关于SQL分组查询的问题

如:表A,
a       b     c     d    
ems1    1     2     2
ems1    1     3     3
ems1    2     4     6
ems1    3     A     H
ems1    2     4     3
ems1    4     B     2

查询表A,要求列出所有的列.执行2次查询,第1次是b列的数据不出现重复的；第2次是b列,d列的数据都不出现重复的.
请给出脚本,谢谢!!!

select * from a a1
where not exists (
select b from a a2  --加表a别名a2
where a2.b=a1.b
group by b
having count(*)>1
)


select * from a a1
where not exists (
select b,d from a a2  --加表a别名a2
where a2.b=a1.b
and a2.d=a1.d
group by b,d
having count(*)>1
)