Show me all the council tax entries.
select * from council_tax
Show me distinct cmi cross reference ids in those.
select cmi_cross_ref_id from council_tax group by cmi_cross_ref_id
What about their corresponding 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