What are the register ids of electoral registries that have the cross reference source system code 'Electoral' or 'Tax'?
select t1.electoral_register_id from electoral_register as t1 join cmi_cross_references as t2 on t1.cmi_cross_ref_id = t2.cmi_cross_ref_id where t2.source_system_code = 'Electoral' or t2.source_system_code = 'Tax'