XQuery with dynamic XPath from XML Sql Server Column - sql-server

Question

I have a log table has xml column which contains log contents
There is also table called logType which is the type of log
I need to create query descripes the xml contents as readable string
I added a column with name logXPath to logtype table
and i created the following query
SELECT contents.value(LogXPath, 'nvarchar(max)')
FROM dbo.Log
JOIN dbo.LogType ON dbo.Log.logTypeID = dbo.LogType.logTypeID
and I got the following error
The argument 1 of the XML data type method "value" must be a string literal
and I searched for a way to do this with no results!!
Is there any do dynamic xpath in Sql Server XML Column?
Edit
for example assume the following schema and data
CREATE TABLE [dbo].[logType]
(
[logTypeID] [int] NOT NULL ,
[logTypeName] [nvarchar](50) NOT NULL ,
[xPath] [nvarchar](MAX) NOT NULL ,
CONSTRAINT [PK_logType] PRIMARY KEY CLUSTERED ( [logTypeID] ASC )
)
GO
INSERT [dbo].[logType]
( [logTypeID] ,
[logTypeName] ,
[xPath]
)
VALUES ( 1 ,
N'Patient Data' ,
N'(/Patient/PatientName)[1]'
)
INSERT [dbo].[logType]
( [logTypeID] ,
[logTypeName] ,
[xPath]
)
VALUES ( 2 ,
N'Clinic Data' ,
N'(/Clinic/ClinicName)[1]'
)
/****** Object: Table [dbo].[log] Script Date: 02/04/2015 13:58:47 ******/
GO
CREATE TABLE [dbo].[log]
(
[logID] [int] NOT NULL ,
[logTypeID] [int] NOT NULL ,
[Contents] [xml] NULL ,
CONSTRAINT [PK_log] PRIMARY KEY CLUSTERED ( [logID] ASC )
)
GO
INSERT [dbo].[log]
( [logID] ,
[logTypeID] ,
[Contents]
)
VALUES ( 1 ,
1 ,
N'<Patient><PatientID>1</PatientID><PatientName>john</PatientName></Patient>'
)
INSERT [dbo].[log]
( [logID] ,
[logTypeID] ,
[Contents]
)
VALUES ( 2 ,
2 ,
N'<Clinic><ClinicID>1</ClinicID><ClinicName>Clinic 1</ClinicName></Clinic>'
)
When I make query like the following ,it gives me the error
SELECT logTypeName ,
[Contents].value(dbo.logType.xPath, 'nvarchar(max)') AS data
FROM dbo.[log]
JOIN dbo.logType ON dbo.[log].logTypeID = dbo.logType.logTypeID

Answer 1

You can build a query dynamically using the table LogType.
declare #SQL nvarchar(max);
set #SQL = 'select case L.logTypeID'+
(
select ' when '+cast(LT.logTypeID as varchar(11))+
' then L.Contents.value('''+LT.xPath+''', ''nvarchar(max)'')'
from LogType as LT
for xml path('')
)+' end as Name from dbo.[Log] as L;';
exec (#SQL);
It will give you a query that looks like this:
select case L.logTypeID
when 1 then L.Contents.value('(/Patient/PatientName)[1]', 'nvarchar(max)')
when 2 then L.Contents.value('(/Clinic/ClinicName)[1]', 'nvarchar(max)')
end as Name
from dbo.[Log] as L;

Answer 2

Okay, so this has been here a while (a year), but this might be helpful...
A VERY helpful TABLE function: http://beyondrelational.com/modules/2/blogs/28/posts/10495/xquery-lab-58-select-from-xml.aspx
Using that function, you can get the values you're after something like this:
Select l.Contents, t.XPath, x.Value
From [log] l With (NoLock)
Inner Join [LogType] t With (NoLock)
on t.LogTypeID=l.LogTypeID
CROSS APPLY XMLTable(l.Contents) AS x
Where REPLACE(REPLACE(REPLACE(t.XPath,'[1]',''),'(',''),')','')=REPLACE('/'+x.XPath,'[1]','')

Answer 3

SQL Server does not allow replacing entire XPath expression with a variable, but you can use sql:variable and sql:column extensions inside the expression (I can't say how exactly without seeing your xml structure and what information you want to query from XML column).
Or, as mentioned above, you can use dynamic SQL:
DECLARE #xpath NVARCHAR(MAX);
SET #xpath = ... //Calculate xpath expression here
DECLARE #sql NVARCHAR(MAX);
SET #sql = N'SELECT contents.value(''' + #xpath + ''', ''NVARCHAR(MAX)''
FROM dbo.Log
JOIN dbo.LogType ON dbo.Log.logTypeID = dbo.LogType.logTypeID';
EXEC sp_executesql #sql;