: I just tested this, using Simple Online XPath Tester with XPath 2.0. Given the following XML input:
<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
<book category="COOKING">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
<book category="CHILDREN">
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<book category="WEB">
<title lang="en">XQuery Kick Start</title>
<author>James McGovern</author>
<author>Per Bothner</author>
<author>Kurt Cagle</author>
<author>James Linn</author>
<author>Vaidyanathan Nagarajan</author>
<year>2003</year>
<price>49.99</price>
</book>
<book category="WEB">
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>
</bookstore>
and the XPath expression
/bookstore/book/((title|author)[1])
I get the following output, which appears to be what you asked for:
<title lang="en">Everyday Italian</title>
<author>J K. Rowling</author>
<title lang="en">XQuery Kick Start</title>
<title lang="en">Learning XML</title>
For XPath 1.0
If you don't have XPath 2.0, as I suspect you don't, and you still want to do it all in XPath, here's what I would do:
/bookstore/book/title[1] | /bookstore/book[not(title)]/author[1]
What does this do? It gives the (first) title of each book that has a title, as well as the (first) author of each book that doesn't have a title.
This expression is not quite as general as what you asked for: it assumes that <title> comes before <author> when it exists, as it does in your sample data. If your data has author before title, then the above expression will still prefer title despite the order.
If you really need the first of the two regardless of whether it's author or title, try
/bookstore/book/title[1][not(preceding-sibling::author)] |
/bookstore/book/author[1][not(preceding-sibling::title)]