What is the cmi cross reference id that is related to at least one council tax entry? List the cross reference id and source system code.
select t1.cmi_cross_ref_id, t1.source_system_code from cmi_cross_references as t1 join council_tax as t2 on t1.cmi_cross_ref_id = t2.cmi_cross_ref_id group by t1.cmi_cross_ref_id having count(*) >= 1