Question: 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.
SQL: 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