What are their cmi cross reference IDs?
SELECT t1.master_customer_id,cmi_cross_ref_id FROM  Customer_Master_Index as t1 join CMI_Cross_References as t2 on t1.master_customer_id = t2.master_customer_id where t1.cmi_details not in  ( 'Schmidt, Kertzmann and Lubowitz' ) 
cmi cross reference IDs refers to cmi_cross_ref_id; their refers to entities not associated with 'Schmidt, Kertzmann and Lubowitz'; join operation is used to link Customer_Master_Index and CMI_Cross_References tables; master_customer_id is a common key in both tables;