group by clause in X++
===

Another weird question, is it possible to specify an expression in a group by clause in X++? basically, how can I do the following:

    while select InventDim **group by substr(InventDim.inventBatchId,1,3)**
    join InventDimId from InventTrans
    where InventTrans.inventDimId == Inventdim.inventDimId && InventTrans.ItemId == _itemId
    {
    MyQty += inventTrans.Qty
    }

thanks for your help
regards,
Thomas