XML is a format for storing EAV, and as such doesn't fit well in any relational database. You can take unstructured data like a text or an XML file and force it into a structured store like SQL, but there isn't a good "fit" any way that I've seen.
Some databases incorporate an enhanced BLOB format specifically for coping with XML data. This is probably the best solution that I've seen for coping with XML within SQL.
-PatP