Here is some code from my VB app - which could be easily transferred to
c# if needed - basically you add a parameter in the web service that
filters your dataset then from InfoPath you specify the parameter in
the XML that gets sent to the service. The biggest problem is making
sure the names spaces and Xpath's are correct. This code calls my web
service for getting a unique ID.

' dataconnection to my webservice is called: UniqueID
Private Function GetNextUniqueID(Optional ByVal KeyName As String =
"InfopathApp") As String
Try
Dim UniqueID As DataObject = thisXDocument.DataObjects("UniqueID")
Dim domtemplate As IXMLDOMDocument3 = UniqueID.DOM

'Set the SelectionNamespaces so that you can find the correct field
domtemplate.setProperty("SelectionNamespaces", _

"xmlns:dfs=""http://schemas.microsoft.com/office/infopath/2003/dataFormSolution"""
& _
           "
xmlns:d=""http://schemas.microsoft.com/office/infopath/2003/ado/dataFields"""
& _
           "
xmlns:dsf=""http://schemas.microsoft.com/office/infopath/2003/dataFormSolution"""
& _
           " xmlns:ns3=""UniqueIDURI""")

'Set Web Service parameters
domtemplate.selectSingleNode("/dfs:myFields/dfs:queryFields/ns3:UniqueID/ns3:applicationName").text
= KeyName

UniqueID.Query()

' get value from: UniqueIDResponse/UniqueIDResult
Return
domtemplate.selectSingleNode("//dfs:dataFields//UniqueIDResponse//UniqueIDResult").text
Catch ex As Exception
' unique connection is not running - create one and HOPE you are unique
Dim today As Date = Now()
Return today.ToString("yyyyMMddhhmmss")
End Try

in my UniqueID webservice I have an input parameter to filter the
dataset:
<WebMethod()> _
Public Function UniqueID(ByVal ApplicationName As String) As String
'' if you are opening a recordset directly with a sql clause you may
want to check for sql injections:
'' ApplicationName = ApplicationName.Replace("'","''")
'' ...