Table schema:
CREATE TABLE Electoral_Register (electoral_register_id VARCHAR, cmi_cross_ref_id VARCHAR); CREATE TABLE CMI_Cross_References (cmi_cross_ref_id VARCHAR, source_system_code VARCHAR)

Question: What are the register ids of electoral registries that have the cross reference source system code 'Electoral' or 'Tax'?
SQL: 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'