Select an element that has another element anywhere inside

Question

How do I select any node a that has node b anywhere inside it?

Given the following three XML documents:

<a>
    <b></b>
</a>

or

 <a>
    <c>
        <b></b>
    </c>
</a>

or

   <a/>

I want the a element in the first two documents to be selected.

Apparently, a[//b] is not a solution.

Answer 1

a[descendant::b]

is more accurate and efficient than

a[.//b]

which is equal to

a[self::node()/descendant-or-self::node()/child::b]

Answer 2

You should try:

//a[.//b]