Need to create group by the value in the value set

Create table xx (ledger_name varchar2(20),Ledger_set varchar2(20);

insert into xx values('LS NUS','N US');

insert into xx values('RS NUS','N US');

insert into xx values('LS NUS','S US');

insert into xx values('RS NUS','S US');

I want to create a value set to show the ledger_name based on the ledger_set entered. But if the ledger_set is not entered then i want to show the distinct values in the ledger_name.

Thanks in Advance

Thanks,

Vedavrath