--不会是这样吧？
selectt * from A where a=b 


selectt * from A where a!=b and ianull(a,'')!='' and ianull(b,'')!=''

select * from a x
where exists (
select 1 from a
where id<>a.id 
and (a=x.a or b=a.b)
)

select * from a x
where a<>''
and b<>''
and not exists (
select 1 from a
where id<>a.id 
and a=x.a
)
and not exists (
select 1 from a
where id<>a.id 
and b=x.b
)

select * from a 
 where a in (select a from a group by a having count(a)>1)
    or b in (select b from a group by b having count(b)>1)
select * from a 
 where (a in (select a from a group by a having count(a)=1)
    or b in (select b from a group by b having count(b)=1))
    and a<>'' and b<>''
 

  现有表A,其中有两个字段a,b,问从表A中抽出数据,要求A.a或A.b有重复，另抽出一张表A.a,A.b无重复且这两项不等于空白.

select a,b from tba
where a<>'' and b<>''
group by a,b

select a,b from tba
where (a<>'' or a is not null) and (b<>'' and b is not null)
group by a,b

select * from ta a 
where (select count(*) from a.a = a and a.b = b ) = 1