"Group By" in data mapper

[lakman1]

I'm struggeling by implementing a group by functionality in data mapper. Has anyone a hint for me, how to group subnodes by property and then to calc the sum of each group?

The file has the structure below. I would like to group by the '@positionNumber' and then sum up the quantities of the subnode 'quantityFirstChoice', in this case 55 + 100 (first and third sub-node).

Thank you,

Lukas

 <ns:positions>

    <ns:position positionNumber="1">

      <ns:quantityFirstChoice>55</ns:quantityFirstChoice>

    </ns:position>

    <ns:position positionNumber="2">

      <ns:quantityFirstChoice>55</ns:quantityFirstChoice>

    </ns:position>

    <ns:position positionNumber="1">

      <ns:quantityFirstChoice>100</ns:quantityFirstChoice>

    </ns:position>

</ns:positions>