What are their respective source system codes?
SELECT t1.master_customer_id,cmi_cross_ref_id,source_system_code  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 ! =   ( 'Schmidt, Kertzmann and Lubowitz' ) 
respective source system codes refers to source_system_code; joining tables Customer_Master_Index and CMI_Cross_References on master_customer_id; filtering out records with specific cmi_details value.