CREATE TABLE customer_master_index (master_customer_id DECIMAL, cmi_details TEXT)
CREATE TABLE business_rates (business_rates_id DECIMAL, cmi_cross_ref_id DECIMAL)
CREATE TABLE parking_fines (council_tax_id DECIMAL, cmi_cross_ref_id DECIMAL)
CREATE TABLE electoral_register (electoral_register_id DECIMAL, cmi_cross_ref_id DECIMAL)
CREATE TABLE rent_arrears (council_tax_id DECIMAL, cmi_cross_ref_id DECIMAL)
CREATE TABLE cmi_cross_references (cmi_cross_ref_id DECIMAL, master_customer_id DECIMAL, source_system_code TEXT)
CREATE TABLE benefits_overpayments (council_tax_id DECIMAL, cmi_cross_ref_id DECIMAL)
CREATE TABLE council_tax (council_tax_id DECIMAL, cmi_cross_ref_id DECIMAL)

Which cmi cross reference id is not related to any parking taxes?
SELECT cmi_cross_ref_id FROM cmi_cross_references EXCEPT SELECT cmi_cross_ref_id FROM parking_fines
SELECT "cmi_cross_ref_id" FROM "cmi_cross_references" EXCEPT SELECT "cmi_cross_ref_id" FROM "parking_fines"