XPath 按属性过滤并返回不同的值

Question

From the following I need to return a distinct list of values with the Category attribute.

<root>
    <classifications>
        <classification name="Category">Category One</classification>
        <classification name="Sub-Category">Sub-Category One</classification>
    </classifications>
    <classifications>
        <classification name="Category">Category Two</classification>
        <classification name="Sub-Category">Sub-Category One</classification>
    </classifications>
    <classifications>
        <classification name="Category">Category One</classification>
        <classification name="Sub-Category">Sub-Category Two</classification>
    </classifications>
    <classifications>
        <classification name="Category">Category Three</classification>
        <classification name="Sub-Category">Sub-Category One</classification>
    </classifications>
    <classifications>
        <classification name="Category">Category One</classification>
        <classification name="Sub-Category">Sub-Category Two</classification>
    </classifications>
    <classifications>
        <classification name="Category">Category Two</classification>
        <classification name="Sub-Category">Sub-Category One</classification>
    </classifications>
</root>

Using classifications/classification[@name="Category"] I get:

Category One
Category Two
Category One
Category Three
Category One
Category Two

What I need is:

Category One
Category Two
Category Three

Answer 1

This XPath will select distinct elements:

//classification[@name = "Category" and not(preceding::classification = .)]

or more specific:

//classification[@name = "Category" 
    and not(preceding::classification[@name = "Category"] = .)
]

Answer 2

For those able to use XPath 2.0 and above:

An alternative approach that may be more flexible, and appears to be more efficient (at least in Saxon 9.8.0) is:

distinct-values(//classification[@name = 'Category'])