求SQL写法：在表中求出一字段有相等值的纪录．

初学SQL，我的表中有一个字段中有很多数据是相等的，也就是这个字段有重复值，有重复的值有多种，每一种又有多个，请问怎样把这个字段有重复值的相关纪录分组查出来，把这个字段有唯一值的纪录过滤掉，谢谢！
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select a.* from tablename as a,(
select fieldname from tablename group by fieldname
having count(*) >1
) as b
where a.fieldname=b.fieldname
order by a.fieldname

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select yourtable.* from yourtable where exists (select 1 from yourtable x where yourtable.fieldname=x.fieldname group by fieldname having count(*)>1)