XPath 按屬性過濾並返回不同的值

Question

從下面我需要返回具有Category屬性的不同值列表。

<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>

使用classifications/classification[@name="Category"]我得到：

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

我需要的是：

Category One
Category Two
Category Three

Answer 1

此 XPath 將選擇不同的元素：

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

或更具體的：

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

Answer 2

對於那些能夠使用 XPath 2.0 及更高版本的人：

另一種可能更靈活且似乎更有效（至少在 Saxon 9.8.0 中）的替代方法是：

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