mysql中的ExtractValue对我不起作用

Question

我有一个xml字符串

set @xml='<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<SeaTest xsi:schemaLocation="https://sea.retain.hawcik.com/schema    /SeaTestSchema
https://sea.retain.hawcik.com/schema/SeaTestSchema1.0.xsd"     xmlns="https://sea.retain.hawcik.com/schema/SeaTestSchema" xmlns:xsi="http://www.w9.org/2999/XMLSchema-instaccc">
<WorkSourcets>
    <TestWorkSourcets>
        <WorkSourcetsName>SITE01</WorkSourcetsName>
        <SourcetsDetails>
            <DuringssUnits>
                <DuringssUnit>
                    <DuName>TEST.NET</DuName>
                    <SourcetsStatus>REQ</SourcetsStatus>
                    <CompleteTestAsText>TEST.COM,SITE01,11/10/2014</CompleteTestAsText>
                    <MentionedDueDate></MentionedDueDate>
                    <MentionedPriority>Normal</MentionedPriority>
                    <DuSpecificAddtionlyElementtes>
                        <Elementte>
                            <ElementteName>REQUESTEDDATE</ElementteName>
                            <ElementteDataType>DATE</ElementteDataType>
                            <ElementteValue>11/10/2014</ElementteValue>
                        </Elementte>
                    </DuSpecificAddtionlyElementtes>
                </DuringssUnit>
            </DuringssUnits>
        </SourcetsDetails>
    </TestWorkSourcets>
</WorkSourcets>
</SeaTest>';

SELECT ExtractValue(@xml, '//DuName'); 

这里我想从DuName标签TEST.NET获取值。但我不知道为什么我会变空。如果上述问题出现任何问题，请告诉我。