How to read data from multiple XML files in SQL Server? - sql

Question

Background :
I want to obtain data from multiple XML files (stored in database) and fetch them into one result set. The basic working solution, with single XML file looks similar to this one :
DECLARE #xml xml
SET #xml =
(SELECT TOP 1 convert(varchar(max), convert(varbinary(max), [XML_FILE]))
FROM [SOME_TABLE])
SELECT
b.value('(./SomeNode/text())[1]','nvarchar(100)')) as [Some_Text],
b.value('(./SomeOtherNode/#VAL)[1]','int')) as [Some_Val]
FROM #xml.nodes('Example/File') as a(b)
Obviously this won't work with SELECT that returns many rows (many XML files). Sub-optimal solution could be achieved using cursor (iterating over collection -> pushing data into temporary table -> SELECT (*) FROM temporary_table) however, I believe thats not necessary and more straightforward solution can be achieved.
Question :
How to fetch data from multiple XML files, obtained via SELECT query, into a single result-set, without using cursor?
FILE_NAME || Value 1 || Value 2 || ...
----------------------------------------------
XML_FILE_1 || Node1Value || Node2Value || ...
XML_FILE_2 || Node1Value || Node2Value || ...

Answer 1

I've found solution thanks to #Shnugo answer.
If the type of xml-container column is different then XML MS-SQL dedicated one, then double CROSS APPLY should be performed. Example below :
DECLARE #mockup TABLE(ID INT IDENTITY, [XML_DATA] VARBINARY(MAX));
INSERT INTO #mockup VALUES('<Example><File><SomeNode>blah</SomeNode><SomeOtherNode VAL="1"/></File></Example>')
,('<Example><File><SomeNode>blub</SomeNode><SomeOtherNode VAL="2"/></File></Example>')
SELECT
ID,
b.value('(SomeNode/text())[1]','nvarchar(100)') as [Some_Text],
b.value('(SomeOtherNode/#VAL)[1]','int') as [Some_Val]
FROM #mockup
CROSS APPLY (SELECT CAST(convert(varbinary(max), [XML_DATA]) as XML)) as RAW_XML(xml_field)
CROSS APPLY RAW_XML.xml_field.nodes('Example/File') as a(b)

Answer 2

For sure the CURSOR approach is not needed and would be wrong entirely...
The general approach should be something like this:
SELECT
b.value('(./SomeNode/text())[1]','nvarchar(100)') as [Some_Text],
b.value('(./SomeOtherNode/#VAL)[1]','int') as [Some_Val]
FROM [SOME_TABLE]
CROSS APPLY [XML_FILE].nodes('Example/File') as a(b);
But there are questions open:
Speaking about xml files is a bit bewildering... I hope to get this correctly, that all these XMLs are living in a table's column.
If the first is true: Are all these XMLs of the same structure? if not you will need some kind of filtering.
is the XML in your table's column a native XML-type already? Your example uses CONVERT extensivly... You will need a native XML in order to use .nodes()
If there's no native XML: Do you have to deal with invalid / uncastable data?
Are there rows with no data but you want to see them anyway? In this case you can try OUTER APPLY instead of CROSS APPLY.
For demonstration a running stand-alone mockup:
DECLARE #mockup TABLE(ID INT IDENTITY, [XML_FILE] XML);
INSERT INTO #mockup VALUES('<Example><File><SomeNode>blah</SomeNode><SomeOtherNode VAL="1"/></File></Example>')
,('<Example><File><SomeNode>blub</SomeNode><SomeOtherNode VAL="2"/></File></Example>')
SELECT
ID,
b.value('(SomeNode/text())[1]','nvarchar(100)') as [Some_Text],
b.value('(SomeOtherNode/#VAL)[1]','int') as [Some_Val]
FROM #mockup
CROSS APPLY [XML_FILE].nodes('Example/File') as a(b)