Every time I demonstrate using XQuery against a table that contains an SQL Server XML data type to a new class, a get about one out of four people with puzzled looks. And they're the ones who know XPath and XSLT. The part that seems weird to them is that a XQuery against what they see as "a collection of documents in a table" produces a one-row answer for each row in the table. You can tell they were expecting a single document (or fragment) answer. So,
SELECT xmlcol.query('/somequery') FROM xmltable
produces a rowset with a single column; there is one row in the "answer" for each row in the table. Eventually someone pipes up with: "well, I want just one answer". Can't you do that?
You can actually get just one answer in a few different ways. The easiest is to query the entire table using SELECT…FOR XML and do the query on the result:
DECLARE @x XML
SET @x = (SELECT xmlcol from xmltable FOR XML RAW, TYPE)
The only problem you can run into with this is the maximum size of an SQL Server XML data type instance, which is ~2gig. You also have to take the "extra" elements that FOR XML generates into account in your final query. You can also do the original XQuery, then do a FOR XML to combine the results together.
But, you can even cut it finer than that. Using an XML.nodes query with CROSS APPLY or OUTER APPLY, you can generate one row in the answer for each node in the sequence that XML.nodes returns (including or ignoring empty-sequence rows, that's what CROSS vs OUTER apply does). These "intermediate rows" from XML.nodes actually contain "virtual documents" with the context node (starting point of the XQuery) possibly pointing somewhere other than the root. You then do XQuery against the virtual document on the left-hand side of the CROSS APPLY. Like this:
SELECT custid, tab.col.query('itemno') –get itemno subelement of lineitem
CROSS APPLY (SELECT * FROM
xmlinv.nodes('//lineitem')) as tab(col)
In this case, if you have a 10-row table, but the 10 XML documents in the table contain 42 lineitem elements, you get 42 rows in the answer. Cool 'eh? If you currently use OPENXML, learn XML.nodes, because this will eventually replace OPENXML in our toolbox. It's MUCH less memory-intensive than OPENXML.
Finally (someone asked this yesterday), if you DO want to start with 1-row/answer per row in table and just throw away the rows that contain empty sequence answer, a simple subquery will do:
SELECT * FROM
(SELECT xmlcol.query('/somequery') AS col FROM xmltable) AS x
WHERE col <> ''
There. Exactly the answer you want…whatever the problem. Have fun.