Stripping data from xml in SQL Server - sql

Question

One of my tables with xml datatype has the following xml information:
<RequestMetaData xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<MetaData Type="DocImport">
<Keywords>
<Key Name="Zone" Value="MIO" />
<Key Name="ClassificationStrategy" Value="NeedClassification" />
<Key Name="Folder" Value="0456e6ca" />
</Keywords>
</MetaData>
<MetaData Type="SourceResponse">
<Keywords>
<Key Name="NotificationResponse_20180427-150426" Value="Received successful response from Source" />
</Keywords>
</MetaData>
</RequestMetaData>
I need to write an SQL query to fetch the value of Classification strategy based on key name.
I have added the xml in a variable #xml and used the following code. It is returning NULL.
select A.b.value('ClassificationStrategy[1]', 'VARCHAR(30)') AS CS
FROM #xml.nodes('/RequestMetaData/MetaData/Keywords') AS A(b)
Can someone please help me with this.

Answer 1

You can read your XML in various ways. Use a simple .value() with an XPath/XQuery expression to retrieve a single value, use .query to retrieve a part of the XML or use .nodes() to return repeated elements as derived table:
DECLARE #xml XML=
N'<RequestMetaData xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<MetaData Type="DocImport">
<Keywords>
<Key Name="Zone" Value="MIO" />
<Key Name="ClassificationStrategy" Value="NeedClassification" />
<Key Name="Folder" Value="0456e6ca" />
</Keywords>
</MetaData>
<MetaData Type="SourceResponse">
<Keywords>
<Key Name="NotificationResponse_20180427-150426" Value="Received successful response from Source" />
</Keywords>
</MetaData>
</RequestMetaData>';
--Read the whole lot
SELECT md.value('#Type','nvarchar(max)') AS MetaDataType
,k.value('#Name','nvarchar(max)') AS KeyName
,k.value('#Value','nvarchar(max)') AS KeyValue
FROM #xml.nodes('/RequestMetaData/MetaData') A(md)
OUTER APPLY md.nodes('Keywords/Key') B(k);
--Get one key's value by name (anywhere in the doc)
DECLARE #keyName VARCHAR(100)='ClassificationStrategy';
SELECT #xml.value('(//Key[#Name=sql:variable("#keyName")]/#Value)[1]','nvarchar(max)');
--Use the meta data type as additional filter (if key names are not unique per doc)
DECLARE #kName VARCHAR(100)='ClassificationStrategy';
DECLARE #mdType VARCHAR(100)='DocImport';
SELECT #xml.value('(/RequestMetaData
/MetaData[#Type=sql:variable("#mdType")]
/Keywords
/Key[#Name=sql:variable("#kName")]
/#Value)[1]','nvarchar(max)');