1  with tbl as (
  2                 select 1 id,'doc1' name,'Alex' username from dual union all
  3                 select 2,'doc2','Kate' from dual union all
  4                 select 5,'doc5','Kate' from dual union all
  5                 select 3,'doc3',null from dual union all
  6                 select 4,'doc1',null from dual
  7                )
  8  ,t as (
  9  select
 10    tbl.*
 11   ,sum(decode(username,null,1,0)) over (order by username,id) rn
 12  from tbl)
 13* select * from t where rn<2
SQL> /

        ID NAME USER         RN
---------- ---- ---- ----------
         1 doc1 Alex          0
         2 doc2 Kate          0
         5 doc5 Kate          0
         3 doc3               1