XPath query: Query not working as expected - xpath

Question

I was learning XPath using the following xml document: http://www.w3schools.com/xpath/xpath_examples.asp
Now, when I execute the query:
bookstore/book/author[contains(.,'G')]
I get the result: Giada De Laurentiis, James McGovern as expected. Now, since contains() returns a boolean value, I expected the following query to return all authors:
bookstore/book/author[true]
however, it returns an empty set. Can somebody explain?

Answer 1

you need bookstore/book/author
UPDATE: to pass true into XPATH you have to use bookstore/book/author[true()]
author[true] just means that you want to get all author element which has true subelement.
You can check it yourself, try expressions
bookstore/book[author1] vs bookstore/book[author]
First one returns you nothing, because there's no book element with author1 subelements. Second one returns you all elements. But if you remove author subnodes from some of books nodes, you'll get only those having author subnode.
So if you take xml like this
<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">
<title lang="en">Harry Potter</title>
</book>
</bookstore>
then
bookstore/book[author] returns
<book category="COOKING">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
and bookstore/book[title] returns
<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">
<title lang="en">Harry Potter</title>
</book>

Answer 2

I expected the following query to return all authors:
bookstore/book/author[true]
however, it returns an empty set. Can somebody explain?
The above expression selects all bookstore/book/author elements that have a child element named true. In the provided XML document no author element has a child named true -- therefore the XPath expression selects nothing.
In a comment the OP asks:
But why does bookstore/book/author[true] not work, since if is
similar to the situation when the contains() always returns true
contains() never returns (the string) "true" -- it returns the boolean value true() -- this is different from the string "true".
Explanation:
You are confused by the fact that the serialization of the boolean value true() to sting is the string "true".
This fact doesnt mean that the string "true" and the boolean value true() are identical.