I wrote an article about the creation of a REST service that maybe can help you.

https://community.intersystems.com/post/creating-rest-service-iris

You can check the ClassMethod TestPost where a data is received from a post call and is sent finally to a business operation where is transformed into a DynamicObject and parsed to another class. This is the business operation:

Class WSTEST.BO.PersonSaveBO Extends EnsLib.REST.Operation
{

Parameter INVOCATION = "Queue";

Method savePerson(pRequest As WSTEST.Object.PersonSaveRequest, Output pResponse As WSTEST.Object.PersonSaveResponse) As %Status
{
	try {
      set person = ##class("WSTEST.Object.Person").%New()
      #dim request as %DynamicObject = {}.%FromJSON(pRequest.JSON)
      set person.PersonId = request.PersonId
      set person.Name = request.Name
      set person.LastName = request.LastName
      set person.Sex = request.Sex
      set person.Dob = request.Dob

      set tSC = person.%Save()
      set pResponse = ##class("WSTEST.Object.PersonSaveResponse").%New()
      set pResponse.PersonId = person.PersonId
      set pResponse.Name = person.Name
      set pResponse.LastName = person.LastName
      set pResponse.Sex = person.Sex
      set pResponse.Dob = person.Dob
      
   }catch{
       Set tSC="Error saving the person"
   }
   Quit tSC
}

XData MessageMap
{
<MapItems>
  <MapItem MessageType="WSTEST.Object.PersonSaveRequest">
    <Method>savePerson</Method>
  </MapItem>
</MapItems>
}

}

UserTable and DataTable are related? I'm guessing that if Condition1 is the relation between the tables you would try this query:

SELECT 
    COUNT(CASE WHEN data.a = "Condition1" then 1 ELSE NULL END) as "ValueA", 
    COUNT(CASE WHEN data.b = "Condition2" then 1 ELSE NULL END) as "ValueB", 
    COUNT(CASE WHEN data.c = "Condition3" then 1 ELSE NULL END) as "ValueC", 
user.id
 FROM UserTable user left join DataTable data on user.id = data.user

In my opinion the best approach is to create a BO for each database connection and invoke them from a BPL, getting the results and doing whatever you want. 

InitialExpression is what you are looking for:
https://docs.intersystems.com/irislatest/csp/docbook/DocBook.UI.Page.cls...

You can send the List by reference to the Python method, populate it inside the method and use the populated list from the objectscript who call it.

Something like this:

set listOfStrings = ##class("%Library.ListOfDataTypes").%New()
do ..TestPython(listOfStrings)
$$$TRACE("What a beautiful trace! There are "_listOfStrings.Count()_" element(s)")
...
...
ClassMethod TestPython(ByRef listOfStrings As %List) [ Language = python ]
{

	import iris
	
	
	listOfStrings.Insert("One")
	listOfStrings.Insert("Two")
	listOfStrings.Insert("Three")
	
	return 1
}