select ...
from customers
where cust_id NOT IN (select ...
                      INTERSECT
                      select ...
                     );

WITH
    CUSTOMERS
    AS
        (SELECT 1 AS CUST_ID, 'ALI' AS CUST_NAME FROM DUAL
         UNION ALL
         SELECT 2, 'ASAD' FROM DUAL
         UNION ALL
         SELECT 3, 'ASAD' FROM DUAL
         UNION ALL
         SELECT 4, 'HUMA' FROM DUAL
         UNION ALL
         SELECT 5, 'NAZIA' FROM DUAL
         UNION ALL
         SELECT 6, 'SHUMAILA' FROM DUAL
         UNION ALL
         SELECT 7, 'PARVEEN' FROM DUAL
         UNION ALL
         SELECT 8, 'KIRAN' FROM DUAL),
    REGIONS
    AS
        (SELECT 1 CUST_ID, 'Faisabad' AREA FROM DUAL
         UNION ALL
         SELECT 6, 'karachi' FROM DUAL
         UNION ALL
         SELECT 9, 'lahore' FROM DUAL
         UNION ALL
         SELECT 11, 'peshawar' FROM DUAL
         UNION ALL
         SELECT 12, 'islamabad' FROM DUAL)
SELECT   
         COALESCE(CUSTOMERS.CUST_ID, REGIONS.CUST_ID) AS CUST_ID, 
         CUSTOMERS.CUST_NAME, 
         REGIONS.AREA
    FROM CUSTOMERS 
         FULL OUTER JOIN REGIONS 
            ON (CUSTOMERS.CUST_ID = REGIONS.CUST_ID)
   WHERE 
         CUSTOMERS.CUST_ID IS NULL 
         OR REGIONS.CUST_ID IS NULL
ORDER BY 
         COALESCE(CUSTOMERS.CUST_ID, REGIONS.CUST_ID)