DECLARE @xml XML
SELECT @xml='<Root>
<Data>
<Item ID="1" AdditionalInfo="Some info">
<SubItem ID="1">1</SubItem>
<SubItem ID="2">2</SubItem>
<SubItem ID="3">3</SubItem>
</Item>
<Item ID="2" AdditionalInfo="Some info">
<SubItem ID="1">1</SubItem>
<SubItem ID="2">2</SubItem>
</Item>
</Data>
</Root>'

SELECT
	y.value('@ID[1]','INT') AS SubItemId,
	y.value('.','INT') AS SubItemValue,
	c.value('@ID[1]','INT') AS ItemId,
	c.value('@AdditionalInfo[1]','varchar(30)') AS AdditionalInfo
FROM @xml.nodes('Root/Data/Item') T(c)
CROSS APPLY
c.nodes('SubItem') X(y)