SQL> select a.a,b.b from test a left join (select * from test where a in(select min(a) from test group by b)) b on a.a=b.a order by a.a;

  A B
--- ----------
  1 zxy
  2
  3
  4 zxz
  5 zxb

SQL> select * from test;

  A B
--- ----------
  1 zxy
  2 zxy
  3 zxy
  4 zxz
  5 zxb