-- Database schema
| Customer_Master_Index : master_customer_id [ INT ] primary_key , cmi_details [ TEXT ] | CMI_Cross_References : cmi_cross_ref_id [ INT ] primary_key , master_customer_id [ INT ] CMI_Cross_References.master_customer_id = Customer_Master_Index.master_customer_id , source_system_code [ TEXT ] | Council_Tax : council_tax_id [ INT ] primary_key , cmi_cross_ref_id [ INT ] Council_Tax.cmi_cross_ref_id = CMI_Cross_References.cmi_cross_ref_id | Business_Rates : business_rates_id [ INT ] primary_key , cmi_cross_ref_id [ INT ] Business_Rates.cmi_cross_ref_id = CMI_Cross_References.cmi_cross_ref_id | Benefits_Overpayments : council_tax_id [ INT ] primary_key , cmi_cross_ref_id [ INT ] Benefits_Overpayments.cmi_cross_ref_id = CMI_Cross_References.cmi_cross_ref_id | Parking_Fines : council_tax_id [ INT ] primary_key , cmi_cross_ref_id [ INT ] Parking_Fines.cmi_cross_ref_id = CMI_Cross_References.cmi_cross_ref_id | Rent_Arrears : council_tax_id [ INT ] primary_key , cmi_cross_ref_id [ INT ] Rent_Arrears.cmi_cross_ref_id = CMI_Cross_References.cmi_cross_ref_id | Electoral_Register : electoral_register_id [ INT ] primary_key , cmi_cross_ref_id [ INT ] Electoral_Register.cmi_cross_ref_id = CMI_Cross_References.cmi_cross_ref_id |
-- -- 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;