Netestoval jsem, ale musí to fungovat:
SELECT case when lag(CUSTOMERS.COMPANY_NAME, 1, null) over
(partition by CUSTOMERS.COMPANY_NAME
order by COMPANY_NAME, COUNT(CALLS.CALL_REF) DESC) is null
then CUSTOMERS.COMPANY_NAME else null end cmp_name,
CUSTOMERS.CUSTOMER_ID AS REF,
CUSTOMERS.COMPANY_NAME,
COUNT (CALLS.CALL_REF) AS TOTAL_CALLS,
CALLERS.CALLER_LAST_NAME AS LAST_NAME,
COUNT(CALLS.CALL_REF)AS CALLS
FROM CUSTOMERS
INNER JOIN CALLERS
ON COMPANY_ID=CUSTOMER_ID
LEFT JOIN CALLS
ON CALLERS.CONTACT_ID=CALLS.CONTACT_ID
GROUP BY CUSTOMERS.CUSTOMER_ID,
CUSTOMERS.COMPANY_NAME,
CALLERS.CALLER_LAST_NAME
ORDER BY REF, COMPANY_NAME, CALLS DESC;