client	creditCard	amount
1	A		10
1	A		20
1	B		40
1	C		5	
1	C		10
1	C		20
2	A		40
2	D		60

client	creditCard 
1	B
2	D

SQL> select client, credit_card, max(amount)
  2  from t1
  3  group by client;
select client, credit_card, max(amount)
               *
ERROR at line 1:
ORA-00979: not a GROUP BY expression

select client, creditCard
  from (select client,
               creditCard,
               dense_rank() over(partition by client order by amount desc) ranking
          from (select client,
                       creditCard,
                       max(amount) amount
                  from prc_table_1
                 group by creditCard, client))
 where ranking = 1;

select * from t1
where amount in (select max(amount) from t1 group by client);

2	A		30

SELECT CLIENT, CREDITCARD, AMOUNT FROM T1
WHERE (CLIENT, AMOUNT) IN (SELECT CLIENT, MAX(AMOUNT) FROM T1 GROUP BY CLIENT);